Study of the conformationally flexible, wide bite-angle diphosphine 4,6-bis(3-diisopropylphosphinophenyl)dibenzofuran in rhodium(I) and palladium(II) coordination complexes

Article abstract of DOI:10.1021/ic102373w

The diphosphine 4,6-bis(3-diisopropylphosphinophenyl)dibenzofuran (abbreviated as ^iPrDPDBFphos) was prepared and studied for its potential as a trans-chelating ligand in transition-metal coordination complexes. In the rhodium norbornadiene comp

Full text of DOI:10.1021/ic102373w

ARTICLE
pubs.acs.org/IC
Study of the Conformationally Flexible, Wide Bite-Angle Diphosphine
4,6-Bis(3-diisopropylphosphinophenyl)dibenzofuran in Rhodium(I)
and Palladium(II) Coordination Complexes
Keying Ding, Deanna L. Miller, Victor G. Young, Jr., and Connie C. Lu*
Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455-0431, United States
S Supporting Information
b
ABSTRACT: The diphosphine 4,6-bis(3-diisopropylphosphino-
phenyl)dibenzofuran (abbreviated as iPrDPDBFphos) was pre-
pared and studied for its potential as a trans-chelating ligand in
transition-metal coordination complexes. In the rhodium norbor-
nadiene complex [(iPrDPDBFphos)Rh(NBD)]BF4, which has
been characterized with multinuclear NMR spectroscopy, X-ray
crystallography, and electrochemical studies, the ligand binds in
cis fashion. In the bis(acetonitrile) complexes of rho-
dium and palladium [(iPrDPDBFphos)M(CH3CN)2](BF4)n
(M = Rh, Pd; n = 1, 2), the ligand adopts a trans coordination geometry. Density functional theory (DFT, M06-L) calculations
predict that the trans conformer is energetically more favorable than the cis by 3.5 kcal/mol. Cyclic voltammograms of the
bis(acetonitrile) Pd(II) and Rh(I) complexes contain reversible and quasi-reversible reduction events, respectively, which are
preliminarily assigned as metal-based redox reactions.
’ INTRODUCTION
and sp3 C-H bonds in the ligand backbone are susceptible to
activation at the transition-metal center because of their
proximity.11 Typically, the more robust trans-spanning diphos-
phines contain relatively rigid backbones, such as the benzophe-
nanthrene π-system in TRANSphos,12 the fused spirobi-
chroman rings in SPANphos,13 or the biphenyl group in BISBI
(Chart 1).14 Another desirable design is to increase the distance
between the ligand backbone and the phosphine donors so that
any undesirable interactions between the transition metal center
and the ligand backbone are minimized.15 To achieve these
criteria, we have prepared a unique, wide bite-angle diphosphine
ligand featuring the diphenyldibenzofuran backbone, from which
the general descriptor of these ligands as DPDBFphos is derived.
We report here the synthesis of the diisopropyl-substituted
phosphine derivative of DPDBFphos and its initial coordination
chemistry with rhodium(I) and palladium(II).
Cis-chelating diphosphine ligands are featured prominently in
homogeneous transition-metal catalysis, whereas trans-chelating
diphosphines are uncommon. The latter are ill-suited to tradi-
tional d8 square planar transition-metal catalysts because the two
remaining sites at the metal center are opposed to one another,
making it difficult to effect canonical catalytic steps such as
migratory insertion and reductive elimination.1,2 Instead, trans-
chelating diphosphines may be interesting for penta- and hexa-
coordinate transition-metal catalysts by occupying the axial sites
and leaving the equatorial sites open for substrate activation and
conversion. In practice, however, “exclusively” trans-chelating
diphosphines are challenging to design.3 Even well-known trans-
spanning diphosphines can support a large range of bite angles,
including both cis and trans structures.4 Nonetheless, these wide
bite-angle diphosphines can be competent and even advanta-
geous as an ancillary ligand for catalysis if the intermediate
species can undergo trans-cis isomerization.3,5-8 Current inves-
tigations seek to explore the unique role of trans-spanning
diphosphines in homogeneous transition-metal catalysis.
Much past efforts have focused on the design and synthesis of
trans-chelating diphosphines.9 This task was initially challenging
because of selectivity issues with metalating ligands wherein the
two phosphine donors are separated by a sufficiently large
distance. Competing reactions can form bimetallic and/or multi-
metallic products,10 in which the diphosphine ligand bridges two
transition-metal centers. The lack of binding selectivity can also
manifest in product mixtures of both cis and trans mononuclear
coordination complexes. Another potential problem is that sp2
’ RESULTS AND DISCUSSION
The two-step ligand synthesis from known starting materials is
depicted in Scheme 1. The first step is a Suzuki coupling of the
diboronic acid dibenzofuran-4,6-bis(boronic acid) and 2.2 equiv
of 3-iodobromobenzene to give 1 in moderate yield (50%).
Compound 1 is the ligand backbone with two reactive aryl bro-
mide linkages. Next, the dibromide 1 is activated with 2 equiv of
nBuLi via lithium-halogen exchange and subsequently quenched
with diisopropylchlorophosphine to install the phosphine donors.
Received: November 29, 2010
Published: February 17, 2011
r
2011 American Chemical Society
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Chart 1. trans-Spanning Diphosphine Ligands with the
Current Ligand Shown in Inset
Scheme 2. Synthetic Routes to iPrDPDBFphos Rh and Pd
Complexes, 3 to 5
norbornadiene ligand exchanges readily with the solvent to form
[(trans-iPrDPDBFphos)Rh(MeCN)2]BF4 4. This reaction is
unusual in that hydrogen is typically required to effect the overall
exchange of norbornadiene with coordinating solvent molecules
(or similarly weak donors) at (diphosphine)rhodium(I) centers.
