Synthesis and characterization of rhodium-aluminum heterobimetallic complexes tethered by a 1,3-bis(diphenylphosphino)-2-propanoxy group

Article abstract of DOI:10.1039/c9dt00938h

We demonstrate the synthesis and characterization of a new class of late transition metal-aluminum heterobimetallic complexes. A bridging ligand which both chelates the transition metal and binds the aluminum via an alkoxide was employed to impart stabili

Full text of DOI:10.1039/c9dt00938h

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Synthesis and characterization of rhodium–
aluminum heterobimetallic complexes tethered by
Cite this: DOI: 10.1039/c9dt00938h
a 1,3-bis(diphenylphosphino)-2-propanoxy group†
Zhongjing Li,a Timothy W. Yokley, a Sheila L. Tran,a Jie Zong,b
a
Nathan D. Schley c and Timothy P. Brewster
*
We demonstrate the synthesis and characterization of a new class of late transition metal-aluminum het-
erobimetallic complexes. A bridging ligand which both chelates the transition metal and binds the alumi-
num via an alkoxide was employed to impart stability to the bimetallic system. Novel rhodium–aluminum
heterobimetallic complexes Rh(DPPE)(DPPP-O-AliBu2Cl) and Rh(DPPP-O-AliBu2)(DPPP-O-AliBu2Cl) are
synthesized and spectroscopically characterized.
Received 2nd March 2019,
Accepted 14th May 2019
DOI: 10.1039/c9dt00938h
rsc.li/dalton
able to activate and transfer hydrogen.17 One catalysis draw-
back of these systems is the limited access of substrate to the
Introduction
Heterobimetallic complexes have gained intense attention due Lewis acidic site. Aluminum is completely caged by the ligand,
to their interesting structures, properties, and applications thereby relegated to inductively adjusting the electronic pro-
such as in theranostics,1 medical imaging,2 and catalysis.3,4 In perties of the metal center – it cannot participate cooperatively
particular, early–late heterobimetallic complexes have been in catalysis without ligand dissociation.
widely explored.5–7 The large polarity difference between metal
Bourissou20 and Emslie21 developed alternative, sterically
centers make them nicely suitable for cooperative heterolytic accessible systems by employing geometrically constrained
bond activations.8,9 In a similar fashion, as the most electropo- phosphine-alane ligands. The complexes obtained by
sitive element in group 13, a highly Lewis acidic aluminum(III) Bourissou cooperatively activated bonds in CO2, CS2, H2, and
center would be expected to impart significant polarity into a PhCONH2, though no catalysis was reported. In Emslie’s
late transition metal heterobimetallic system.
systems, a 1,1′-bis(phosphino)ferrocene ligand was employed
Aluminum-containing heterobimetallics are rare in com- to facilitate formation of a Pt–Al bond. In the first catalytically
parison to early/late transition metal systems and when com- useful example of a Z-type late metal-aluminum heterobimetal-
pared to analogous complexes containing its less acidic Group lic system, Iwasawa employed a terpyridine pincer ligand with
13 congener, boron.10–15 The scarcity of aluminum heterobi- two coordinating phosphorus arms to cage Pd and Al.22 The
metallic complexes is largely due to the challenging synthesis. resulting complexes are effective for hydrosilylation of CO2.
Successful syntheses typically require careful ligand design to Later, a Ni/Al complex reported by Nakao was found to facili-
buttress Al to the late transition metal. For example, represen- tate C–H functionalization of pyridines.23 Interestingly, a
tative complexes developed by Lu feature a hepta-dentate formal metal-aluminum bond is not required for cooperative
ligand that cages Al and the transition metal to facilitate for- catalysis to occur, merely proximity of the two sites. For
mation of a Z-type bond.15–19 Lu’s aluminum complexes have example, hydrocarbamoylation of alkenes was achieved in a
been found capable of N2 activation,15 with gallium analogues bridged Ni/Al system reported by Cramer.24 The limited
number of well characterized examples of late metal-alumi-
num heterobimetallics, combined with their recently demon-
aDepartment of Chemistry, The University of Memphis, 3744 Walker Avenue,
strated catalytic utility, makes the development of new hetero-
Smith Chemistry Building, Memphis, Tennessee 38152, USA.
bimetallic constructs an area ripe for exploration.
