Rhodium complexes of a new structurally adaptive PNN-pincer type ligand

Article abstract of DOI:10.1021/om400151e

A new PNN-pincer type ligand with pyrazolyl and diphenylphosphine flanking donors on a diarylamido anchor has been prepared. Its bis(tert-butyl isocyanide)rhodium(I) complex exhibits hemilabile behavior in solution, and its solid-state structure verified the elusive κ²P,N coordination mode for this type of ligand. Reactions between (PNN)Rh(CN^tBu) ₂ and iodomethane afford both fac- and cis,mer-[(PNN)Rh(CN ^tBu)₂(Me)](I), which further showcases the structural versatility of the ligand.

Full text of DOI:10.1021/om400151e

Communication
pubs.acs.org/Organometallics
Rhodium Complexes of a New Structurally Adaptive PNN-Pincer
Type Ligand
Sarath Wanniarachchi, Jeewantha S. Hewage, Sergey V. Lindeman, and James R. Gardinier*
Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53201-1881, United States
S
* Supporting Information
ABSTRACT: A new PNN-pincer type ligand with pyrazolyl and
diphenylphosphine flanking donors on a diarylamido anchor has been
prepared. Its bis(tert-butyl isocyanide)rhodium(I) complex exhibits
hemilabile behavior in solution, and its solid-state structure verified the
elusive κ2P,N coordination mode for this type of ligand. Reactions
between (PNN)Rh(CNtBu)2 and iodomethane afford both fac- and
cis,mer-[(PNN)Rh(CNtBu)2(Me)](I), which further showcases the
structural versatility of the ligand.
ver the past few decades, there has been intense interest
(corresponding to a five-coordinate species) could be located
Oin metal complexes of multidentate “hemilabile” ligands on either respective potential energy surface”. Given our
experience with diarylamido-anchored ligands with six-mem-
bered chelate rings similar to complex B, we were keenly aware
that, although less common than the mer coordination mode,
the fac coordination mode is sometimes observed.7 Such a
possibility casts doubt on the structural nature of the reported
(PNN)Rh(CNR)2 complexes. Thus, we set out to exploit the
crystallinity of pyrazolyl-containing ligand systems to structur-
ally verify the elusive κ2P,N coordination mode of the
(PNN)Rh(CNR)2 complexes. In this paper, we document
useful coupling reactions to obtain a new pyrazolyl-containing
ligand with a PNN donor set. We also describe the syntheses of
various rhodium(I) complexes and the hemilability of one
complex. The variability in metal coordinating behavior of the
new pincer-type ligand is also illustrated through examination
of [(PNN)Rh(Me)(CNtBu)2](I).
The synthetic route to the new ligand and its
carbonylrhodium(I) complex is outlined in Scheme 1. A CuI-
catalyzed amination reaction between 2-pyrazolyl-4-toluidine
(H(pzAnMe))8 and diiodobenzene affords 2-iodo-N-(4-methyl-
2-(1H-pyrazol-1-yl)phenyl)benzenamine, (H(NIPh- pzAnMe)), a
precursor (top right of Scheme 1) that is used in the final step
of the ligand construction. A high-yielding Pd0-catalyzed
coupling reaction between H(NIPh-pzAnMe) and HPPh2
provides the desired ligand with a PNN donor set (the crystal
structure of H(PNN) and further synthetic discussion are
found in the Supporting Information). The reaction between
Rh(CO)2(acac) and H(PNN) in acetone afforded a high yield
of (PNN)Rh(CO) (1). Complex 1 is air stable in the solid state
as well as in solution, and no special precautions were required
for its handling. Although all attempts to obtain crystals of 1
suitable for X-ray diffraction have been stymied by its
propensity to form microcrystalline needles, the NMR spectral
where one ligating arm readily dissociates or is forcibly
displaced by an incoming nucleophile.1 The identification and
study of such hemilabile ligands has been important both for
the development of new catalytic reactions and for the
discovery of new materials for sensing applications.2 A majority
of hemilabile ligands are bidentate3 with both “hard” and “soft”
Lewis donors. Other ligands such as tris(pyrazolyl)borate,
tris(pyrazolyl)methanes, and related “scorpionates”, which
typically bind metals in a facial terdentate manner with six-
membered chelate rings, have also been shown to be hemilabile
with certain metals.4 There has been a growing interest in
complexes of hemilabile “pincer” ligands (typically anionic
terdentate species that bind metals with five-membered rings),
because certain examples have been found to mediate
remarkable chemical transformations.5 The van der Vlugt
group recently reported on the hemilabile/”flexidentate”
character of bis(isopropyl isonitrile)rhodium(I) complexes of
two PNN-pincer (A and C, Chart 1) and one pincer-type (B,
Chart 1) ligand.6 The authors provided compelling spectro-
scopic evidence that various (PNN)Rh(CNR)2 complexes
contained four-coordinate rhodium with κ2P,N ligands, but in
no case was a complex structurally authenticated. Instead,
computational studies were used to support the assertion that
one ligand arm was dissociated, since “no minimum
Chart 1. Rhodium(I) Complexes of Hemilabile PNN-Pincer
Ligands Reported by the van der Vlugt Group6
Received: February 24, 2013
Published: May 3, 2013
© 2013 American Chemical Society
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a
Scheme 1.