A notable exception is the direct substitution of NBD by an ether
group at rhodium(I); however, in this case, the ether group was
covalently linked to the phosphine ligand.19
The 31P NMR spectra of 3, 4, and 5 are all consistent with
chemically equivalent phosphorus nuclei. The 31P chemical shifts
are similar for the Rh and Pd bis(acetonitrile) complexes, 4 and 5,
respectively (31P for 4 in CDCl3: 46.4 ppm; 5 in CD3CN: 43.1
ppm). The Rh norbornadiene complex 3 has an upfield phos-
phorus shift of 28.8 ppm, but this difference likely reflects the
different binding modes of the ligand in the two Rh complexes
(vide infra). The Rh-P coupling constants are also quite
different for the two rhodium compounds, with JRh-P = 148
Hz for 3 and JRh-P = 127 Hz for 4. The latter coupling constant
falls into the expected range for Rh trans-diphosphine complexes
(JRh-P = 118 to 135 Hz).3 The Rh norbornadiene complex 3
exhibits a sharp proton NMR at room temperature (Supporting
Scheme 1. Synthetic Route to the Diphosphine iPrDPDBF-
phos 2
1
Information, Figure 1). In the H NMR spectrum, half of the
protons in the diphenyldibenzofuran backbone are unique. For
the four isopropyl groups, only two distinct methyl resonances
and one methine signal are observed; while for the norborna-
diene ligand, three resonances are seen, one signal for each
proton type. A C2v symmetric structure is consistent with these
NMR characteristics.
The diphosphine ligand 2, which will be referred to as iPrDPDBF-
phos, was obtained in good yield (80%).
The room-temperature proton NMR spectra of the Rh and Pd
bis(acetonitrile) compounds, 4 and 5, respectively, are sharp in
the aromatic region but broad in the aliphatic region. Upon
cooling, these broad peaks sharpen, revealing two distinct
methine protons and four methyls for the ligand isopropyl
groups, as well as two additional methyl peaks, one for each
bound acetonitrile. Except for these acetonitrile groups, exactly
half of the protons in 4 are unique. Curiously, the two acetonitrile
methyl shifts are dramatically different at 2.44 and 1.04 ppm in 4
(CDCl3, -15 °C), signifying quite different chemical environ-
ments. Likewise, Figure 1 compares the 1H NMR spectra of the
palladium complex 5 in two solvents, CD3CN and CD2Cl2, at
room and low temperatures. Both room temperature spectra
show extremely broad aliphatic peaks, which upon cooling to
-30 °C in CD3CN resolve nicely into two distinct methine and
four unique methyl peaks. However, no signals for bound
CH3CN molecules are observed, likely because of the complete
exchange of CD3CN with the protio-solvent molecules at the
palladium center. To obtain the chemical shifts of the bound
Initial metalation studies of the ligand focused on the noble
transition metals because (1) we anticipated the metal-ligand
complexes to be diamagnetic, which facilitates characterization
by multinuclear NMR spectroscopy; (2) both rhodium and
palladium diphosphine complexes are well-known catalysts for
several bond-forming reactions; and (3) in specific cases, dipho-
sphine ligands with wide bite angles (100 to 160°) have enhanced
the efficiency of Pd-catalyzed C-C and C-N cross coupling
reactions5,6,8,15 or increased the regioselectivity in Rh-mediated
hydroformylations.16-18 Metalations of 2 with [Rh(NBD)2]BF4
(where NBD = norbornadiene) and [Pd(MeCN)4](BF4)2 pro-
ceed cleanly to give [(cis-iPrDPDBFphos)Rh(NBD)]BF4
3
and [(trans-iPrDPDBFphos)Pd(MeCN)2](BF4)2 5, respectively
(Scheme 2). The coordination modes of the ligand in complexes
3-5 were determined by single-crystal X-ray diffraction studies
(vide infra). Compound 3 is stable in acetonitrile at room
temperature (rt); however, upon heating to 80 °C, the
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Figure 1. Variable-temperature proton NMR spectra (400 MHz) of
[(iPrDPDBFphos)Pd(NCMe)2](BF4)2 5 in CD3CN and CD2Cl2.
Figure 2. Solid-state structure of the cation [(iPrDPDBFphos)Rh-
(NBD)] 3 at 50% probability level. Hydrogen atoms, BF4 counteranion,
and extra solvent molecules were omitted for clarity. Selected bond dis-
tances (Å): Rh-P1 2.3887(9), Rh-P2 2.3524(9), Rh-C40 2.204(5),
Rh-C41 2.190(5), Rh-C42 2.160(4), Rh-C43 2.159(4); selected
bond angle (deg): P1-Rh-P2 104.52(3); selected torsion angles
(deg): Rh-P1-C15-C14 -54.3(3), Rh-P2-C21-C20 -73.3(2),
C7-C6-C13-C14 34.3(5), C8-C9-C19-C20 -14.6(5).
acetonitrile molecules, the low temperature proton NMR experiment
was performed in the non-coordinating solvent CD2Cl2. At
-60 °C, the methyl peaks of the two bound CH3CN molecules
are observed at 2.84 and 1.40 ppm (along with some free CH3CN
at 1.97 ppm). Hence, in both the Pd and Rh bis(acetonitrile)
compounds, a difference of ∼1.4 ppm is observed in the chemical
shifts of the two acetonitrile methyl groups.