E-mail: tbrwster@memphis.edu
bDepartment of Chemistry, The Pennsylvania State University, University Park,
PA 16802, USA
Our laboratory has been exploring the synthesis of Ir–Al
and Rh–Al heterobimetallic complexes, with the aim of exploit-
ing these two metal centers for cooperative activation of small
molecules in a manner reminiscent of a Frustrated Lewis
Pair.25–27 A “transition metal first” synthetic strategy in which
the reactive aluminum center is added to the bimetallic
cDepartment of Chemistry, Vanderbilt University, Nashville, Tennessee 37235, USA
†Electronic supplementary information (ESI) available: NMR spectra of newly
synthesized compounds, mass spectrometry data, and X-ray data. CCDC
1900599–1900601. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c9dt00938h
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species following the synthesis of the transition metal com- to avoid unintentional formation of [Rh(DPPE)2]+, [Rh
ponent was developed to overcome the inherent synthetic (DPPP-OH)2]+, DPPP-OH bridged dimers,32 or other species.
difficulty of working with moisture sensitive compounds.28 In First, Rh(DPPE)(CO)Cl was obtained by reacting DPPE with
recent experiments,
a
proof-of-concept result has been [Rh(CO)2Cl]2.31 The 31P{1H} NMR spectrum of Rh(DPPE)(CO)Cl
obtained demonstrating that the heterobimetallic Ir–Al and showed two sets of dd peaks that are characteristic of Rh–P
Rh–Al complexes developed in our laboratory can facilitate the and P–P splitting (see Fig. S3 in the ESI†). Further treatment of
activation of small molecules.29
Rh(DPPE)(CO)Cl with DPPP-OH resulted in the formation of 1.
We now report the synthesis and characterization of a new The 31P{1H} NMR spectrum of 1 displayed a diagnostic, two
series of Rh–Al heterobimetallic complexes. In our initial signal pattern for the two phosphorus in DPPE and the two
paper,28 we reported a slow, thermal decomposition of the phosphorus in DPPP-OH respectively, as shown in Fig. S6 in
rhodium-containing heterobimetallic complexes. Analysis of a the ESI.† The two phosphorus centers in each DPPE or DPPP
decomposition product informed us that dissociation of the are chemically equivalent (δ 59.68 ppm and 16.78 ppm,
bridging pyridone ligand from rhodium was responsible for respectively) but magnetically inequivalent, leading to an AA′
the observed decomposition. To alleviate this concern we envi- BB′X spin system wherein each phosphorus atom is coupled to
sioned replacing the pyridine moiety with
phosphine ligand. In the work reported below, 1,3-bis(diphe-
a
chelating the other three and the I = 1/2 rhodium center.
The observed 31P{1H} coupling patterns within 1 are
nylphosphino)-2-propanol (DPPP-OH)30 has been utilized second-order as a result of the coupling between two nuclei
to tether rhodium and aluminum. As in our previous work, with identical chemical shifts [2JP–P ≫ 10 × (Δδ)]. Thus, the
to ensure stable attachment, aluminum will be linked to the coupling constants cannot be extracted by simply using peak
bridging ligand through a thermodynamically strong Al–O spacing and relative peak intensity. However, the pattern can be
bond.