Figure 1. Structure of (PNN)Rh(CNtBu)2 in 3·0.2(pentane), with
hydrogen atoms removed for clarity. Thermal ellipsoids are shown at
the 50% probability level. Selected bond distances (Å): Rh1−P1,
2.2521(5); Rh1−N1, 2.0728(14); Rh1−C51, 1.8884(18); Rh1−C61,
1.9825(18); C51−N2, 1.156(8); N2−C52, 1.472(8); C61−N3,
1.141(9); N3−C62, 1.457(9); Selected bond angles (deg): P1−
Rh1−N1, 82.42(4); P1−Rh1−C51, 93.70(6); C51−Rh1−C61,
88.40(7); N1−Rh1−C61, 95.48(6); N1−Rh1−C51, 176.10(7); P1−
Rh1−C61, 177.63(5); C51−N2−C52, 177.3(8); C61−N3−C62,
173.3(17).
a
Key: (i) cat. CuI, 1.2 Cs2CO3, p-dioxane, Δ 16 h; (ii) 1.2 HPPh2, 0.5
mol % Pd2(dba)3, 1 mol % Xantphos, 1.2 NEt3, p-dioxane, Δ, 15 h;
(iii) Rh(CO)2(acac), acetone, Δ, 15 min.
data of 1 are in accord with the structural formulation depicted
in Scheme 1. The C−O stretching frequency for 1, νCO 1957
cm−1, is comparable to νCO 1960 cm−1 reported for the related
PNP derivative of Mayer and Kaska with a diarylamido anchor
and two PPh2 flankers.9 Another related complex, (NNN)Rh-
(CO) (2; NNN has two pyrazolyl flanking donors attached to
the same diarylamido backbone as in 1), has a C−O stretching
frequency of νCO 1954 cm−1,10 which indicates only a slight
increase in back-bonding in comparison to 1. These
comparable results corroborate our previous findings that the
para aryl substituents (rather than flanking donors) dictate the
electronic properties of the metal complexes of diarylamido-
anchored pincer ligands. The 13C NMR spectrum of 1 shows a
doublet of doublets signal at δC 193 ppm (1JRh−C = 67 Hz and
2JP−C = 18 Hz) for the rhodium-bound carbonyl; that for 2
showed a doublet resonance at δC 193 ppm (1JRh−C = 71 Hz).
The similarity of chemical shift and coupling constant between
1 and 2 suggests that 1 has a square-planar coordination
geometry about rhodium with trans-disposed amido and
carbonyl groups like that in the structurally characterized 2.
The 31P NMR spectrum of 1 shows a doublet resonance at δP
61 ppm (1JP−Rh = 167 Hz), which is shifted downfield from the
singlet resonance at δP = −20 ppm for H(PNN) and the
doublet resonance at δP 41.8 ppm (1JP−Rh = 135.1 Hz) reported
for Mayer and Kaska’s PNP derivative.9
Dynamic behavior for 3 in solution is evident from an
examination of variable-temperature NMR spectral data. The
resonances for various nuclei show different temperature-
dependent line broadening and changes in chemical shifts
depending on the type of resonance. Figure 2 shows an overlay
Figure 2. Overlays of the 31P NMR spectra (left) and representative
portion of the 1H NMR spectra (right) of 3 in acetone-d6 obtained at
various temperatures.
1
of 31P NMR spectra and a representative portion of the H
NMR spectra for 3 in acetone-d6 at various temperatures (more
complete spectra can be found in the Supporting Information).