The fluxionality of the bis(acetonitrile) complexes 4 and 5 at
room temperature suggests some torsional flexibility, that is, the
N-M-N vector can rotate in the plane that is perpendicular to
the P-M-P vector. Rotation can only partially occur since
complete rotation of the N-M-N vector would result in
collision between an acetonitrile molecule and the dibenzofuran
backbone. Replacing acetonitrile with a bulkier nitrile analogue
would further hinder this rotational freedom. To test this
N2 situated near the dibenzofuran backbone while N1 is spatially
directed toward the isopropyl groups.
Space-filling images of structures 4 and 5 reveal that the CH3C
group attached to N2 is sandwiched between the π-systems of
the two aryl linkers, which are slightly canted relative to the
dibenzofuran backbone (Supporting Information, Figure 3). In
these two structures, the distance between the carbon atom of the
methyl group and the centroids of the aryl linker rings ranges
from 3.857 to 4.283 Å. This is indeed significant since the total
sum of a methyl C-H bond (1.0 Å), the van der Waal radius of a
π-system (1.7 Å), and that of a hydrogen atom (1.2 Å) is roughly
3.9 Å. The ring current associated with these aryl linkers should
shield this methyl group,21 which explains the upfield shift of one
of the acetonitrile methyl resonances by ∼1.4 ppm in the proton
NMR spectra of 4 and 5. The coalescence of the isopropyl
resonances in the proton NMR spectrum at room temperature
reflects slow exchange of the two acetonitrile molecules as well as
the ligand isopropyl groups. The fluxional process is likely a
combination of some torsional rotation of the N-M-N vector
and back-and-forth canting of the aryl linkers as shown in
t
hypothesis, the corresponding BuCN analogue of 4 was pre-
pared by adding 2 equiv of tBuCN to 3, and as anticipated, the
resulting proton NMR spectrum (Supporting Information,
Figure 2) is sharp at rt, with the four methyl and two methine
signals all well-resolved. Interestingly, the tBu protons are shifted by
over 1 ppm (1.41, 0.34 ppm, CDCl3, 300 MHz). For comparison,
the tBu protons in free tBuCN are observed at 1.35 ppm.
Molecular Structures from X-ray Crystallography. Com-
pound 3 can be crystallized from various solvent combinations,
but single crystals of sufficient quality for X-ray diffraction studies
were obtained from vapor diffusion of benzene into a 1,2-
difluorobenzene solution (Figure 2). Single crystals of 4 and 5
were grown from their concentrated acetonitrile solutions by
cooling to -25 °C or vapor diffusion of ether at rt, respec-
tively. Crystallographic details are given in Table 1. In the trio of
crystal structures, all metal centers have square planar geometry
with the iPrDPDBFphos ligand occupying either cis- or trans-
binding sites.
In the bis(acetonitrile) complexes, the ligand is a trans
chelator with P-M-P angles of 177.39(4)° in 4 (Figure 3)
and 174.59(3)° in 5 (Figure 4). Molecular structures of bis-
(acetonitrile) Rh and Pd complexes are uncommon with two
phosphine donors and rare for the more restrictive case of trans-
chelating phosphines. The only exception is Chisholm’s
[trans-(PCy3)2Rh(MeCN)2]BF4 complex,20 and its Rh-P and
Rh-N bond distances of 2.341(2) and 1.984(2) Å, respectively,
compare reasonably with those of 4 (ave. Rh-P 2.304 Å, ave.
Rh-N 1.966 Å). An approximate mirror plane that bisects the
P-P vector and contains the oxygen atom of the dibenzofuran
backbone equalizes the two halves of the molecule. The bound
acetonitriles, which lie in the mirror plane, are inequivalent with
t
Scheme 3. In the BuCN analogue of 4, the aryl linkers, would
be effectively locked in place because of unfavorable steric
interactions with the bulkier tBu group.
The crystal structure of the Rh norbornadiene complex 3
stands apart in that the diphosphine ligand is a cis chelator with a
P-Rh-P angle of 104.52(3)°. The Rh center is slightly distorted
from square planar geometry with a small twist angle between the
two Rh-ligand planes (P-Rh-P and Rh(NBD)) of 12.8°. While
crystal structures of (diphosphine)Rh(NBD) cations are com-
mon, it is rare to find such structures featuring wide bite-angle
diphosphines. Indeed, among the 66 known examples in the
Cambridge Structural Database,22 the average P-Rh-P bond
angle is 89.8° with a standard deviation of 5.8°. The bite angles
range from 82.9 to 103.7°. Hence, even as the cis conformer,
iPrDPDBFphos would be well considered a wide bite-angle
diphosphine. The Rh-P and Rh-C bond distances are not
