Monomeric Chini-Type Triplatinum Clusters Featuring Dianionic and Radical-Anionic π*-Systems

Article abstract of DOI:10.1002/anie.201604903

Owing to their unique topologies and abilities to self-assemble into a variety of extended and aggregated structures, the binary platinum carbonyl clusters [Pt₃(CO)₆]_n^2?(“Chini clusters”) continue to draw significant interest. Herein, we report the isolation and structural characterization of the trinuclear electron-transfer series [Pt₃(μ-CO)₃(CNAr^Dipp2)₃]^n?(n=0, 1, 2), which represents a unique set of monomeric Pt₃clusters supported by π-acidic ligands. Spectroscopic, computational, and synthetic investigations demonstrate that the highest-occupied molecular orbitals of the mono- and dianionic clusters consist of a combined π*-framework of the CO and CNAr^Dipp2ligands, with negligible Pt character. Accordingly, this study provides precedent for an ensemble of carbonyl and isocyanide ligands to function in a redox non-innocent manner.

Full text of DOI:10.1002/anie.201604903

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International Edition: DOI: 10.1002/anie.201604903
German Edition: DOI: 10.1002/ange.201604903
Platinum Clusters
Monomeric Chini-Type Triplatinum Clusters Featuring Dianionic and
Radical-Anionic p*-Systems
Brandon R. Barnett, Arnold L. Rheingold, and Joshua S. Figueroa*
Dedicated to Professor Christopher C. Cummins on the occasion of his 50th birthday
2À
Abstract: Owing to their unique topologies and abilities to self-
While the oligomeric members of [Pt3(CO)6]n with n =
2–8 have all been isolated and structurally character-
ized,[7–11,15] the parent species [Pt3(CO)6]2À (n = 1) has
eluded complete characterization. Although reportedly syn-
thesized in situ via reduction of [Pt3(CO)6]22À with Na/K alloy,
it could not be crystallized or precipitated from solution, with
characterization relying solely upon IR and atomic absorption
spectroscopies. Interestingly, although several mixed carbon-
yl/phosphine clusters of the type Pt3(m-CO)3(PR3)3 have been
long-known,[16–20] their reduction to the corresponding plati-
nates has not been reported, potentially implying that the
weak p-accepting properties of triorganophosphines are
insufficient to stabilize the presence of two negative charge
equivalents.[21] Given our success in isolating isocyano ana-
logues to classical carbonyl metalates using sterically encum-
bering m-terphenyl isocyanides,[22–25] we report herein the
synthesis and structural characterization of K2[Pt3(m-
CO)3(CNArDipp2)3] (ArDipp2 = 2,6-(2,6-(i-Pr)2C6H3)2C6H3), as
well as the open-shell monoanion K[Pt3(m-CO)3(CNArDipp2)3].
These anionic clusters serve as isolobal mimics to [Pt3(CO)6]2À
as well as putative [Pt3(CO)6]CÀ, with the highest occupied
orbital shown to be primarily located on the ligand p*
framework. Occupation of this highly delocalized p-symmet-
ric orbital results in the aggregate set of isocyanide and
carbonyl ligands functioning in a redox non-innocent fashion
akin to multidentate redox active ligands.[26–28]
assemble into a variety of extended and aggregated structures,
the binary platinum carbonyl clusters [Pt3(CO)6]n (“Chini
2À
clusters”) continue to draw significant interest. Herein, we
report the isolation and structural characterization of the
trinuclear electron-transfer series [Pt3(m-CO)3(CNArDipp2)3]nÀ
(n = 0, 1, 2), which represents a unique set of monomeric Pt3
clusters supported by p-acidic ligands. Spectroscopic, compu-
tational, and synthetic investigations demonstrate that the
highest-occupied molecular orbitals of the mono- and dia-
nionic clusters consist of a combined p*-framework of the CO
and CNArDipp2 ligands, with negligible Pt character. Accord-
ingly, this study provides precedent for an ensemble of
carbonyl and isocyanide ligands to function in a redox non-
innocent manner.
