Preparation and reactivity of a monomeric octahedral platinum(IV) amido complex

Article abstract of DOI:10.1021/ic800843b

Synthesis and isolation of the monomeric octahedral platinum(IV) amido complex (NCN)PtMe₂NHPh have been accomplished upon deprotonation of the amine complex [(NCN)PtMe₂(NH₂Ph)][OTf]. The preliminary reactivity of the amido ligand has been explored.

Full text of DOI:10.1021/ic800843b

Inorg. Chem. 2008, 47, 6124-6126
Preparation and Reactivity of a Monomeric Octahedral Platinum(IV)
Amido Complex
Colleen Munro-Leighton,† Yuee Feng,† Jubo Zhang,† Nikki M. Alsop,† T. Brent Gunnoe,*,†
Paul D. Boyle,† and Jeffrey L. Petersen‡
Department of Chemistry, North Carolina State UniVersity, Raleigh, North Carolina 27695-8204,
and C. Eugene Bennett Department of Chemistry, West Virginia UniVersity,
Morgantown, West Virginia 26506
Received May 8, 2008
Synthesis and isolation of the monomeric octahedral platinum(IV)
amido complex (NCN)PtMe2NHPh have been accomplished upon
deprotonation of the amine complex [(NCN)PtMe2(NH2Ph)][OTf].
The preliminary reactivity of the amido ligand has been explored.
An important distinction between amido, alkoxo, oxo, nitrene,
and related ligands coordinated to middle/late transition-metal
centers in high versus low oxidation state is the predilection of
the former toward odd-electron radical chemistry, while the
latter class of complexes typically undergoes “even-electron”
reactivity.10 For example, paramagnetic ruthenium(III) hydroxo,
iron(III) alkoxo, and other high oxidation oxo and imido complexes
initiate C-H bond cleavage via net bond homolysis (i.e.,
hydrogen-atom abstraction).7,23,24 In contrast, diamagnetic
octahedral ruthenium(II) and iron(II) complexes react with C-H
bonds via even-electron acid/base reactions,2,12–15 and ruthe-
nium(II) amido/hydroxo, iridium(III) methoxo, and rhodium(I)
aryloxo systems activate C-H bonds through even-electron
net1,2-addition of C-H bonds across the M-X (X ) OR, NHR)
ligands.25–28
The combination of a Lewis acidic metal center with a
nucleophilic/basic heteroatomic ligand provides an opportunity
for metal-mediated transformations. Along these lines, octahe-
dral and d6 group 10 complexes (Ni, Pd, and Pt) with amido,
alkoxo, and related ligands represent a potentially interesting
class of systems. The 4+ formal oxidation state should result
in a Lewis acidic metal in addition to a filled set of dπ orbitals.
Thus, such complexes might be expected to undergo even-
electron chemistry, with the heteroatomic ligand acting as a base
or nucleophile. Although four-coordinate palladium(II) amido,
alkoxo, and related systems are key intermediates in important
Several studies have revealed that formally anionic hetero-
atomic ligands (e.g., amido, alkoxo, and related ligands)
coordinated to late transition metals in low oxidation states can
be quite basic and/or nucleophilic.1–11 For example, ruthe-
nium(II) complexes can initiate intermolecular deprotonation
of some C-H bonds to form amine/carbanion ion pairs,2,12–16
and an octahedral iron(II) parent amido complex has been
suggested to undergo an intermolecular nucleophilic addition
to free carbon monoxide.18 Recently, we have demonstrated
that monomeric copper(I) amido, alkoxo, and sulfido complexes
catalyze conjugate addition of amines, alcohols, and thiols to
electron-deficient olefins including some vinyl arenes.19–22 The
formal disruption of ligand-to-metal π donation due to filled
dπ orbitals likely plays an important role in the reactivity of
these systems by enhancing the nucleophilicity and basicity of
amido, alkoxo, and related ligands.4,8–10
* To whom correspondence should be addressed. E-mail: brent_gunnoe@
ncsu.edu.
†
North Carolina State University.
West Virginia University.
‡
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6124 Inorganic Chemistry, Vol. 47, No. 14, 2008
10.1021/ic800843b CCC: $40.75  2008 American Chemical Society
Published on Web 06/21/2008