remarkable among this set. The former (2.3887(9), 2.3524(9) Å)
are slightly longer than the average (2.31 Å, SD = 0.04), and the
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Table 1. Crystallographic Data for 3-5
3 0.64(C6H6) 0.36(C6H4F2)
4 2(CH3CN)
5 CH3CN
3
3
3
3
formula
crystal size, mm3
C
49H55.3BF4.7OP2Rh
C44H54BF4N4OP2Rh
0.40 ꢀ 0.30 ꢀ 0.20
904.93
C42H51B2F8N3OP2Pd
0.40 ꢀ 0.30 ꢀ 0.20
955.82
0.45 ꢀ 0.30 ꢀ 0.20
925.19
fw, g/mol
space group
a, Å
P1
Pbca
P21/n
12.888(1)
13.145(1)
14.702(2)
103.481(2)
108.628(2)
104.214(2)
2153.0(4)
2
25.253(6)
11.484(3)
30.963(7)
90
11.5799(9)
11.3663(9)
33.729(3)
90
b, Å
c, Å
R, deg
β, deg
90
95.942(2)
90
γ, deg
V, Å3
90
8979(3)
4415.5(6)
4
Z
8
T, K
173(2)
0.71073(2)
1.339
0.71073(2)
1.438
F calcd. g cm-3
refl. collected/2σmax
unique refl./I > 2θ(I)
no. params./restraints
λ, Å /μ (KR), mm-1
R1/GooF
1.427
81982/55.0
9711/6233
584/168
82062/50.1
7955/6217
522/6
42213/50.1
7836/6233
556/6
0.71073/0.528
0.0458/1.048
0.0984
0.71073/0.505
0.0537/1.296
0.1669
0.71073/0.563
0.0419/1.045
0.0890
wR2 (I > 2σ(I))
residual density, e Å-3
0.725/-0.631
1.347/-0.695
1.108 /-0.556
Figure 3. Solid-state structure of the cation [(iPrDPDBFphos)Rh-
(NCMe)2] 4 at 50% probability level. Hydrogen atoms, BF4 counter-
anion, and extra solvent molecules were omitted for clarity. Selected
bond distances (Å): Rh-P1 2.309(1), Rh-P2 2.299(1), Rh-N1
1.964(4), Rh-N2 1.968(4); selected bond angles (deg): P1-Rh-P2
177.39(4), N1-Rh-N2 179.1(2), P1-Rh-N1 90.0(1), P1-Rh-N2
90.0(1), P2-Rh-N1 92.5(1), P2-Rh-N2 87.4(1); selected torsion
angles (deg): Rh-P1-C15-C14 -45.4(4), Rh-P2-C21-C20
35.3(4), C7-C6-C13-C14 43.8(7), C8-C9-C19-C20 -43.1(6).
Figure 4. Solid-state structure of the dication [(iPrDPDBFphos)Pd-
(NCMe)2] 5 at 50% probability level. Hydrogen atoms, BF4 counter-
anions, and extra solvent molecules were omitted for clarity. Selected
bond distances (Å): Pd-P1 2.3594(9), Pd-P2 2.3521(9), Pd-N1
2.002(3), Pd-N2 1.963(3); selected bond angles (deg): P1-Pd-P2
174.59(3), N1-Pd-N2 178.0(1), P1-Pd-N1 91.68(8), P1-Pd-N2
87.65(8), P2-Pd-N1 92.41(8), P2-Pd-N2 88.14(8); selected tor-
sion angles (deg): Pd-P1-C15-C14 -49.5(3), Pd-P2-C21-C20
45.1(3), C7-C6-C13-C14 52.1(5), C8-C9-C19-C20 -50.5(5).
latter (2.204(5), 2.190(5), 2.160(4), 2.159(4) Å) are slightly
shorter (2.21 Å, SD = 0.03), but they are all within one or two
standard deviation(s) of their corresponding average values.
The structure of 3 has no clear symmetry elements, although
within the molecular fragment “(iPrDPDBFphos)Rh” (i.e., ignor-
ing NBD), a 2-fold rotation axis is present that contains the Rh
and O atoms. The ligand backbone is twisted so that the torsion
angle defined by (aryl)C-P-P-C(aryl) is 87.0°. In contrast,
this torsion angle is only 9.3° in the trans compound 4. The
observation of just two distinct methyl resonances for the
isopropyl groups in 3 (versus four in 4) by NMR belies the
asymmetry of the solid-state structure. Rotation along both
Rh-P bonds with concurrent adjustment of the phosphine
substituents would effectively swing the P2Rh(NBD) fragment
back and forth relative to the dibenzofuran frame. If such a motion
is rapid, then the averaged molecule would appear to have C2v
symmetry, which is consistent with its proton NMR spectrum.
Cyclic Voltammetry. The electrochemical behavior of com-
plexes 3, 4, and 5 was investigated with cyclic voltammetry.
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Scheme 3. Fluxional Processes That Exchange the Two Bound Acetonitrile Molecules, Represented Simply as L1 and L2, in
[(trans-iPrDPDBFphos)Rh(NCMe)2]BF4 4
Figure 6. Stacked cyclic voltammograms of [(iPrDPDBFphos)Rh-
Figure 5. Cyclic voltammograms of [(iPrDPDBFphos)Pd(NCMe)2]-
(NCMe)2](BF4)2 4 in 1,2-difluorobenzene and acetonitrile (100 mV/
(BF4)2 5 at various scan rates (10, 50, 100 mV/s; 0.1 M [nBu4N]PF6 in
s; 0.1 M [nBu4N]PF6).
MeCN). Currents were multiplied by (scan rate)-1/2
.
Recording cyclic voltammograms of the Rh norbornadiene
complex 3 in acetonitrile proved unsuccessful because after only
a few cycles, the voltammograms were remarkably similar to that
of 4, the Rh bis(acetonitrile) complex. Henceforth, the cyclic
voltammograms were collected in 1,2-difluorobenzene with 0.1
M [nBu4N]PF6. Surprisingly, in contrast to 4, no oxidative events
were observed, and instead, a reduction was seen at -1.66 V
versus Fc/Fcþ (Figure 7). A full sweep of the ligand iPrDPDBF-
phos was recorded as a control (Supporting Information,
Figure 4), and none of these events is ascribable to the ligand.