H
ighly reduced carbonyl metalates have long interested
organometallic chemists owing to their ability to place
a relatively electropositive transition-metal center in a for-
mally negative oxidation state.[1,2] This phenomenon is made
possible in large part due to the strong p-acidic properties of
the carbonyl ligand and has allowed for the isolation of many
mono- and multi-nuclear binary carbonyl metalates from
most members of the transition series.[3–6] With regard to
topology and electronic structure, one of the most intriguing
classes of such complexes are the so-called “Chini clusters,”
[Pt3(CO)6]n
.
2À [6–8] In these clusters, triangulo-Pt3 units bearing
Exposure of a diethyl ether solution of two-coordinate
both terminal and bridging carbonyl ligands stack one on top
of the other along a common C3 axis to form columnar
structures bearing an overall doubly anionic charge. The
higher nuclearity oligomers (n = 5–8) can self-assemble into
continuous chains, with the dimensionality of long-range
ordering being highly dependent upon the identity of the
charge-balancing cations.[9–11] Many of the higher ordered
structures display remarkable conductive properties, leading
to interest in their use as tunable conductive materials based
off of a molecular platform.[12,13] As such, a furthered under-
standing of the electronic structure and reactivity profiles
available to these clusters continues to be of importance.[14]
Pt(CNArDipp2 [29,30] to 1 atm CO gas results in displacement of
)
2
one isocyanide ligand and aggregation to afford the trinuclear
cluster Pt3(m-CO)3(CNArDipp2
(1, Scheme 1). Despite the
)
3
presence of excess CO, further CO-for-isocyanide substitution
is not observed even upon stirring for several days. Crystallo-
graphic characterization of 1 (Figure 1A) reveals an equi-
lateral triangulo-Pt3 core with Pt–Pt distances (mean =
2.6496(2) Š) similar to those seen for Pt3(m-CO)3(PR3)3.[16–20]
Trinuclear 1 adopts nearly ideal D3h site symmetry, with the
À
À
Pt CO and Pt CNR bond vectors all essentially coincident
with the Pt3 plane (maximum torsion angle = 8.568). In
accordance with its high symmetry in the solid state, the
ꢀ
solution FTIR spectrum of 1 displays only a single n(C N)
À1
=
and n(C O) mode (2118 and 1834 cm , respectively; E’
symmetric in the D3h point group).
[*] B. R. Barnett, Prof. A. L. Rheingold, Prof. J. S. Figueroa
Department of Chemistry and Biochemistry
University of California, San Diego
Remarkably, cyclic voltammetry of 1 in THF (Figure S3.1
in the Supporting Information) shows the presence of two
reversible reduction events centered at À1.95 V (DEp =
86 mV) and À2.60 V (DEp = 81 mV) versus Fc (Fc = [(h5-
C5H5)2Fe]), suggesting the accessibility of the anionic clusters
[Pt3(m-CO)3(CNArDipp2)3]nÀ (n = 1, 2). Chemical reduction of
9500 Gilman Drive MC 0358, La Jolla, CA 92193 (USA)
E-mail: jsfig@ucsd.edu
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201604903.
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1 in benzene solution using an excess of
KC8 results in smooth formation of a deep
purple diamagnetic product as assayed by
1H and 13C{1H} NMR spectroscopy and
proposed
to
be
K2[Pt3(m-
CO)3(CNArDipp2)3] (K2[1]; Scheme 1).