COMMUNICATION
catalytic C-N and C-O bond-forming processes29 and square-
planar nickel(II), palladium(II), and platinum(II) amido com-
plexes are known,30,31 examples of fully characterized octahe-
dral and monomeric metal(IV) (metal ) nickel, palladium, or
platinum) amido complexes are virtually unknown. The prepa-
ration and reactivity of tri-µ-amidobis[triammineplatinum(IV)]
complexes have been reported,32–35 Peters and co-workers studied
a platinum(IV) complex with a chelating pincer-type amido ligand,
and Goldberg et al. have recently reported the synthesis and
reactivity of platinum(IV) sulfonamide complexes.36,37
Figure 1. ORTEP of 3 (50% probability; most H atoms and the BArF
4
The reaction of previously reported (NCN)PtMe2Br38 [NCN
) 2,6-(pyrazolyl-CH2)2C6H3] with silver triflate in refluxing
THF results in the formation of (NCN)PtMe2OTf (1; eq 1). The
1H NMR spectrum of complex 1 displays patterns similar to
those of (NCN)PtMe2Br but with shifted resonances. Thus, we
presume that the coordination geometry of 1 is analogous to
that of (NCN)PtMe2Br with a facially coordinated NCN ligand
and the triflate trans to the aryl unit. Complex 1 exhibits thermal
stability because heating 1 in C6D6 at temperatures up to 120
°C for 20 h does not result in decomposition (1H NMR
spectroscopy).
counterion are excluded for clarity). Selected bond lengths (Å): Pt1-N5
2.213(2), N5-C17 1.440(3), Pt1-C8 2.022(3). Selected bond angles (deg):
Pt1-N5-C17 121.0(2), C8-Pt1-N5 177.59(9).
the 13C NMR spectrum (in C6D6). The amido H atom resonates
at 2.88 ppm as a broad singlet with Pt satellites visible as
shoulders. In comparison, the amido H atom of TpRuL2(NHPh)
[L ) P(OMe)3 or PMe3], also six-coordinate d6 anilido
complexes, resonates at 1.98 and 2.80 ppm, respectively.17
The reaction of 1 with aniline in methylene chloride produces
the cationic complex [(NCN)PtMe2(NH2Ph)][OTf] (2). The poor
solubility of 2 in standard organic solvents led us to attempt a
counterion exchange to improve the solubility. The reaction of
complex 1 with aniline in CH2Cl2 followed by the addition of
A single crystal of 4 suitable for an X-ray diffraction study
was grown from a saturated benzene solution, and the solid-
state structure is shown in Figure 2. The Pt1-N5 bond length
for complex 4 is 2.087(2) Å, which is 0.126 Å shorter than the
Pt1-N5 bond length [2.213(2) Å] in the cationic aniline
complex 3. As is expected for platinum(IV) versus ruthe-
nium(II), the Pt1-N5 bond of 4 is slightly shorter than the
Ru-Namido bond [2.101(2) Å] of TpRu{P(OMe)3}2(NHPh).17
The anilido lone pair of complex 4 can potentially back-donate
into the π* orbitals of the anilido phenyl ring, which would
shorten the Namido-Cphenyl bond. Indeed, the N5-C17 bond
length of complex 4 is 1.376(3) Å, which is shorter than
1.440(3) Å for the cationic complex 3; however, the orientation
of the phenyl substituent of the anilido ligand of 4 is not oriented
tomaximizeoverlapwiththeamidolonepair[thePt1-N5-C17-C18
dihedral angle is 35.6(3)°]. The twisting of the phenyl ring out
of the Hamido-N5-C17 plane might be due to a steric interaction
between C18(H) and the methyl ligand C15(H). Cowan and
Trogler have reported the solid-state structure for the platinu-
m(II) amido complex trans-PtH(NHPh)(PEt3)2.40 Consistent
with a comparison between PtII and PtIV, the PtII-Namido bond
was found to be longer than that for 4 at 2.125(5) Å, with a
Namido-Cphenyl bond shorter than that of 4 at 1.343(8) Å. Espinet
et al. have also reported the solid-state structure for a platinu-
m(II) amido complex, albeit with a substituted anilido ligand.41
NaBArF [ArF
)
3,5-(CF3)2C6H3] cleanly produces
4
[(NCN)PtMe2(NH2Ph)][BArF ] (3; eq 2). The solid-state struc-
4
ture of complex 3 has been determined from a single-crystal
X-ray diffraction study (Figure 1). The Pt1-N5 bond length is
2.213(2) Å. Though a search of the Cambridge Structural
Database did not yield any examples of platinum(II) aniline
complexes, Radlowski et al. have reported a platinum(II)
complex with two chelating amine ligands with the lengths of
the four Pt-N bonds between 2.023(9) and 2.073(9) Å.39
Deprotonation of 2 with BuLi produces the platinum(IV)
amido complex (NCN)PtMe2(NHPh) (4; eq 3). A single
resonance is observed for the two symmetry-equivalent methyl
ligands at 1.42 ppm (singlet with Pt satellites, 2JPt-H ) 72 Hz)
in the 1H NMR spectrum and -8.6 ppm (1JPt-C ) 688 Hz) in
(29) Mindiola, D. J.; Hillhouse, G. L. J. Am. Chem. Soc. 2001, 123, 4623–
4624.
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S. P. J. Am. Chem. Soc. 1997, 119, 12800–12814.
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1997, 265, 17–22.
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Inorganic Chemistry, Vol. 47, No. 14, 2008 6125