effectively modeled based on parameterized chemical shift and
coupling constants, as shown in Fig. 1 below, allowing for full
coupling analysis. As is often observed in highly second-order
spin systems, the sign of the coupling constant J has a remark-
able effect on the observed pattern. In this system, cis 2JP–P coup-
Results and disscussion
Synthesis
ling constants were found to be negative (parallel spins lower in
2
The general synthesis procedures are summarized in Scheme 1 energy) while the trans JP–P coupling constants were positive
2
(for further information, see Experimental details). The overall (opposite spins lower in energy). Measurement of negative JP–P
strategy is similar to our reported procedure,28 where alumi- values is not uncommon in bis-phosphine systems.33
num was introduced at the end of the reaction sequence by
Treatment of 1
with AliBu3 resulted in the target
reacting tri-isobutyl aluminum (AliBu3) with an aluminum- heterobimetallic complex Rh(DPPE)(DPPP-O-AliBu2Cl) (2).
binding hydroxyl group on pre-assembled transition metal Unfortunately, the well-known complex [Rh(DPPE)2]Cl34,35 was
complexes, thereby taking advantage of the easy formation of formed as a byproduct during the synthesis of 2. The two com-
aluminum–oxygen bonds and eliminating the need to further plexes are not easily separated, and the bis-DPPE impurity is
manipulate air- and moisture-sensitive aluminum compounds. present as a minor component in our product spectra (see
In the synthesis of Rh(DPPE)(DPPP-OH)Cl (1), the assembly Fig. S7 in ESI†). After extensive recrystallization, the mole per-
was designed to install DPPE and DPPP-OH sequentially on to centage of [Rh(DPPE)2]Cl could be reduced to below 5% (calcu-
rhodium from readily accessible Rh2(CO)4Cl2.31 The reaction lated from integration of the 31P{1H} NMR spectrum of the
conditions were empirically optimized and carefully controlled product mixture). The 31P{1H} NMR spectrum of 2 displayed a
Scheme 1 Synthetic procedure for Rh(DPPE)(DPPP-O-AliBu2Cl) and Rh(DPPP-O-AliBu2)(DPPP-O-AliBu2Cl).
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Fig. 1 Modeled (blue, above) and experimental (red, below) fine pattern of the 31P{1H} NMR spectrum for the two phosphorus atoms in the DPPE
ligand (PB, P’B in table) in complex 1. An analogous peak is found for the DPPP-OH ligand. Inset: P–P and Rh–P coupling constants; PA, PA’
(DPPP-OH), PB, PB’ (DPPE). For full spectrum, see Fig. S3 in the ESI.†
similar, diagnostic, coupling pattern to that observed in 1.
In an effort to eliminate the formation of unwanted inse-
Aluminum coordination causes the second order multiplet for parable byproducts, we then shifted our attention to the syn-
the DPPP-O-AliBu2Cl phosphorus resonance to shift slightly thesis of a new target complex, Rh(DPPP-O-AliBu2)(DPPP-O-
upfield to δ 13.90 ppm from the resonance observed in the AliBu2Cl) (4), where we replaced the troublesome DPPE ligand
parent complex (δ 16.78 ppm, see Fig. 2). The DPPE resonance with a second equivalent of DPPP-OH. The precursor, [Rh
remains largely unchanged at δ 59.37 ppm. Full coupling con- (DPPP-OH)2]Cl (3), was synthesized in a straightforward manner
stant information for this AA′BB′X system can be found in the by treating [Rh(COD)Cl]2 31 with 4.2 equivalents of DPPP-OH fol-
Experimental details and in the ESI.†
lowed by washing COD and excess DPPP-OH away with a mixed
The 1H NMR spectrum of 2 also contains three critical diag- solvent system of pentane/dichloromethane (v/v = 5/1). Though
nostic features that unambiguously confirm our assignment of we were unable to obtain X-ray quality crystals of the chloride
the structure of the major reaction product. First, the inte- complex, anion metathesis with Na(BArF24) in dichloromethane
gration of the signals corresponding to alane protons clearly (BArF24 = tetrakis(3,5-trifluoromethylphenyl)borate) followed by
indicated the loss of one equivalent of its alkane substituent. slow precipitation from a solution of [Rh(DPPP-OH)2][BArF24] (3-
Second, the remaining methylene protons between δ 0.65 and BArF) in chloroform layered with pentane did yield suitable crys-
0.75 are now diastereotopic, coupled to each other with a tals of the BArF24 analogue (Fig. 4). The complex is found to
geminal coupling constant of 13.6 Hz. This strongly indicates have a distorted square planar geometry, with cis P–Rh–P angles
successful docking to the alkoxide and the formation of a of 88.51° and 88.18° within the metallacycles and 98.33° and
tetrahedral aluminum center via chloride coordination. 98.67° between the metallacyles (sum of angles = 373.69°). The
Third, the signal arising from the methine protons shifted two hydroxide functionalities also are located in equatorial posi-
downfield due to steric deshielding36 after reaction between tions of their respective pseudo-chair metallacycles and are
iBu3Al and the hydroxyl group on DPPP-OH. These phenomena found to be mutually cis within the spiro ring system.