Hence, this feature is tentatively assigned as a Rh(I)/Rh(0)
redox couple. The reduction is best described as quasi-reversible
because the current ratio ipc/ipa approaches unity only at suffi-
ciently fast scan rates (>500 mV/s) while the ipa of the corre-
sponding secondary oxidation at -0.5 V diminishes. This scan-
rate behavior may be consistent with an unstable product that is
generated electrochemically, which then converts into another
species at a rate comparable to that of the experimental time
scale.
Theoretical Calculations. Because metalation reactions of
iPrDPDBFphos produced both cis and trans coordination com-
plexes, we were interested in unraveling the ligand’s inherent
propensity to adopt one conformation over the other via
theoretical investigations. Casey and Whiteker previously studied
the natural bite angle of chelating diphosphines using molecular
mechanics calculations.26 The work has been continued by
van Leeuwen,4 but more recently he has reported that MM2
calculations can give gross errors in determining bite angles.27
Figure 5 shows the CV of the Pd complex 5 in 0.1 M [nBu4N]PF6
acetonitrile solution at various scan rates (10, 50, 100 mV/s).
Scanning at 10 mV/s, a quasi-reversible electron-transfer wave is
observed at -0.20 V with a peak-to-peak separation of 90 mV (vs
Fc/Fcþ). At the faster scan rate of 100 mV/s, the peak potential
shifts cathodically to -0.23 V while the peak-to-peak separation
increases to 135 mV. This trend is consistent with a quasi-
reversible process wherein the electron-transfer kinetics are
sufficiently slow so that at faster scan rates the redox event
appears less reversible. Because Pd(II) is well-known to undergo
reduction to Pd(0) during catalysis, this event is assigned as a
Pd(II)/Pd(0) redox couple.
Figure 6 shows the cyclic voltammogram of the Rh bis-
(acetonitrile) complex 4 in two different solvents, acetonitrile
and 1,2-difluorobenzene. Sullivan and Meyer have previously
demonstrated the utility of 1,2-difluorobenzene as an inert and
non-coordinating solvent in electrochemical studies of transition
metal complexes that bind solvent molecules as ligands during
electrochemical experiments.23 In both solvents, an irreversible
oxidative event is observed near 500 mV. This oxidative event is
followed by an irreversible reductive feature that shifts dramati-
cally in the different solvents, -0.29 V in 1,2-difluorobenzene
and -0.73 V in acetonitrile. We speculate that the irreversible
feature near 0.5 mV is a two-electron oxidation of Rh(I) to
Rh(III), a common oxidation state for mononuclear Rh com-
plexes. However, oxidation to a mononuclear Rh(II) species is
also possible.24,25
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Figure 8. Relaxed potential energy scan of various bite angles for
iPrDPDBFphos in the model cation [(iPrDPDBFphos)H]þ. Two land-
scapes, twisted and non-twisted, were calculated independently from the
initial geometries of the cis and trans structures, respectively (indicated
by arrows).
Figure 7. Cyclic voltammograms of [(iPrDPDBFphos)Rh(NBD)]BF4
3 at various scan rates (50, 100, 250, 500 mV/s; 0.1 M [nBu4N]PF6 in
1,2-difluorobenzene). Currents were multiplied by (scan rate)-1/2
.
For the specific case of SPANphos, van Leeuwen applied
semiempirical (AM1) and DFT (B3LYP) methods, but the
ligand conformational space was explored only for the spirobi-
chroman backbone, and the phosphine groups were omitted
entirely.27 These studies were complemented with calculations of
the ligand-metal complex, but the latter cannot disentangle the
ligand’s inherent preference from the electronic/geometric bias
of the metal center.
calculated. The trans conformer was lower in energy than the cis
by 3.5 kcal/mol.
Future Outlook. A conformationally flexible diphosphine
ligand is reported that can bind to a transition metal center in
both trans and cis modes. DFT calculations indicate that a
changeover from cis to trans geometry can provide a 3.5 kcal/
mol driving force. This may be the underlying reason why the
conversion of [(cis-iPrDPDBFphos)Rh(NBD)]þ compound to
the corresponding trans bis(acetonitrile) complex does not
require hydrogen to remove the norbornadiene group, and
instead occurs by ligand exchange. Future directions will seek
to couple the driving force associated with cis-to-trans conversion
to metal-mediated reactivity. Cyclic voltammograms suggest the
feasibility of zero-valent Rh and Pd species, and ongoing work is
focused on preparing reduced coordination complexes.
We chose to model the protonated ligand [H(iPrDPDBF-
phos)]þ to minimize the electronic bias of the connecting
atom by using a spherical partner. Akin to Casey’s calcula-
tions, the P-H bond distances were kept constant at 2.30 Å to
mimic the Rh-P bond distance. The initial geometries were
taken from the solid-state structures of [(cis-iPrDPDBFphos)-
Rh(NBD)]BF4 3 and [(trans-iPrDPDBFphos)Rh(MeCN)2]-
(BF4)2 4. A relaxed potential energy scan was performed with
geometry optimizations at each step for a targeted range of bite
angles, ∼90 to 180 degrees with a step size of 10 degrees. We
employed the M06-L DFT functional in the Gaussian09 program
because it has been reported to give more accurate energies for
non-covalent interactions and transition metal systems com-
pared to B3LYP.28
’ EXPERIMENTAL SECTION
General Considerations. Unless otherwise stated, all manipula-
tions were performed under a dinitrogen atmosphere in a VAC Atmo-
sphere glovebox or using standard Schlenk techniques. Standard solvents
were deoxygenated by sparging with dinitrogen and dried by passing
through activated alumina columns of a SG Water solvent purification
system. 1,2-Difluorobenzene was degassed, dried with activated alumina
(8-14 mesh size), and stored over activated 4 Å molecular sieves.
