Four-coordinate, trigonal pyramidal Pt(II) and Pd(II) complexes

Article abstract of DOI:10.1021/ja105284p

We report herein the characterization of electrophilic, trigonal bipyramidal {[SiP₃^R]Pt(L)}⁺ cations ([SiP ₃^R] = [(2-R₂PC₆H₄) ₃Si]; R = Ph, ⁱPr) that feature weakly coordinated ligands including CH₂Cl₂, Et₂O, toluene, and H ₂. A cationic toluene adduct that shows a close platinum aryl C-H σ-contact is perhaps most noteworthy in this context. For the isopropyl-substituted ligand, [SiP₃^iPr], it has proven possible to exclude the fifth axial donor to afford the rigorously four-coordinate, trigonal pyramidal (TP) complex {[SiP₃ ^iPr]Pt}⁺. An isostructural TP palladium complex {[SiP ₃^iPr]Pd}⁺ is also accessible. Prototypical four-coordinate d⁸ platinum and palladium complexes are square planar. The TP d⁸ cations described herein are hence geometrically distinct.

Full text of DOI:10.1021/ja105284p

Published on Web 09/21/2010
Four-Coordinate, Trigonal Pyramidal Pt(II) and Pd(II) Complexes
Charlene Tsay, Neal P. Mankad, and Jonas C. Peters*
California Institute of Technology, DiVision of Chemistry and Chemical Engineering, Pasadena, California 91125,
and Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
Received June 16, 2010; E-mail: jpeters@caltech.edu
Scheme 1
Abstract: We report herein the characterization of electrophilic,
trigonal bipyramidal {[SiP3R]Pt(L)}+ cations ([SiP3R] ) [(2-
R2PC6H4)3Si]; R ) Ph, iPr) that feature weakly coordinated ligands
including CH2Cl2, Et2O, toluene, and H2. A cationic toluene adduct
that shows a close platinum aryl C-H σ-contact is perhaps most
noteworthy in this context. For the isopropyl-substituted ligand,
[SiP3iPr], it has proven possible to exclude the fifth axial donor to
afford the rigorously four-coordinate, trigonal pyramidal (TP)
complex {[SiP3iPr]Pt}+. An isostructural TP palladium complex
{[SiP3iPr]Pd}+ is also accessible. Prototypical four-coordinate d8
platinum and palladium complexes are square planar. The TP d8
cations described herein are hence geometrically distinct.
C-H σ-complexes have mostly been detected transiently or
indirectly, with a notable exception being a Rh(I) σ-methane
complex characterized by NMR at low temperatures.8,9 The para
hydrogen of 6 was located in the difference map at 1.02 Å from
Electrophilic platinum(II) compounds have been studied exten-
the para carbon atom, in contrast with three of the other aromatic
sively due to their ability to activate C-H bonds such as those in
hydrogens, which were located at an average of 0.96 Å from their
methane and other cheap hydrocarbons.1,2 As these species are
respective carbon atoms.10 The para hydrogen was refined freely
almost always square planar, it is of interest to consider how other
to a distance of 2.672 Å from the Pt center while the para carbon
geometries might impact the electrophilicity and reactivity of Pt(II)
is 2.766 Å from Pt; the Pt-Hpara-Cpara angle is 84.0°. These values
centers. Using the tris(phosphino)silyl scaffold [SiP3 ]- ([SiP3 ]-
R
R
fall slightly outside the normal range for intramolecular agostic
interactions.11 The closest contacts between the toluene molecule
and the ligand phenyl rings range from 2.301 to 3.336 Å, suggesting
that host-guest interactions may play a role in stabilizing the
observed toluene adduct structure, as has been proposed for certain
structurally characterized metal-alkane adducts.12 Solvents in
which 6 is soluble, for example CD2Cl2 and CDCl2F, appear to
3
) [(2-R2PC6H4)3Si]-; R ) Ph, Pr) we sought to access trigonal
bipyramidal (TBP) platinum(II) cations featuring weakly coordi-
nated ligands in the axial site. We report herein the characterization
i
of TBP [SiPR ]Pt-L cations, including a toluene adduct that presents
3
a close aryl C-H contact to the platinum center. We moreover
establish that it is possible to completely exclude the weak axial
donor to afford a rigorously four-coordinate, trigonal pyramidal (TP)
Pt(II) species. An isostructural complex is accessible for Pd(II).
These species expand the small number of isolable d8 complexes
that populate a TP geometry.4,5
The neutral Pt(II) complexes [SiP3Ph]Pt-CH3 (1), [SiP3Ph]Pt-Cl (2),
and [SiP3Ph]Pt-H (3) are readily accessible by Si-H activation of the
phenyl-substituted ligand [SiP3Ph]H with (COD)PtMe2, (COD)PtCl2,
and Pt(PPh3)4, respectively. These yellow complexes exhibit 3-fold
symmetry in solution at room temperature and have TBP geometries
in the solid state with an average τ5 ) 0.88 (see Supporting Information
for all crystallographic details).6 Protonation of 1 in CH2Cl2 at room
temperature with H(OEt2)2BArF4 (BArF4 ) B[3,5-(CF3)2C6H3]4) pro-
vides access to a series of cationic solvento adducts (Scheme 1). The
crystallographically characterized diethyl ether {[SiP3Ph]Pt(OEt2)}-
{BArF } (4) and dichloromethane {[SiP3Ph]Pt(Cl2CH2)}{BArF } (5)
4
4
adducts are yellow-orange while the toluene adduct {[SiP3Ph]Pt-
(toluene)}{BArF } (6) is red-brown, consistent with d-d transitions
4
2
2
2
from a dxy/dx -y HOMO to a dz LUMO.
The solid-state structure of 6 (Figure 1) suggests possible
coordination of the toluene to the cationic Pt center through the
para aryl C-H bond rather than the π-cloud of the aryl ring. Such
an interaction is of particular interest for an electrophilic platinum
species as studies have suggested that aryl C-H bond activations
proceed via an initial aryl C-H σ-complex that isomerizes to more
stable, and in certain cases observable, π-arene adduct species.7
Figure 1. Solid-state structures of (A) 5, (B) 6, (C) 10, and (D) 16. Thermal
ellipsoids drawn at 50% probability. Hydrogen atoms and BArF anions
4
omitted for clarity. Ligand phenyl rings abbreviated in A and B.
9
10.1021/ja105284p  2010 American Chemical Society
J. AM. CHEM. SOC. 2010, 132, 13975–13977 13975