are consistent with nitrate-bound aluminum centers in the Interestingly, the deviation from planarity observed in the solid
heterobimetallic complexes previously described in our labora- state structure of 3-BArF is not observed in the solid state struc-
tory.28 The 1H NMR spectrum was modeled using simple, first- ture of [Rh(DPPP)2][Cl]37 (DPPP = diphenylphosphinopropane,
order coupling parameters to obtain the relevant coupling structure found in Fig. 4, sum of angles = 360.1°) suggesting
information (as shown in Fig. 3). Full coupling parameters can that the steric demand of the hydroxyl group has a measurable
be found in the Experimental details.
impact on the observed structural metrics.
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Fig. 2 Truncated 31P{1H} NMR spectrum of 2 showing the shift in the DPPP-O-R resonance on moving from R = H (bottom) to R = Al(iBu2)Cl (top).
Fig. 3 Modeled (top) and measured (bottom) 1H NMR spectrum of aluminum alkyl resonances in 2.
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reactivity. The 1H NMR spectrum (Fig. S12†) is quite broad,
but features a number of upfield (δ < 0.5 ppm) resonances con-
sistent with the presence of the aluminum alkyl. The 31P{1H}
NMR spectrum of 4 at room temperature features a sharp
doublet and a broad secondary feature (as shown in Fig. S11,
ESI†). Variable temperature NMR in tetrahydrofuran-d8 was
carried out to further elucidate the observed broadness of the
31P{1H} spectrum, as shown in Fig. 6, below.
Two processes were observed as temperature changes. The
first process is evident at very low temperature. Two broad
peaks (∼δ 17.44 and δ 11.72 ppm) with clear separation could
be observed at −85 °C via 31P{1H} NMR spectroscopy. As the
temperature is raised these two peaks began to broaden and
shift with coalescence observed at approximately −15 °C. In the
second dynamic process, a doublet with a coupling constant of
133 ppm, first observable at −55 °C, first sharpens significantly
with rising temperature. Above room temperature, this signal
begins to broaden slightly and shift downfield. Though we were
unable to increase the temperature above 65 °C on our NMR
spectrometer, it appears that the broad, underlying feature
begins to coalesce with the sharp doublet at high temperature.
The distinction between the two features is evident in the room
temperature spectrum shown in Fig. S11 (ESI†). Importantly,
signals arising from the precursor complex 3 are not observed
in the product spectrum suggesting that the observed behavior
is intrinsic to the reaction product mixture, 4.
Though the exact nature of the dynamic species in solution
has not yet been rigorously established, we have significant evi-
dence that the formulaic assignment of complex 4 is correct.
First, ESI-MS (Fig. S13 and S14 in ESI†) of 4 under different
ionization conditions showed different patterns that could be
assigned as fragments arising from 4. Under all ionization con-
ditions, a peak at 960.08 from precursor [Rh(DPPP-OH)2]+ (m/z
= 960) was seen suggesting the Al–O bond as a likely fragmen-
tation location (the starting material was not observed by NMR
spectroscopy). Higher-mass fragments assignable to [Rh(DPPP-
O-AliBu)(DPPP-O-AliBu)·H2O]+ (m/z = 1144, water arises from
injection port contamination) and [Rh(DPPP-O-AliBu2)(DPPP-
O-AliBu2)]+ (m/z = 1240) were also easily identified, confirming
that aluminum has docked successfully. High Resolution Mass
Spectrometry provided further validation for the formation of
the desired product (Fig. S15, ESI†). At no point in any ESI-MS
experiment were mass fragments greater than the parent ion
of 4 observed, strongly suggesting that 4 is monomeric, rather
than dimeric or oligomeric, in solution.