Deuterated solvents were purchased from Cambridge Isotope Labora-
tories, Inc., degassed via freeze-pump-thaw cycles, and stored over
activated 4 Å molecular sieves. Proton, 13C, and 31P NMR spectra were
recorded on Varian 300, 400, and 500 MHz spectrometers at ambient
temperature unless otherwise stated. Chemical shifts were referenced to
residual solvent, except for 31P NMR data, for which an external reference
of 85% H3PO4 set to 0 ppm was used. Because of the proton equivalence
within these systems, the reportedintegrations should be doubledto match
the expected absolute values. Elemental analyses were performed by
Columbia Analytical Services. The ligand precursor dibenzofuran-4,6-
The plot of relative total energy versus bite angle is shown in
Figure 8. Two landscapes were computed. The twisted landscape
was computed using the cis structure 3, and the non-twisted
landscape was computed with the trans analogue 4. For the
former, a shallow potential well is calculated with a minimum at
140 degrees. The minimum of the non-twisted landscape occurs
at 180 degrees and is 1.78 kcal/mol lower in energy than the
twisted minimum. In both landscapes, the accessible bite angle
range (within 3 kcal/mol) is large, from 140 to 180 degrees for
the non-twisted geometry and 120 to 170 degrees for the twisted
geometry. However, it is likely that the energies for the twisted
landscape are overestimated (vide infra).
Single point energy calculations were also performed to
compare the ligand conformations in the two structures of 3
and 4. First, the geometry of the two complexes were optimized
(M06-L, def2-TZVP for Rh, P, N, def2-SV(P) for C, H, O). In
both compounds, the calculated Rh-P bond distances were
comparable to experimental values (within 0.04 Å), the Rh-C/
Rh-N bond distances within 0.01 Å, and the P-Rh-P bond
angles within 4 degrees. Next, all atoms except those comprising
iPrDPDBFphos were deleted, and single point energies were
30
bis(boronic acid)29 and the metal precursor [Pd(CH3CN)4](BF4)2
were prepared according to published procedures.
4,6-Bis(3-bromophenyl)dibenzofuran (1). Dibenzofuran-4,6-bis-
(boronic acid) (4.59 g, 17.94 mmol), 3-iodobromobenzene (11.16 g,
39.46 mmol), Pd(PPh3)4 (1.04 g, 0.90 mmol), Na2CO3 (4.25 g, 40
mmol), DME (280 mL), and deoxygenated dH2O (28 mL) were
combined in a 500 mL resealable flask. The flask was sealed, and the
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Inorganic Chemistry
ARTICLE
mixture was heated at 90 °C for 12 h. After the reaction solution cooled
to rt, the solvent was removed under reduced pressure, and the
remaining residue was dissolved in benzene (250 mL), stirred for 30
min, and washed with water (3 ꢀ 50 mL) in a separatory flask under air.
The organic layer was separated, dried over MgSO4, and filtered through
an alumina plug to produce a colorless solution. The solvent was
removed under reduced pressure to give a white powder (4.46 g, 50%
yield). 1H NMR (300 MHz, CDCl3): δ 8.12 (br t, J = 1.5 Hz, 1H), 7.98
(dd, J = 7.5 and 0.9 Hz, 1H), 7.93 (ddd, J = 7.8, 0.9, and 0.6 Hz, 1H), 7.65
(dd, J = 7.5 and 0.9 Hz, 1H), 7.55 (dq, J = 8.1 and 0.9 Hz, 1H), 7.45 (app
q, J = 7.8 Hz, 2 H). 13C NMR (75.5 MHz, CDCl3): δ 153.3, 138.3, 131.7,
130.9, 130.5, 127.4, 126.8, 125.1, 124.3, 123.8, 123.0, 120.7.
sharpened upon cooling to -15 °C. However, the aryl peaks grew
broader with decreasing temperature, so their chemical shifts will not be
reported with the low temperature characterization. 1H NMR(400
MHz, CDCl3, -15 °C): δ 2.62 (br, 1H, methine of PiPr2), 2.44 (s,
1.5H, Rh(CH3CN)(C0H3CN)), 2.26 (br m, 1H, methine of PiPr2), 1.43
(br, 3H, methyl of PiPr2), 1.33 (br, 3H, methyl of PiPr2), 1.17 (br, 3H,
methyl of PiPr2), 1.04 (s, 1.5H, Rh(CH3CN)(C0H3CN)), 0.92 (br, 3H,
methyl of PiPr2). 13C NMR (101 MHz, CD2Cl2): δ 153.6, 138.3 (t, J =
10.2 Hz), 137.4 (t, J = 6.6 Hz), 131.5, 131.2, 130.8, 129.4, 127.9 (t, J = 16
Hz), 126.2, 125.9, 124.4, 120.6, 23.0 to 22.0 (br m), 20.4 (br m), 18.9
(br), 15.6 (br), 4.3 to 3.0 (br m). At rt, the aliphatic carbon peaks were
not well-resolved. ESI-MS(þ): m/z = 737 [(M - BF4)þ]. Anal. Calcd.
for C40H48BF4N2OP2Rh: C, 58.27; H, 5.87; N, 3.40. Two independent
crystalline samples were tested. Found: (first) C, 58.81; H, 6.00; N, 4.05;
(second) C, 58.88; H, 6.34; N, 3.72.