At 223 K, the 31P NMR spectrum consists of a doublet at 45.8
ppm with JRhP = 138.5 Hz. When the temperature is raised
above 243 K, the doublet resonance shifts slightly downfield
and becomes broader until coupling can no longer be detected
above 303 K. As can be seen in Figure 2 and Figures S4 and S5
(Supporting Information), similar behavior occurs for reso-
Complex 1 reacts with excess (4 equiv or more) of CNtBu to
give analytically pure (PNN)Rh(CNtBu)2 (3). As reported for
other similar complexes, complex 3 is air sensitive both in the
solid state and in solution. Thus, 3 needs to be stored and
handled under an inert atmosphere. Single crystals of 3 suitable
for X-ray diffraction were grown by extracting the initial
product mixture of 1 and excess CNtBu with pentane and
allowing the pentane-soluble portion to stand under nitrogen
for several hours. The structure of 3 shown in Figure 1 verifies
the κ2P,N coordination mode of the ligand. The rhodium is in a
square-planar geometry, where the sum of angles about the
metal is 360°. The isocyanide ligand trans to the amido exhibits
a shorter Rh−C bond (1.888(2) Å) and a marginally longer
unsaturated C−N bond (1.156(8) Å) in comparison with that
trans to the phosphine arm (Rh−C, 1.983(2) Å; C−N,
1.141(9) Å). The Rh−C bond distances are the ranges found
for other charge-neutral rhodium(I) organoisocyanide com-
plexes.12−16 The Rh−N and Rh−P bonds in 3 are similarly
unremarkable.
1
nances in the H NMR spectra, but with notable differences.
The resonances for the pyrazolyl, tolyl, and one of the tert-butyl
group (upfield signal) hydrogens exhibit the greatest line
broadening and changes in chemical shifts, followed by
resonances for the PC6H4N group. The resonances for the
t
hydrogens of the (C6H5)2P group and the other Bu group
exhibit negligible changes with temperature. The rate constant
of the dynamic process can be extracted by measuring W1/2, the
line broadening in excess of the natural line width, according to
the relation k = πW1/2. As detailed in the Supporting
Information, Eyring analyses (Figure S6) of the temperature
dependence of the line broadening/rate constant derived from
1
the 31P NMR resonance and the H NMR resonances for
pyrazolyl, tolyl, and upfield tBu hydrogens afforded the
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following activation parameters: ΔG⧧ = 14.3 ( 0.1) kcal/mol,
ΔH⧧ = 9 ( 2) kcal/mol, and ΔS⧧ = −19( 5) cal/(K mol).
The negative value for activation entropy suggests a highly
organized transition state. On the basis of experimental
observations and theoretical calculations (OP86/Def2-SV-
(P))11 that show a five-coordinate conformer is only 5.1 kcal
higher in energy than a four-coordinate structure (Figure S10,
Supporting Information), we attribute the dynamic process to
be a result of reversible coordination of the hemilabile pyrazolyl
arm (k298 = 229 s−1), as in Scheme 2. Such a proposition
Scheme 2. Plausible Process Responsible for Temperature-
Dependent NMR Line Broadening in the Spectra of 3
Figure 3. Structure of cis,mer-[(PNN)Rh(Me)(CNtBu)2](I)·C6H6,
(mer-4·C6H6). Selected bond distances (Å): Rh1−P1, 2.2420(6);
Rh1−N1, 2.0684(18); Rh1−N11, 2.0969(19); Rh1−C51, 1.949(2);
Rh1−C61, 2.074(2); Rh1−C71, 2.101(2); N2−C51, 1.151(3); N2−
C52, 1.466(3); N3−C61, 1.146(3); N3−C62, 1.470(3); Selected bond
angles (deg): P1−Rh1−N11, 166.78(5); N1−Rh1−C51, 175.30(9);
C61−Rh1−C71, 175.08(9); P1−Rh1−C61, 95.51(6); C71−Rh1−
N11, 85.21(8); N1−Rh1−C71, 86.48(9); N11−Rh1−N1, 85.34(7);
P1−Rh1−N1, 82.67(6); C51−Rh1−C71, 88.86(9); C51−Rh1−C61,
93.40(9); C61−Rh1−N1, 91.20(8). Hydrogen atoms and benzene
molecule have been omitted for clarity.
rationalizes the observed trends in the disparate broadening of
resonances and chemical shift changes in the NMR spectra.