Fig. 4 Top: ORTEP representations of 3-BArF. Left: Carbons 2–6 of
each phenyl ring omitted for clarity. Right: Full structure. Bottom:
ORTEP representation of [Rh(DPPP)2][Cl]. For all structures, ellipsoids
shown at 50% probability. Hydrogen atoms and anion omitted from
each structure for clarity.
Interestingly, the synthesis of 3 from [Rh(CO)2Cl]2 was not
successful. CO dissociation from Rh was found to be incom-
plete, leading to the formation of the penta-coordinate
complex [Rh(DPPP-OH)2(CO)]Cl (5). An X-ray crystal structure
of this unexpected product was obtained by layering a chloro-
form solution of the complex with pentane (Fig. 5). Complex 5
displays a trigonal bipyramidal structure (sum of equatorial
angles = 360.1°, Paxial–Rh–Paxial = 173.76°) with the carbonyl
ligand occupying one of the equatorial positions. As in the
structure of 3-BArF above, the two hydroxide functionalities
also are located in equatorial positions of their respective
pseudo-chair metallacycles and are found to be mutually syn
within the spiro ring system.
Treatment of 3 with AliBu3 resulted in the target heterobi-
metallic complex Rh(DPPP-O-AliBu2)(DPPP-O-AliBu2Cl) (4).
Unfortunately, solid-state characterization of 4 has not been
possible due to its dynamic nature and its intrinsically high
Further support for a monomeric product was obtained
from DOSY (Fig. S18 and S19†). Complexes in the product
mixture 4 all diffuse at the same rate, indicating a uniform
product size. Importantly, they also diffuse at the same rate as
complex 3, which is known to be monomeric from the solid
state structure of 3-BArF. This suggests that 3 and 4 are of
similar size. Were 4 dimeric or oligomeric, it would be
expected to diffuse much more slowly than 3.
Fig. 5 ORTEP representations of 5. Ellipsoids shown at 50% probability.
Hydrogen atoms and chloride counterion omitted for clarity. Left:
Carbons 2–6 of each phenyl ring omitted for clarity. Right: Full
structure.
With this knowledge in hand, we propose a reasonable
explanation for the dynamic behavior observed in the NMR
spectra of 4. We first postulate that the two distinct fluxional
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Fig. 6 Variable temperature 31P{1H} NMR of 4. Temperature recorded across a range from −85 °C to 65 °C.
processes derive from two different, isomeric products. This is
Additionally, we postulate that the dynamic behavior
consistent with complex 3 which exists as a mixture of syn and observed for each isomer relates to chloride exchange between
anti isomers (Fig. 7). It is quite likely, given the low tempera- the aluminum centers. As can be clearly seen in Fig. 7, the two
ture of the synthesis of 4 from 3 that interconversion between DPPP-O-Al ligands in 4 should be chemically inequivalent, as
the syn and anti forms is not possible during synthesis. Thus, one contains a bound chloride and the other does not. One
observing two distinct species with distinct dynamic behavior would, thus, expect an AA′BB′X coupling pattern in the phos-
is not unexpected. At high temperatures, where dissociative phorus NMR as was observed for complexes 1 and 2. What is
interconversion may become feasible, the two species would instead observed at room temperature is a sharp doublet, indi-
be expected to interchange. This is consistent with the cating that all four phosphorus centers are equivalent (via
observed high temperature broadening in Fig. 6. Notably, any exchange). The most likely explanation for this is chloride
dissociative pathways available above room temperature could scrambling. The different molecular geometries of the syn and
also facilitate intramolecular scrambling of DPPP-O-AliBu2 anti isomers would cause differential activation barriers to
ligands with DPPP-O-AliBu2Cl ligands further contributing to exchange, thus resulting in their unique dynamic behavior.