Retention of the triangulo-Pt3(m-CO)3-
(CNR)3 motif is suggested by peaks at
d = 242.2 and d = 194.5 ppm in the 13C-
{1H} NMR spectrum corresponding to
bridging carbonyl and terminal isocya-
nide ligands, respectively. This new spe-
cies gives rise to a 195Pt NMR signal at d =
À4271 ppm, which is strikingly similar to
that of 1 (d = À4319 ppm) given the wide
chemical shift range inherent in 195Pt
NMR.[31] FTIR spectra in C6D6 show
À1
=
a single n(C O) band at 1722 cm , as
ꢀ
well as broad n(C N) stretching modes
centered at 2043 and 1985 cmÀ1. These
bands are shifted to significantly lower
energies relative to 1 and can be com-
pared with the bridging and terminal
n(CO) modes reported for in situ gener-
Scheme 1. Synthesis of clusters 1, K[1], and K2[1].
Figure 1. Molecular structures of 1 (A), [K2(DME)3][1] (B) and [K(THF)4][1] (C). Some iso-propyl groups have been removed for clarity.
D) Experimental (black) and simulated (gray-dashed) X-band powder EPR spectrum of K(THF)4[1] recorded at 294 K.
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ated [Pt3(CO)6]2À (1945, 1740 cmÀ1, THF).[8]
in K(THF)4[1] is essentially isostructural to that of neutral
1 (Table 1). The coordination geometry about each platinum
Structure determination on single crystals of K2[1] grown
from DME/n-hexane at À358C confirmed the identity of this
species to be the dianion K2(DME)3[Pt3(m-CO)3(CNArDipp2)3]
(K2(DME)3[1], Figure 1B). Each Pt3 unit exists as a discreet
entity and no oligomerization to higher nuclearity, stacked
structures is observed. Most notably, despite the introduction
of two negative charge equivalents, the structural features of
the [Pt3(m-CO)3(CNArDipp2)3] core in K2(DME)3[1] are essen-
À
À
center remains almost perfectly planar, while the Pt Pt, Pt
À
CCNR, and Pt CCO bond lengths are basically unchanged from
those in 1 and K2(DME)3[1] (Table 1). The room temperature
solid-state EPR spectrum of K(THF)4[1] displays axial
symmetry, with g? = 2.020 and gk = 1.974 (Figure 1D). The
small degree of g anisotropy and the minimal deviation of
these values from 2.002 (g of a free electron) differ from EPR
spectra typically seen for Pt-based S = 1/2 metalloradi-
cals.[33–35] However, they are very much in accord with spectra
exhibited by systems containing closed-shell platinum centers
bound to radical ligands.[36,37] Noticeably, no coupling to the
195Pt nucleus (I = 1/2, 33.8% abundant) is resolved, suggesting
that minimal spin density resides at the Pt3 core.
À
tially unchanged relative to 1. The Pt C bond vectors remain
nearly coincident with the Pt3 plane, with only very minor
À
À
À
perturbations of the Pt Pt, Pt CCNR, and Pt CCO distances
apparent (Table 1). The two potassium counterions sit
Table 1: Selected bond lengths from the solid-state structures of
complexes 1, K(THF)4[1], and K2(DME)3[1].
Density Functional Theory (DFT) studies were under-
taken in order to interrogate the electronic structure of the
clusters [Pt3(m-CO)3(CNArDipp2)3]nÀ (n = 0, 1, 2). The opti-
mized geometries of the truncated models [Pt3(m-CO)3-
(CNArPh2)3]nÀ ([1*]nÀ, n = 0, 1, 2) satisfactorily reproduce
the important metrical parameters of the corresponding
structurally characterized triplatinum clusters 1, K(THF)4[1]
and K2(DME)3[1] (Figures S4.1–3 and Tables S4.1–3).
Remarkably, the calculated HOMO of [1*]2À (Figure 2A)
Complex
Mean d(Pt-Pt)
Mean d(Pt-CCNR
)
Mean d(Pt-CCO)
[Š]
[Š]
[Š]
1
2.6496(2)
1.917(3)
1.924(6)
1.896(4)
2.062(2)
2.047(4)
2.044(3)
K(THF)4[1] 2.6448(3)
K2(DME)3[1] 2.6359(2)
directly over the centroid on either face of the Pt3 triangle
and are chelated by one or two molecules of DME.