[(iPrDPDBFphos)Pd(CH3CN)2](BF4)2 (5). The ligand 2 (58 mg, 106
μmol) and [Pd(CH3CN)4](BF4)2 (47 mg, 106 μmol) were dissolved in
tetrahydrofuran (THF) and stirred overnight at rt. All volatiles were
removed under reduced pressure. The residue was rinsed with diethyl
ether and dried under reduced pressure, affording a pale yellow solid
(157 mg, 80% yield). Pale yellow needle crystals were grown from vapor
diffusion of Et2O into a CH3CN solution of 5. 31P NMR (202 MHz,
CD3CN): δ 43.1. 1H NMR (500 MHz, CD3CN): δ 8.55 (br t, J = 5 Hz,
1H), 8.25 (dd, J = 3.6 and 5.7 Hz, 1H), 7.92 (d, J = 7.8 Hz, 1H), 7.79
(t, J = 7.8 Hz, 1H), 7.712 (m, 1H), 7.61 (m, 2H), 3.08 to 2.69 (br m’s, 3.5
H, P(CH(CH3)2)2 and Pd(CH3CN)(C0H3CN)), 1.72 to 0.80 (br m’s,
13.5H, P(CH(CH3)2)2 and Pd(CH3CN)(C0H3CN). The broad peaks
in the aliphatic region sharpen upon cooling down to -60 °C. Only the
aliphatic resonances are reported at the lower temperature as the aryl
peaks do not shift significantly. 1H NMR (400 MHz, CD2Cl2, -60 °C):
δ 2.86 (br, 1H, methine of PiPr2), 2.86 (s, 1.5 H, Pd(CH3CN)-
(C0H3CN)), 2.74 (br, 1H, methine of PiPr2), 1.53(br, 3H, methyl of
PiPr2), 1.42 (s, 1.5 H, Pd(CH3CN)(C0H3CN)), 1.23 (br, 3H, methyl of
PiPr2), 1.00 (br, 3H, methyl of PiPr2).13C NMR (75 MHz, CD3CN): δ
154.0, 139.1 (t, J = 6.3 Hz), 137.1 (t, J = 8.6 Hz), 134.3, 132.8, 131.5,
131.3 (t, J = 4.0 Hz), 126.3, 125.9, 125.3, 122.6, 122.3, 122.1, 24.1 (t, J =
12.6 Hz), 21.7 to 19.2 (br, m), 18.7, 18.3 to 15.1 (br, m). At rt, the
aliphatic carbon peaks were not well-resolved. ESI-MS (þ): m/z = 740
[(M - BF4)þ]. Anal. Calcd. for C42H51B2F8N3OP2Pd (5 þ CH3CN):
C, 52.77; H, 5.38; N, 4.40. Found: C, 52.68; H, 5.17; N, 4.33.
X-ray Crystallographic Data Collection and Refinement of
the Structures. Single crystals of 3 to 5 were placed onto the tip of a
0.1 mm diameter glass capillary and mounted on a Bruker SMART
Platform CCD diffractometer for data collection at 173(2) K. The data
collection was carried out using MoKR radiation (graphite mono-
chromator). The data intensity was corrected for absorption and decay
(SADABS).31 Final cell constants were obtained from least-squares fits
of all measured reflections. The structure was solved using SHELXS-97
and refined using SHELXL-97.32 A direct-methods solution was calcu-
lated which provided most non-hydrogen atoms from the E-map. Full-
matrix least-squares/difference Fourier cycles were performed to locate
the remaining non-hydrogen atoms. All non-hydrogen atoms were
refined with anisotropic displacement parameters. Hydrogen atoms
were placed in ideal positions and refined as riding atoms with relative
isotropic displacement parameters. Crystallographic data for com-
pounds 3 to 5 are listed in Table 1.
Physical Methods. Mass spectrometry (MS) data was acquired
on a Bruker BioTOF ESI-MS under positive mode. Cyclic voltammetry
experiments were performed inside a glovebox with a CHInstruments
Model 600D potentiostat/galvanostat. A single-chamber cell was set up
with a glassy carbon working electrode (3 mm diameter), a Pt wire as the
auxiliary electrode, and a reference electrode consisting of a silver wire in
10 mM AgNO3 solution (0.1 M [nBu4N]PF6 in CH3CN). All measure-
ments were calibrated to an internal ferrocene standard.
4,6-Bis(3-diisopropylphosphinophenyl)dibenzofuran (iPrDPDBFphos,
2). To a solution of 1 (1.59 g, 3.32 mmol) in THF (100 mL) cooled in a
dry ice/acetone bath, nBuLi (4.15 mL, 6.64 mmol, 1.6 M in hexane) was
added dropwise. The bright yellow mixture was stirred at -78 °C for 1.5
h and then quenched slowly with diisopropylchlorophosphine (1.06 mL,
6.64 mmol). The resulting colorless solution was stirred for 4 h at
-78 °C and then evaporated to dryness under reduced pressure. After
moving into a glovebox, the residue was extracted with diethyl ether
(100 mL) and filtered through a plug of Celite. Colorless crystals (1.45 g,
80% yield) were obtained by cooling an Et2O/pentane solution (15 mL,
2:1) to -25 °C. 31P NMR(121 MHz, CDCl3): δ 12.3. 1H NMR (500
MHz, CDCl3): δ 8.01 (m, 1H), 8.00 (dd, J = 7.5 and 1.0, 1H), 7.93 (d,
J = 7.5 Hz, 1H), 7.65 (dd, J = 7.5 and 1.0 Hz, 1H), 7.53 (m, 2H), 7.48
3
(app t, J = 8 Hz, 1H), 2.15 (septet of d, PCH(CH3)2, JHH = 7.0, 2JHP
=
2.0 Hz, 2H), 1.13 (dd, PCH(CH3C0H3), 3JHP = 15 Hz, 3JHH = 7.0 Hz,
6H), 0.97 (dd, PCH(CH3C0H3), 3JHP = 11.0 Hz, 3JHH = 7.0 Hz, 6H).