Also, the possible presence of both four- and five-coordinate
isomers of 3 at room temperature provides an explanation for
the greater than expected number of CN stretches observed in
the IR spectra. Theoretical calculations indicate that two CN
stretches are expected at 2164 and 2101 cm−1 (in a 0.99
intensity ratio) for four-coordinate 3 and at 2147 and 2075
cm−1 (in an intensity ratio of 1.05) for five-coordinate 3. The
experimentally observed CN stretching frequencies for 3 in
benzene occur at 2156, 2102, and 2065 cm−1 with relative
intensities of 1.9:1:1.2. Thus, the relatively high intensity of the
high-energy band may be a result of two overlapping bands. It is
noted that complexes A and B (Chart 1) each had three CN
stretches (near 2157, 2080, and 2040 cm−1); data for C were
not reported.6 Finally, the possibility that the solution dynamic
process involves dissociation of one CNtBu is disfavored, owing
to the negative value for activation entropy. Furthermore, the
NMR resonances for free CNtBu or for (PNN)Rh(CNtBu)
(IA) were not observed. As also noted in the Supporting
Information, we have spectroscopically characterized IA as a
synthetic intermediate along the way to 3. The spectroscopic
signatures of IA and its mixtures with 3 are different from the
variable-temperature NMR spectral data.11
Figure 4. Structure of the cation in fac-[(PNN)Rh(Me)(CNtBu)2]-
(I)·H2O (fac-4·H2O). Selected bond distances (Å): Rh1−P1,
2.2765(6); Rh1−N1, 2.0577(19); Rh1−N11, 2.176(2); Rh1−C51,
1.955(2); Rh1−C61, 2.040(2); Rh1−C71, 2.107(2); N2−C51
1.147(3); N2−C52 1.473(3); N3−C61 1.145(3); N3−C62
1.466(3). Selected bond angles (deg): N1−Rh1−C51, 175.79(9);
P1−Rh1−C61, 174.33(7); C71−Rh1−N11, 167.91(8); P1−Rh1−
N11, 96.46(5); N1−Rh1−C71, 87.62(9); N11−Rh1−N1, 81.98(7);
P1−Rh1−N1, 81.01(6); C51−Rh1−C71, 89.96(10); C51−Rh1−C61,
88.92(9); C61−Rh1−N1, 94.46(9). Hydrogen atoms, the iodide
anion, and the water molecule have been omitted for clarity.
The structural adaptability of the new PNN-pincer type
ligand is displayed by rhodium(III) complexes derived from 3.
Thus, as per Scheme 3, the reaction between 3 and MeI
produced easily separable mixtures of cis,mer (hereafter referred
to simply as mer, since the trans,mer isomer has not yet been
detected) and fac isomers of [(PNN)Rh(Me)(CNtBu)2](I), 4.
The mer isomer has some solubility in benzene, in contrast to
the fac isomer, thereby allowing separation. Figures 3 and 4
show the structures of the cations in mer-4 and fac-4,
respectively. In these structures, the Rh−N1 distance of
2.068(2) Å (mer-4) and 2.058(2) Å (fac-4) are among the
longest such bonds found in related (pincer)RhIII derivatives,17
rivaling 2.064(2) Å in trans-(NNN)RhCl2(PEt3). In fact, the
Rh1−N1 distance in mer-4 with a formal rhodium(III) center is
close to the 2.0728(14) Å found in 3, with a rhodium(I) center.
For both fac- and mer-4, the CNtBu trans to the amido group
has shorter Rh1−C51 and longer C51−N2 bonds versus the
analogous bonds in the other CNtBu group (trans to the
phosphine). This observation may be indicative of the greater
π-donating abilities of the diarylamido versus the triarylphos-
phine group that increases the metal back-bonding to the trans-
CNtBu ligand. This effect is also apparent in 3. It is also worth
noting that the Rh−Npz bond in fac-4 is longer than that in mer-
Scheme 3. Reaction between MeI and 3 in CH2Cl2
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4 or 3, which might be related to the constrained ligand
geometry in this coordination mode and the donating abilities
of the ligand trans to the pyrazolyl nitrogen.
crystallographic data. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail for J.R.G.: james.gardinier@marquette.edu.
Interestingly, the ratio of fac- to mer-4 obtained from the
preparative reactions depends on the solvent and time allotted
for reaction, as indicated by NMR spectroscopy (and X-ray
crystallography). When the reaction was performed in
dichloromethane, a 3:1 fac:mer ratio was immediately obtained.
That is, upon addition of MeI to a CD2Cl2 solution of 3, the
■
Notes
The authors declare no competing financial interest.
original 31P NMR doublet resonance at δP 46.0 ppm (JP−Rh
=
ACKNOWLEDGMENTS
J.R.G. thanks the NSF (CHE-0848515) for financial support.
■
141 Hz) was immediately replaced by two new doublet
resonances at δP 57.8 ppm (JP−Rh = 121 Hz) and δP 54.3 ppm
(JP−Rh = 106 Hz) in a 3:1 ratio. The former resonance with the
larger coupling constant is due to mer-4, while the latter
resonance with the smaller coupling constant is due to fac-4.
Over time, the resonance for the mer isomer grows at the
expense of that for the fac isomer. When the reaction between 3
and MeI was performed in a limited amount of benzene, pure
fac-4 (59%) immediately precipitated as a yellow solid; the
soluble portion contained dark orange mer-4 (40%). The
reversal in isomer ratio in C6D6 in comparison to the reaction
performed in CD2Cl2 is kinetic in nature. After the benzene-
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coordination mode in its bis(organoisocyanide)rhodium(I)
complex was structurally verified, a mode suggested by the
van der Vlugt group for similar complexes. Hemilabile behavior
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ASSOCIATED CONTENT
■
S
* Supporting Information
Text, figures, tables, and CIF files giving experimental and
computational details, NMR spectra, further discussion, and
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