In contrast to Chiniꢀs [Pt3(CO)6]2À,[8] K2[1] exhibits
remarkable kinetic stability. As a solid, it can be stored for
weeks in a glovebox freezer at À358C without change, while
[D6]benzene solutions of K2[1] at 258C undergo decomposi-
tion over the course of several weeks to give free CNArDipp2 as
1
the sole isocyanide-containing species according to H NMR
spectroscopy. This kinetic stability results in an exceedingly
simple synthetic method whereby analytical quality K2[1] can
be obtained in excellent yields (99%) via simple lyophiliza-
tion of the benzene solvent after filtration of the reaction
mixture.
While Chini clusters of many nuclearities have been
isolated, a common feature among all of them is an overall
charge of 2À. Their reactions with oxidizing agents have been
proposed to proceed via single-electron transfer to transiently
form [Pt3(CO)6]nCÀ, followed by dimerization and further
2À
aggregation to yield [Pt3(CO)6]2n stacked clusters.[32] How-
Figure 2. DFT Calculated HOMOs of [1*]2À (A), [Pt3(CO)6)]2À (B), and
[Pt3(m-CO)3(PMe3)3]2À (C). BP86/def2-TZVP/ZORA.
ever, such monoradical species have not been observed to
date during the course of these oxidations. The apparent
resistance of K2[Pt3(m-CO)3(CNArDipp2)3] (K2[1]) toward
aggregation, as well as the clean electrochemical data for 1,
suggested the potential accessibility of the open-shell mono-
anion [Pt3(m-CO)3(CNArDipp2)3]CÀ. Accordingly, simple com-
proportionation of 1 and K2[1] in [D8]THF (Scheme 1)
produces a dark green solution, which gives rise to only
and SOMO of [1*]À consist of the in-phase combination of the
out-of-plane p* orbitals of the carbonyl and isocyanide
ligands (a2’’ in D3h symmetry) with only a modest contribution
from Pt-based orbitals (20.5% for [1*]2À HOMO; 25.7% for
[1*]À SOMO).[38] The primarily ligand-based parentage of this
1
broad H NMR signals indicative of the presence of a para-
À
magnetic species (meff = 1.7(2) mB, Evans method). Analysis by
orbital is in accord with the minimal shortening of the Pt C
bond distances upon reduction of 1 (Table 1), despite the
À1
=
ꢀ
FTIR spectroscopy shows n(C O) (1775 cm ) and n(C N)
(2061, 2022 cmÀ1) bands intermediate in energy with respect
to those of 1 and K2[1]. Storage of this solution at À358C
yields black crystals of K(THF)4[Pt3(m-CO)3(CNArDipp2)3]
(K(THF)4[1]) as determined by X-ray diffraction (Fig-
ure 1C). As noted for K2(DME)3[1], the anionic fragment
=
ꢀ
sizeable shift of the IR n(C O) and n(C N) bands to lower
energies, which are usually attributable to increased metal-to-
ligand p-backdonation. It is also consistent with semi-
empirical extended Hückel calculations on [Pt3(CO)6]2À,
which predict a HOMO that is primarily CO p* in charac-
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ter.[39–41] Indeed, DFT calculations on [Pt3(CO)6]2À yield
a HOMO that is entirely analogous to that of [1*]2À, namely
the a2’’ combination of bridging and terminal CO out-of-plane
p* orbitals (Figure 2B). This confirms that K2[1] indeed
functions as an isolobal analogue of [Pt3(CO)6]2À, while K[1] is
only symmetry-adapted linear combination of Pt 5d orbitals
for the [Pt(m-CO)3L3] (L = CO, CNR) fragment with a2’’
À
symmetry (Figure S4.6–8) has significant Pt Pt bonding
character, which serves to significantly lower it in energy.