line broadening and complicating the analysis of the high-
Though we believe that our explanation above is the most
temperature spectra. We favor dissociative (as opposed to plausible given the data in hand, other possibilities for
associative) ligand exchange as
a mechanism due to dynamic behavior exists. First, each diastereomer could
precedent38,39 and the steric bulk of the phosphines.
undergo slow chair–chair interconversion. Given the signifi-
Fig. 7 Possible diastereomers of 4.
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cant preference for the bulky O-AliBu2Cl to occupy an equator- calcium hydride and stored over molecular sieves in the glove-
ial position off the six-membered metallacycle, we believe that box. Deuterated solvents were purchased from Cambridge
this is unlikely to be observed. Other potential dynamic behav- Isotope Laboratories, dried over molecular sieves, and stored
ior derives from the aluminum center itself. To maintain in the glovebox prior to use. DPPE was purchased from Sigma-
charge balance, at least one aluminum must be coordinatively Aldrich. DPPP-OH was synthesized following literature pro-
unsaturated. It could then engage in a multitude of potential cedure.30 NaBArF24 was synthesized by the method of
stabilizing interactions. First, it could dimerize via a bridging Bergman40 and purified using the modification reported by
alkyl with a neighboring aluminum center. We consider this Peters.41 All other reagents and solvents used were commer-
possibility unlikely due to the steric bulk of the isobutyl group cially available and used without further purification.
(AliBu3 does not exist as a dimer), the DPPP-O ligand, and the Aluminum and phosphine reagents were stored in a nitrogen
1
lack of evidence for dimer formation by ESI-MS and DOSY. glovebox prior to use. H and 13C NMR spectra were recorded
The aluminum center could also engage in weak, intra- on a 400 MHz JEOL spectrometer and referenced to the
molecular, Z-type bonding with the transition metal center. residual solvent peak.42 Additional NMR data were obtained
However, to attain such a geometry, the 6-membered metalla- on a 400 MHz JEOL spectrometer and referenced to an appro-
cycle would be forced to adopt a boat geometry, which is likely priate external standard (31P: H3PO4 in D2O (0.00 ppm)).
energetically unfavorable even with a stabilizing Z-type bond.
Spectra were modeled using the MestReNova 10 software
Finally, we attempted to extend the synthetic protocols uti- package. ESI-MS was measured on a ThermoFisher LTQ-XL ion
lized here to other aluminum moieties. Interestingly, no other trap mass spectrometer from dichloromethane or tetrahydro-
commercially available aluminum alkyls led to the formation furan solution. High resolution mass spectra were recorded on
of readily identifiable heterobimetallics. AlEt3, AlEt2Cl, and a Waters Synapt High Resolution Mass Spectrometer housed at
AliBu2Cl, all facilitated transformation of the Rh precursors the University of Memphis. Elemental analysis was performed
into a mixture of intractable products. The unique reactivity of by Atlantic Microlabs, Inc. and is reported as the average of
AliBu3 is attributed to the bulky size of isobutyl groups, which duplicate runs.
provide efficient steric protection of the Al center. Chloride,
which in addition to its small steric profile can readily bridge
Rh(DPPE)(CO)Cl
between two aluminum centers, further complicates syntheses This procedure was adapted from the synthetic protocol for
from AlEt2Cl, and AliPr2Cl.