C O M M U N I C A T I O N S
Scheme 2
substitute the toluene at temperatures as low as -100 °C, frustrating
our attempts to characterize the Pt-toluene interaction in 6 by
solution NMR spectroscopy.
Figure 2. DFT-calculated (A) HOMO and (B) LUMO of 10.
The yellow Pt(II) complexes, [SiP3iPr]Pt-CH3 (7), [SiP3iPr]Pt-Cl
(8), and [SiP3iPr]Pt-H (9), can be synthesized in a manner analogous
to that of the phenyl ligand system. The hydride 9 has been
structurally characterized, and the hydride position was found in
the difference map and refined freely, providing a τ5 value of 0.94.
sor and exhibited an electronic spectrum similar to that of 10.15
While the platinaboratrane complex was formulated as Pt(0), its
close electronic relationship to 10 is evident and the two systems
are isoelectronic.16 Minimization of cationic 10 by DFT methods
affords a structure very similar to that observed in the solid state.
The HOMO and LUMO orbitals obtained from single-point
Protonation of 7 with H(OEt2)2BArF in dichloromethane solution
4
results in an immediate color change to purple (λmax ) 494 nm, ε
) 964 cm-1 M-1), in contrast to the yellow-orange color of solvento
calculations of 10 plotted in Figure 2A,B are consistent with the
8
1
2
d formulation. A high-lying LUMO of dz parentage with a lobe
adducts such as 5. While the H NMR spectrum retains the same
general characteristics as those of the other [SiP3iPr]Pt-X com-
pounds, the 31P NMR resonance is dramatically shifted downfield
to 102.1 ppm, in contrast to ca. 22 to 46 ppm for the five-coordinate
protruding into the empty axial site opposite the silyl anchor is
well separated from an in-plane HOMO of dxy parentage. The
electrophilicity of 10, and the corresponding {[SiP3Ph]Pt}+ system
which scavenges very weak ligands, lends further credence to the
d8 formulation.
[SiPR ]Pt-X complexes described above. The 195Pt NMR resonance
3
was located indirectly by a 195Pt/1H 2D HMBC experiment at -2818
ppm. For comparison, the 195Pt NMR resonances of TBP 7, 8, and
9 are at -6324, -4439, and -6039 ppm, respectively.
We have begun to canvass related palladium systems for
comparison. In brief, the yellow complexes [SiP3iPr]Pd-CH3 (14)
and [SiP3iPr]Pd-Cl (15) have been synthesized and spectroscopically
The solid-state structure of a single crystal grown from dichlo-
F
characterized. Protonation of 14 with H(OEt2)2BAr4 in dichlo-
romethane solution reveals the four-coordinate, trigonal pyramidal
complex {[SiP3iPr]Pt}{BArF } (10) (Figure 1). The structure of 10
romethane solution immediately affords a deep red solution (λmax
) 463 nm) with a corresponding 31P NMR resonance at 53 ppm.
An X-ray diffraction experiment on a single crystal of the product
grown from dichloromethane confirms the identity of this spe-
cies as a four-coordinate trigonal pyramidal Pd(II) species,
4
j
has been refined in P1 to an R1 of 0.0358 and a wR2 of 0.0767,
and the only disordered positions concern the -CF3 groups on the
BArF anion. The closest approach to the platinum center, other
4
than the Si and three P-atoms of the immediate coordination sphere,
is an isopropyl methyl hydrogen that is at 3.344 Å, larger than the
sum of their van der Waals radii and well outside the range of a
plausible agostic interaction. These data are consistent with the
{[SiP3iPr]Pd}{BArF } (16) (Figure 1).17,18 Reactivity studies of 10
4
and 16, and of related Ni(II) complexes, are presently underway.
Acknowledgment. This work was generously supported by the
NSF (CHE-0750234). We are grateful to Paul F. Oblad for
conducting preliminary synthetic studies and to Dr. Peter Mu¨ller
and Dr. David VanderVelde for assistance with XRD and NMR
studies, respectively.
1
solution H NMR spectrum, which exhibits a single methine and
two methyl resonances at RT. The platinum center deviates
minimally from the plane of the phosphines, as the P-Pt-P angles
sum to 358°. The τ4 value of 10 is 0.855, where τ4 is 0.851 for an
ideal trigonal pyramid.13
The bulky nature of the isopropyl-substituted [SiPR ] ligand
3
Supporting Information Available: Experimental, spectroscopic,
X-ray, and computational details. This material is available free of
charge via the Internet at http://pubs.acs.org.
precludes cationic 10 from binding solvents such as benzene,
dichloromethane, diethyl ether, and THF. Indeed, XRD analysis
of crystals grown from a THF solution of 10 afforded the same
solid-state structure as those grown from CH2Cl2. The electrophilic
character of 10 is manifest in its reactivity toward H2 and a series
of N-donor ligands. Exposing 10 to an atmosphere of dihydrogen
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