N-heterocyclic phosphenium ligands as sterically and electronically-tunable isolobal analogues of nitrosyls

Article abstract of DOI:10.1021/ic202581v

The coordination chemistry of an N-heterocyclic phosphenium (NHP)-containing bis(phosphine) pincer ligand has been explored with Pt ⁰ and Pd⁰ precursors. Unlike previous compounds featuring monodentate NHP ligands, the resulting NHP Pt and Pd complexes feature pyramidal geometries about the central phosphorus atom, indicative of a stereochemically active lone pair. Structural, spectroscopic, and computational data suggest that the unusual pyramidal NHP geometry results from two-electron reduction of the phosphenium ligand to generate transition metal complexes in which the Pt or Pd centers have been formally oxidized by two electrons. Interconversion between planar and pyramidal NHP geometries can be affected by either coordination/dissociation of a two-electron donor ligand or two-electron redox processes, strongly supporting an isolobal analogy with the linear (NO ⁺) and bent (NO^-) variations of nitrosyl ligands. In contrast to nitrosyls, however, these new main group noninnocent ligands are sterically and electronically tunable and are amenable to incorporation into chelating ligands, perhaps representing a new strategy for promoting redox transformations at transition metal complexes.

Full text of DOI:10.1021/ic202581v

Article
pubs.acs.org/IC
N-Heterocyclic Phosphenium Ligands as Sterically and Electronically-
Tunable Isolobal Analogues of Nitrosyls
Baofei Pan, Zhequan Xu, Mark W. Bezpalko, Bruce M. Foxman, and Christine M. Thomas*
Department of Chemistry, Brandeis University, 415 South Street MS 015, Waltham, Massachusetts 02454, United States
S
* Supporting Information
ABSTRACT: The coordination chemistry of an N-heterocyclic
phosphenium (NHP)-containing bis(phosphine) pincer ligand has
been explored with Pt0 and Pd0 precursors. Unlike previous
compounds featuring monodentate NHP ligands, the resulting
NHP Pt and Pd complexes feature pyramidal geometries about the
central phosphorus atom, indicative of a stereochemically active
lone pair. Structural, spectroscopic, and computational data suggest
that the unusual pyramidal NHP geometry results from two-
electron reduction of the phosphenium ligand to generate
transition metal complexes in which the Pt or Pd centers have
been formally oxidized by two electrons. Interconversion between
planar and pyramidal NHP geometries can be affected by either coordination/dissociation of a two-electron donor ligand or two-
electron redox processes, strongly supporting an isolobal analogy with the linear (NO+) and bent (NO−) variations of nitrosyl
ligands. In contrast to nitrosyls, however, these new main group noninnocent ligands are sterically and electronically tunable and
are amenable to incorporation into chelating ligands, perhaps representing a new strategy for promoting redox transformations at
transition metal complexes.
Chart 1. Analogy between NHPs and Nitrosyls in
Coordination Chemistry
INTRODUCTION
■
Since the isolation of the first stable N-heterocyclic carbene
(NHC) by Arduengo and co-workers in 1991,1 NHCs have
been widely studied in the field of transition metal chemistry2−4
and catalysis.5,6 As the “carbon copies” of NHCs, far less
attention has been paid to their isovalent group 15 analogues,
N-heterocyclic phosphenium cations (NHP+s).7−9 In contrast
to NHCs, which are considered to be strong σ-donors and
weak-π acceptors, theoretical investigations of the electronic
properties of NHP cations have shown that their bonding is
dominated by π-acceptor character, with only weak σ-donation
to the transition metal.10−12 Owing to the electronically inverse
properties of these two ligand families, NHPs are expected to
exhibit reciprocal reactivity in transition metal chemistry.
One of the most interesting aspects of NHP+ cations is their
analogy to NO+ in coordination chemistry, not only because of
both ligands’ cationic charge but also because of their similar
ability to adopt two different transition metal binding modes
(Chart 1). Most commonly, when binding to electron poor
metal fragments, NHPs adopt a planar geometry at the
phosphorus atom, acting as both σ donors and π acceptors and
resulting in a metal−phosphorus double bond (e.g., Cp-
(CO)2Mo(NHPMe)).13 On the other hand, when binding to
electron rich metal fragments, NHPs adopt a pyramidal
geometry, which suggests that there is a nonbonding lone
pair on the central phosphorus atom (e.g., Cp*(CO)2Fe-
(NHPMe)).14 This leads to two limiting descriptions: (1) the
electrophilic phosphenium is acting exclusively as a two-
electron acceptor ligand to the metal (NHP+/Mn), or (2) the
NHP has formally oxidized the metal center and is acting as an
X-type phosphido-ligand (NHP−/Mn+2). In this sense, a
convincing analogy between NHPs and nitrosyls can be
Received: November 30, 2011
Published: March 14, 2012
© 2012 American Chemical Society
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established, highlighting the potential noninnocent behavior of
these ligands. Key advantages of NHPs over their nitrosyl
counterparts are the ability to tune steric and electronic