the analogous iridium complex.32 RhCl3·3H2O (1.37 g,
5 mmol) was dissolved in 100 mL methanol. The mixture was
heated to reflux under CO for 24 hours. The bright yellow solu-
tion was cooled to room temperature and methanol was
removed by vacuum. The dark red solid was dissolved in
Conclusions
Highly polarized heterobimetallic complexes are a promising 50 mL benzene, to which a benzene solution (100 mL) of 1,2-
catalyst family for the cooperative activation of polarizable sub- bis(diphenylphosphino)ethane (2.0 g, 5 mmol) was added
strates. In an effort to overcome the instability of the pyridone dropwise. The yellow precipitate was collected and recrystal-
bridge in previously reported late transition metal-aluminum lized from dichloromethane/hexanes, affording 2.3 g yellow
1
complexes from our laboratory28 and prevent decomposition powder as product. Yield: 80%. H NMR (400 MHz, CDCl3) δ
in the presence of a substrate, a synthetic route to a novel het- 7.71–7.83 (m, 8 H), 7.40 (m, 12 H), 2.42–2.49 (m, 2 H), 2.15–2.22
erobimetallic complex, 2, which is bridged via a ligand which (m, 2 H). 31P{1H} NMR (162 MHz, CDCl3) δ 71.17 (dd, J = 159.6,
chelates the transition metal was developed. Thorough spectro- 34.8 Hz), 49.46 (dd,
J = 127.2, 34.8 Hz). Anal. calc.
scopic characterization indicated the presence of a small C27H24OP2RhCl: C, 57.42; H, 4.28. Found: C, 57.39; H, 4.40.
amount of [Rh(DPPE)2]Cl in the product mixture. The thermo-
Rh(DPPE)(DPPP-OH)Cl (1)
dynamic stability of this undesired byproduct led us to develop
a synthetic route to 4. This complex was found to have a To a stirred dichloromethane (5 mL) solution of DPPP-OH
dynamic structure in solution. Application of these exciting (215 mg, 0.5 mmol) at −78 °C, Rh(DPPE)(CO)Cl (280 mg,
complexes toward small molecule activation is now under 0.5 mmol) in dichloromethane (10 mL) was added dropwise
investigation.
over a period of 1 hour. The mixture was allowed to stir for
16 hours as the temperature was slowly raised from −78 °C to
room temperature. The crude product was purified by recrys-
tallization from tetrahydrofuran at −35 °C. 370 mg yellow
Experimental section
powder was collected as product. Yield: 77%. 1H NMR
Procedures were performed in
a
nitrogen-filled Inert (400 MHz, CDCl3) δ 7.51 (m, 4H), 7.40–6.96 (m, 28H), 6.82
Technologies glovebox or using standard Schlenk techniques (m, 8H), 3.83 (s, 1 H), 3.66–3.69 (m, 1 H), 2.90 (m, 2H),
unless otherwise specified. Extra dry benzene, pentane, tetra- 2.71 (m, 2H), 2.06 (m, 2H), 1.86 (m, 2H). 31P{1H} NMR
hydrofuran, diethyl ether, and dichloromethane were pur- (162 MHz, CDCl3) δ 59.68 (DPPE, JRh = 133.8 Hz, Jtrans = 266.5
chased from commercial suppliers and stored over molecular Hz, Jcis, B–B′ = −29.5 Hz, Jcis, B–A′ = −38.5 Hz), 16.78 (DPPP-OH,
sieves in the glovebox prior to usage. Toluene was dried over JRh = 128.9 Hz, Jtrans = 266.5 Hz, Jcis, A–A′ = −49.2 Hz, Jcis, A–B′
=
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−38.5 Hz). Anal. calc. C53H50OP4RhCl: C, 65.95; H, 5.22. 1079.1557. Found: 1079.0964. See ESI† for further ESI-MS
Found: C, 58.79; H, 5.02. Elemental analysis is consistent with analysis.
two molecules of dichloromethane per formula unit (C, 58.20,
H, 4.80).
X-Ray crystallography
A suitable crystal of each sample was selected for analysis and
Rh(DPPE)(DPPP-O-AliBu2Cl) (2)
mounted in a polyimide loop. All measurements were made on
a Rigaku Oxford Diffraction Supernova Eos CCD with filtered
Cu-Kα or Mo-Kα radiation at a temperature of 100 K. Using
Olex2,43 the structure was solved with the ShelXT structure solu-
tion program using Direct Methods and refined with the ShelXL
refinement package44 using Least Squares minimization.