Importantly, the resulting energy gap between these d orbitals
and the a2’’-symmetric p* manifold is substantial (4.40–
4.54 eV), which precludes significant orbital mixing. Remark-
ably, owing to the primarily ligand-based nature of the a2’’
HOMO/SOMO found for [1*]2À and [1*]À, it can be noted
that population of this orbital in K2[1] and K[1] results in the
CO/CNArDipp2 ligand set acting in a formally redox non-
innocent fashion. Distinct from these ligands merely func-
tioning as p-acids, the successive reduction on going from 1 to
K(THF)4[1] and K2[1] yields clusters best described as
containing a (Pt3)0 core supported by a ligand set that, as an
aggregate, bears a singly- or doubly-reduced p*-manifold. In
essence, combining the out-of-plane p* orbitals of all six
ligands in-phase renders them accessible in energy, allowing
the a2’’ orbital to function as an electron reservoir reminiscent
of multidentate redox non-innocent ligand systems bearing
extended p-systems.[26]
isolobal with the monoanion [Pt3(CO)6]CÀ, a putative fleeting
2À
intermediate in the oxidation of [Pt3(CO)6]2À to [Pt3(CO)6]2
.
The all-p-acid ligand sets employed in K2[1] and [Pt3(CO)6]2À
can be contrasted with the hypothetical dianions of the
phosphine-substituted triplatinum clusters Pt3(m-CO)3-
(PR3)3.[16–20] DFT calculations for [Pt3(m-CO)3(PMe3)3]2À illus-
trate that the primarily s-donating nature of the trimethyl-
phosphine ligands precludes delocalization of electron den-
sity onto the terminal ligands in the cluster. For this complex,
the HOMO is again a2’’-symmetric, but consists primarily of
the out-of-plane p* orbitals of the bridging carbonyl ligands
(Figure 2C). The relative localization of electron density onto
the bridging ligands in [Pt3(m-CO)3(PMe3)3]2À, along with the
strongly s-donating nature of the phosphine ligands, likely
renders the reduction of Pt3(m-CO)3(PR3)3 to the correspond-
ing dianion more difficult than the reduction of 1 to K2[1], and
serves to highlight the central role played by the all-p-acid
ligand sets of K2[1] and [Pt3(CO)6]2À in stabilizing the negative
charge equivalents.
The robust and highly-reduced nature of K2[Pt3(m-
CO)3(CNArDipp2)3] (K2[1]) allows it to undergo well-defined
reactivity with main-group and transition-metal electrophiles
(Scheme 2). Addition of a benzene solution of Ph3SnCl or
Et3SiCl to K2[1] proceeds with elimination of KCl and
formation of the diamagnetic triplatinum clusters K[Pt3(m3-
SnPh3)(m-CO)3(CNArDipp2)3] (2, Figure 3A) or K[Pt3(SiEt3)-
(m-CO)3(CNArDipp2)3] (3, Figure 3B) as determined by X-ray
diffraction. In 2, the triphenylstannyl ligand is bound in a m3
fashion, a binding mode that is unprecedented for triorga-
Interestingly, the modest contribution of platinum-based
atomic orbitals to the a2’’-symmetric HOMO of [1*]2À is
mostly 6pz in parentage, while the molecular orbital contains
only 0.8% 5d character. Similarly small 5d contributions are
apparent in the SOMO of [1*]À (1.3% 5d) and the HOMO of
[Pt3(CO)6]2À (1.4% 5d), an effect we believe can be traced to
the intratriangular Pt Pt bonding interactions.[42] Indeed, the
À
Scheme 2. Reactivity of K2[1] with electrophiles.
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demonstrate that while eÀ equiva-
lents are housed in the ligand-based
p* manifold of K2[1], electrophilic
functionalization can proceed at
the platinum centers of the trian-
gulo-Pt3 core.