13C NMR (126 MHz, CDCl3): δ 153.5, 136.1 (d, J = 8 Hz), 135.4 (d, J =
18 Hz), 134.7 (d, J = 20 Hz), 133.8 (d, J = 17 Hz), 129.6, 128.3 (d, J = 7
Hz), 127.4, 126.0, 125.2, 123.5, 119.9, 23.0 (d, J = 11 Hz), 20.1 (d, J = 18
Hz), 19.1 (d, J = 9 Hz). ESI-HRMS-TOF m/z: [M þ H]þ calc. for
C36H43OP2, 553.2789; found, 553.2813.
[(iPrDPDBFphos)Rh(NBD)]BF4 (3). A 20 mL scintillation vial was
charged with 2 (101 mg, 183 μmol), Rh(NBD)2(BF4) (68 mg, 183
μmol) and 15 mL CH2Cl2. The red solution was stirred at rt for 4 h and
then evaporated to dryness under reduced pressure. The residue was
washed with diethyl ether (2 ꢀ 5 mL) to give a red-orange powder (134
mg, 90% yield). Crystals were grown from vapor diffusion of pentane into
a CHCl3 solution of 3. Single crystals suitable for X-ray analysis were
obtainedfromvapor diffusion ofC6H6 into a 1,2-difluorobenzenesolution
of 3 at rt. 31P NMR(121 MHz, CDCl3): δ 28.8 (d, JRhP = 148 Hz). 1H
NMR (500 MHz, CDCl3): δ 8.44 (br d, J = 8.5 Hz, 1H), 8.06 (dd, J = 8.0
and 1.0 Hz, 1H), 7.82 (d, J = 7.5 Hz, 1H), 7.77 (d, J = 7.5 Hz, 1H), 7.70 (t,
J = 7.5 Hz, 1H), 7.66 (br t, J = 7.0 Hz, 1H), 7.54 (t, J = 7.8 Hz, 1H), 5.08
0
(br, 2H, NBD), 4.05 (br, 1H, NBD), 2.35 (br, 2H, PCHCH3CH3 ), 1.89
(s, 1H, NBD), 1.29 (br, 6H, PCH(CH3C0H3), 1.02 (br, 6H, PCH
(CH3C0H3). 13C NMR (75.5 MHz, CD2Cl2): δ 153.5, 136.9 (t, J = 5.0
Hz), 133.4 (t, J = 6.3 Hz), 131.8 (m), 130.9, 130.1, 130.0, 126.7, 125.3,
124.5, 124.4, 121.4, 75.6, 69.0, 53.5, 26.5 (m, J = 9.8 Hz), 21.4, 20.4. ESI-
MS(þ): m/z = 747 [(M - BF4)þ]. Anal. Calcd. for C43H50BF4OP2Rh:
C, 61.89; H, 6.04; N, 0. Found: C, 62.09; H, 5.92; N, 0.0.
[(iPrDPDBFphos)Rh(CH3CN)2]BF4 (4). Compound 3 (110 mg, 132
μmol) was dissolved in acetonitrile (10 mL) and heated for 2 h at 80 °C.
All volatiles were evaporated under reduced pressure. The residue was
washed with diethyl ether (3 ꢀ 3 mL) and dried under reduced pressure,
affording a bright yellow powder (93 mg, 85% yield). Yellow needle
crystals were grown from concentrated acetonitrile solution at -25 °C.
31P NMR(121 MHz, CDCl3): δ 46.4 (d, JRhP = 127 Hz). 1H NMR (500
MHz, CDCl3): δ 8.57 (br t, J = 5.0 Hz, 1H), 8.08 (dd, J = 6.0 and 3.0 Hz,
1H), 7.63 (d, J = 8.0 Hz, 1H), 7.58 (t, J = 8.0 Hz, 1H), 7.53 (m, 2H),
7.45 (m, 1H), 2.53 to 2.35 (br m’s, 3.5H, methine of PiPr2 and Rh(CH3-
CN)(C0H3CN)), 1.45 to 0.95 (br m’s, 13.5H, methyl of PiPr2 and
Rh(CH3CN)(C0H3CN)). The broad peaks in the aliphatic region
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ARTICLE
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and phosphorus atoms, triple-ζ-quality basis sets with one set of
polarization functions were used (def2-TZVP) with additional ECP
bases for rhodium.35 The carbon, oxygen, and hydrogen atoms were
described by smaller polarized split-valence def2-SV(P) basis sets.36
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Corresponding Author
*E-mail: clu@umn.edu.
’ ACKNOWLEDGMENT
The authors gratefully acknowledge Dr. Benjamin Lynch,
Dr. Nicholas Labello, Dr. Boris Averkiev, and the Minnesota
Supercomputing Institute for computing support/resources.
Elodie Marlier and Dr. Letitia Yao provided assistance with
NMR spectroscopic studies. Professors Karsten Meyer
(University of Erlangen) and Philippe Buhlmann (University
of Minnesota) are thanked for helpful suggestions. We would
also like to thank Sigma-Aldrich for a donation of [Rh(NBD)2]-
BF4. Funding for this work was provided by the University of
Minnesota. Single crystal X-ray diffraction experiments were
performed at the X-ray Crystallographic Laboratory in the
Chemistry Department of UMN.
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