To a stirred tetrahydrofuran (3 mL) solution of Rh(DPPE)
(DPPP-OH)Cl formed in situ from Rh(DPPE)(CO)Cl (141 mg,
0.25 mmol) and DPPP-OH (110 mg, 0.25 mmol) at −78 °C,
iBu3Al (50 mg, 0.25 mmol) was added. The mixture was
allowed to stir for 1 hour and warmed from −78 °C to room
temperature. After the reaction, tetrahydrofuran was removed
by vacuum. The resultant yellow powder was dissolved in
0.5 mL tetrahydrofuran and precipitated by the addition
pentane. This recrystallization was repeated twice to remove
[Rh(DPPE)2]Cl and improve purity to greater than 95%. 170 mg
bright yellow powder was collected as product. Impurity pre-
vented acceptable microanalysis from being obtained. Yield:
62%. 1H NMR (400 MHz, tetrahydrofuran-d8) δ 6.60–7.36 (m,
Conflicts of interest
The authors declare no competing financial interest.
Acknowledgements
56H), 5.36 (m, 1 H), 2.91 (m, 2H), 2.74 (m, 2 H), 2.62 (m, 2 H), This material is based upon work supported by the National
1.52 (d, J = 6.6 Hz), 1.50 (d, J = 6.6 Hz), 0.72 (dd, Jgem = 13.5 Hz, Science Foundation under Grant No. CHE-1531466 and a grant
J = 6.6 Hz), 0.68 (dd, Jgem = 13.5 Hz, J = 6.6 Hz). 31P{1H} NMR from The University of Memphis College of Arts and Sciences
(162 MHz, tetrahydrofuran-d8) δ 59.37 (DPPE, JRh = 107.2 Hz, Research Grant Fund. This support does not necessarily imply
Jtrans = 205.5 Hz, Jcis, B–B′ = −22.6 Hz, Jcis, B–A′ = −29.2 Hz), 58.20 endorsement by the University of Memphis of research con-
(Rh(DPPE)2Cl, d, J = 132.8 Hz), 13.90 (DPPP-OH, ddd, JRh
=
clusions. The authors thank Prof. Nathan J. DeYonker
101.2 Hz, Jtrans = 205.5 Hz, Jcis A–A′ = −39.4 Hz, Jcis, A–B′ = −29.2 (University of Memphis) and Prof. D. Michael Heinekey
Hz). HRMS calculated for [C53H50Al2O4P4Rh]+ (product of alkyl (University of Washington) for helpful discussions. Prof. Paul
hydrolysis) m/z calc.: 971.1554. Found: 970.8866 (see Fig. 17a Simone and Mr. Drake Williams (University of Memphis) are
and b in ESI†).
thanked for assistance with obtaining HRMS data. Z. L. thanks
the University of Memphis College of Arts and Sciences and
Department of Chemistry for a Postdoctoral Fellowship.
[Rh(DPPP-OH)2]Cl (3)
To a stirred dichloromethane (5 mL) solution of [Rh(COD)Cl]2
(49 mg, 0.1 mmol), DPPP-OH (170 mg, 0.4 mmol) in dichloro-
methane (5 mL) was added dropwise. The mixture was stirred
at room temperature for 30 min before the solution was con-
centrated by vacuum to ∼1 mL. The product was obtained via
precipitation with pentane. This recrystallization was repeated
twice to yield 122 mg of yellow powder. This product exists as
a mixture of two isomers, as determined by 31P{1H} NMR spec-
troscopy and confirmed by combustion analysis. Yield: 61%.
31P{1H} NMR (162 MHz, CDCl3) δ 14.14 (d, J = 134 Hz), 13.39
(d, J = 134 Hz). Anal. calc. C54H52O2P4RhCl: C, 65.17; H, 5.27.
Found: C, 64.88; H, 5.42.
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