In conclusion, the kinetic stabi-
lization afforded by encumbering
m-terphenyl isocyanides, in concert
with their isolobal relationship to
CO, allows for the isolation of
a robust analogue to Chiniꢀs [Pt3-
(CO)6]2À dianion. Remarkably, the
corresponding monoanion is also
isolable and was shown to contain
one unpaired electron located pri-
marily on the aggregate ligand p*
system. Importantly, this study pro-
vides precedent for a set of isocya-
nide and carbonyl ligands to act in
a redox non-innocent fashion. We
are exploring additional reactivity
patterns of these Pt3 clusters that
harness this unique electronic struc-
ture feature.
Figure 3. Molecular structure of A) K[Pt3(m3-SnPh3)(m-CO)3(CNArDipp2)3] (2), B) K(Et2O)[Pt3(SiEt3)(m-
CO)3(CNArDipp2)3] (3), C) K(Et2O)2[Pt3(m3-AuPPh3)(m-CO)3(CNArDipp2)3] (4), and D) Pt3(m3-AuPPh3)2(m-
CO)3(CNArDipp2)3 (5). Some iPr groups have been removed for clarity.
Acknowledgements
We are grateful to the U.S. National
nostannyl ligands.[43] Contrastingly, the triethylsilyl ligand in 3
Science Foundation (CHE-1464978) for support of this work
and for a Graduate Research Fellowship to B.R.B. We thank
Dr. Mohand Melaimi for assistance with cyclic voltammetry
measurements, as well as Prof. Michael J. Tauber and Douglas
W. Agnew for assistance with EPR measurements. The W. M.
Keck Foundation is also gratefully acknowledged for use of
computing resources at the W. M. Keck Laboratory for
Integrated Biology II. J.S.F is a Camille Dreyfus Teacher-
Scholar (2012-2017).
À
is terminally bound to a single Pt center, with the Pt Si bond
vector being nearly orthogonal to the Pt3 plane. The
isocyanide bound to the silyl-ligated platinum center bends
significantly out of the plane toward the opposite face of the
cluster. However, despite the asymmetry evident in the solid-
state structure of 3, only a single sharp set of -ArDipp2
1
resonances are detected by H NMR spectroscopy at 208C,
indicating that exchange of the silyl ligand between the three
Pt centers is fast on the NMR timescale. Dianionic K2[1] also
reacts with one or two equivalents of AuCl(PPh3), providing
dark blue K(Et2O)2[Pt3(m3-AuPPh3)(m-CO)3(CNArDipp2)3] (4,
Keywords: cluster compounds · isocyanides ·
non-innocent ligands · platinum
Figure 3C)
or
forest
green
Pt3(m3-AuPPh3)2(m-
CO)3(CNArDipp2
)
3
(5, Figure 3D). In both clusters, the
How to cite: Angew. Chem. Int. Ed. 2016, 55, 9253–9258
Angew. Chem. 2016, 128, 9399–9404
-AuPPh3 fragments symmetrically bridge the three Pt centers
affording a Pt3Au tetrahedron (4) or a Pt3Au2 trigonal
bipyramid (5). While reminiscent of the formation of
[Pt3(m3-AuPCy3)(m-CO)3(PCy3)3]+, which is produced by cap-
ping the corresponding neutral Pt3 cluster with a [AuPCy3]+
fragment,[44] it should be noted that the two additional valence
electrons possessed by K2[1] compared to Pt3(m-CO)3(PCy3)3
allows for the isolation of an anionic Pt/Au cluster, of which
only one other structurally characterized example is
known.[45] These extra electrons also allow for the formation
of 5 via addition of a second equivalent of [AuPPh3]+,
a transformation which is not accessible to [Pt3(m3-AuPCy3)-
(m-CO)3(PCy3)3]+[46] and has not been reported for any
phosphine-substituted Pt3(m-CO)3(PR3)3 cluster. Accordingly,
these reactions leading to the formation of complexes 2–5
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Received: May 19, 2016
Published online: June 27, 2016
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