COMMUNICATION
Figure 3. ORTEP of 5 (30% probability; H atoms are excluded for clarity.)
Selected bond lengths (Å): Pt1-C17 2.062(5), C17-C18 1.180(8), C18-C19
1.459(7), Pt1-C3 2.065(5). Selected bond angles (deg): Pt1-C17-C18
179.4(6), C17-C18-C19 178.1(7), C3-Pt1-C17 177.3(2).
Figure 2. ORTEP of 4 (50% probability; most H atoms are excluded for
clarity). Selected bond lengths (Å): Pt1-N5 2.087(2), N5-C17 1.376(3),
Pt1-C1 2.046(2). Selected bond angles (deg): Pt1-N5-C17 130.6(1),
C1-Pt1-N5 174.65(7).
12 h at 60 °C (eq 5). An X-ray structure has been obtained
from a single crystal of 5, and the data reveal a pseudooctahedral
coordination geometry (Figure 3). The Pt1-C17 bond length
is 2.062(5) Å, and the acetylide fragment is nearly linear with
Pt1-C17-C18 and C17-C18-C19 bond angles of 179.4(6)°
and 178.1(7)°, respectively.
Although the p-iodo substitution precludes a direct comparison,
the Pt-Namido bond for trans-[PtCl{NH(p-IC6H4)}(PEt3)2] was
found to be slightly shorter than that for 4 at 2.006(4) Å.
The downfield region of the 1H NMR spectrum of complex
4 in toluene-d8 is consistent with Namido-Cphenyl bond rotation
that is rapid on the NMR time scale: a single triplet is observed
for the m-anilido protons (7.20 ppm) with overlapping triplet
and doublet for the p- and o-anilido protons (6.59-6.64 ppm),
respectively. We conducted variable-temperature NMR studies
in an effort to explore the possibility of hindered Pt-Namido and
N-Cphenyl bond rotation.42 Upon cooling to -50 °C, the onset
of broadening was observed for the resonances due to the amido
phenyl group; however, cooling to -70 °C did not result in
decoalescence of these resonances, and we were not able to
obtain kinetic data for hindered N-C bond rotation.
We have previously reported that TpRuL2X (X ) OH, NH2,
or NHPh) complexes possess nucleophilic amido/hydroxo
ligands.2,7,43 In order to directly compare the nucleophilicity
of TpRu(PMe3)2(NHPh) and complex 4, we compared the rate
of nucleophilic substitution reactions with EtBr (eq 4). At 80
°C, complex 4 reacts with EtBr to produce EtN(H)Ph and
(NCN)PtMe2Br with kobs ) 1.4(1) × 10-4 s-1 (t1/2 ∼ 1.4 h).
This corresponds to ∆Gq of 27.0 kcal/mol, smaller than the
corresponding ∆Gq of 29.0 kcal/mol (80 °C) for
TpRu(PMe3)2(NHPh) (t1/2 ∼ 22 h). Thus, the relative rates of
reaction of ethyl bromide with TpRu(PMe3)2(NHPh) and
complex 4 are not dramatically different, while copper(I) anilido
complexes show substantially enhanced reactivity with ∆Gq
values of ∼22 kcal/mol at room temperature.
In order to test the thermal stability of complex 4, we heated
a C6D6 solution of 4 to 120 °C. After 90 h, ethane and methane
were observed in minimal quantities (by 1H NMR spectroscopy)
with little change to the resonances due to 4. The formation of
neither aniline nor additional Pt species was observed. Thus,
complex 4 is thermally robust. The reaction of complex 4 with
H2 at 80 °C initially results in the formation of a second complex
with the {(NCN)PtMe2} fragment (this complex could not be
isolated) with concomitant production of free aniline. Continued
heating for 90 h results in a decrease in the resonances due to
(NCN)PtMe2 systems (1H NMR) and production of free aniline
(eq 6). Thus, we presume that dihydrogen is being activated to
release aniline and form insoluble Pt decomposition products.
In summary, we have synthesized a new platinum(IV) amido
complex. Despite the thermal stability of 4, the anilido complex
reacts with acidic C-H bonds and H2. Simple nucleophilic
substitution reactions with EtBr place the nucleophilicity of the
anilido of 4 between that of TpRu(PMe3)2NHPh, which reacts
more slowly than 4, and monomeric Cu-NHPh complexes,
which react more rapidly than 4.4,8–10
The basicity of the amido and hydroxo ligands for the series
of ruthenium complexes has also been examined through
reactions with organic substrates with acidic C-H bonds. For
example, the reaction of TpRu(PMe3)2NHPh with phenylacety-
lene at 80 °C results in the formation of TpRu(PMe3)2(Ct CPh)
and free aniline.2 Similarly, the reaction of 4 with phenylacety-
lene produces free aniline and (NCN)PtMe2(Ct CPh) (5) after
Acknowledgment. We acknowledge the NSF (CAREER
Award Grant CHE 0238167) for support.
Supporting Information Available: Details of the synthesis and
characterization of all complexes, reactivity studies, and complete
data from the X-ray crystal diffraction studies. This material is
available free of charge via the Internet at http://pubs.acs.org.
(42) Variable-temperature NMR studies of TpRu(L)(L′)(NHPh) revealed
an inverse correlation for bond rotational barriers of Ru-Namido and
Namido-Cipso. See ref 17.
(43) Blue, E. D.; Davis, A.; Conner, D.; Gunnoe, T. B.; Boyle, P. D.; White,
P. S. J. Am. Chem. Soc. 2003, 125, 9435–9441.
IC800843B
6126 Inorganic Chemistry, Vol. 47, No. 14, 2008