properties via derivatization and the ability to incorporate these
ligands into chelating frameworks. These strategies may also
impart stability to metal NHP complexes and protect the
phosphenium unit from nucleophilic attack.
Scheme 1
The ability of NHPs to adopt a different binding mode with
vastly different donor/acceptor properties may enhance the
redox activity of transition metal NHP complexes. Given the
analogy to nitrosyl ligands, which can undergo a two electron
redox transformation (NO+ + 2e−→ NO−),15 such redox
interconversion is also anticipated to be applicable for NHPs.
There are examples of transition metal complexes containing
both planar and pyramidal NHP ligands in the literature;13,14
however, to the best of our knowledge, there is no report of
interconversion between these two species via two electron
redox chemistry. Gudat and co-workers proposed a pyramidal-
to-planar conformational change via the loss of a CO ligand
((NHP)Co(CO)4 → (NHP)Co(CO)3), but the pyramidal
geometry of the NHP in (NHP)Co(CO)4 was not structurally
confirmed.16 Incorporation of NHPs into chelating ligands
should prove advantageous in stabilizing transition metal
species with different NHP binding modes; however, in
contrast to a rapidly growing number of NHC-containing
chelating ligands in transition metal chemistry,17,18 multi-
dentate ligands featuring NHPs are noticeably absent from the
literature.
Recently our group reported the synthesis and character-
ization of the first example of an NHP-containing pincer ligand
in which the central NHP unit is linked to two phosphine side
arms via aryl linkers.19 The coordination of this pincer-NHP
ligand to transition metal halide starting materials proved
problematic as a result of the tendency of halides to migrate to
the electrophilic NHP center to form halophosphine
complexes.19 A successful route to NHP-metal complexes was
realized via sodium halide extrusion from the anionic metal
complex Na[Co(CO)4] and a chlorophosphine precursor to
generate [PPP]Co(CO)2 (PPP = NHP-diphosphine ligand).20
Attempts to promote a pyramidal-to-planar interconversion of
the NHP via CO extrusion from [PPP]Co(CO)2 was
unsuccessful.20 Herein, we turn our attention to treatment of
the [PPP]+ ligand with halide-free Pd0 and Pt0 starting
materials, uncovering more support for the analogy with
nitrosyl ligands, namely, ligand conformational changes
promoted by redox changes or coordination of additional
two-electron donor ligands.
Figure 1. Displacement ellipsoid (50%) representation of 1. For clarity
the PF6 counterions and all hydrogen atoms have been omitted.
−
Relevant interatomic distances (Å) and angles (deg): 1: Pd1−Pd1′,
2.7500(5); Pd1−P102, 2.1616(15); Pd1−P102′, 2.4982(16); Pd1−P2,
2.3547(8); Pd1−P4′, 2.3659(8); P2−Pd1−P4′, 122.24(3); P2−Pd1−
P102, 84.54(4); P102−Pd1−P102′, 104.85(5); P102′−Pd1-P4,
84.66(4); P2−Pd1−P102′, 123.31(5); P4−Pd1−P102, 140.84(4).
structurally characterized Pd-NHP complexes reported by
Jones and co-workers (2.1227(10) Å, 2.1167(25) Å).21 In
complex 1, each NHP moiety acts as an NHP+ phosphenium
ligand toward one Pd center, with a nearly planar angle
(164.5°) between the N−P−N plane and the short P(NHP)−Pd
bond vector. The other Pd center weakly donates electron
density into the empty p orbital on the same central
phosphorus, resulting in an elongated bonding interaction
along with a bent angle (122.5°) between the N−P−N plane
and the long P(NHP)−Pd bond vector (Figure 2). These
geometric features can be best attributed to a semibridging
NHP arrangement, analogous to a semibridging carbonyl
ligand. The tetrahedral geometry of both Pd centers is in
agreement with the NHP+ phosphenium Pd0 assignment for 1.
RESULTS AND DISCUSSION
■
Phosphenium Ligand Addition to M0 Starting
Materials. Treatment of the ligand [PPP]+ with Pd(PPh3)4
at room temperature for 12 h generates the Pd dimer
[(PPP)Pd]2[PF6]2 (1, Scheme 1). The 31P NMR spectrum of
1 features a downfield signal at 288.3 ppm and an upfield signal
at 14.1 ppm in a 1:2 integral ratio, which is consistent with a
coordinated central NHP unit and two coordinated aryl
phosphine arms. The solid state structure of 1 reveals an
asymmetric geometry. As shown in Figure 1, each Pd center has
a short bond to one of the NHP phosphorus atoms (Pd−P102:
2.1616(15) Å) and an elongated contact to the other NHP
phosphorus atom (Pd−P102′: 2.4982(16) Å). The short
P(NHP)−Pd bond is consistent with double bond character
and comparable to the P(NHP)−Pd distances in the two
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−
Figure 2. Displacement ellipsoid (50%) representation of 2, 3, and 4. For clarity, all hydrogen atoms, PF6 counteranions, and solvate molecules
have been omitted. Relevant interatomic distances (Å) and angles (deg): 2: Pd−P2, 2.2535(6); Pd−P1, 2.3099(5); Pd−P3, 2.3384(5); Pd−P4,
2.4148(5); P1−Pd−P2, 86.128(23); P2−Pd−P3, 83.307(23); P3−Pd−P4, 97.750(22); P1−Pd−P4, 98.041(22); P1−Pd−P3, 161.993(19); P2−
Pd−P4, 154.619(20). 3: P2−Pt, 2.2600(7); P1−Pt, 2.3137(7); P3−Pt, 2.3190(6); P4−Pt, 2.3787(6); P1−Pt−P2, 86.081(23); P2−Pt−P3,
81.367(23); P3−Pt−P4, 102.287(21); P1−Pt−P4, 103.973(21); P1−Pt−P3, 147.173(22); P2−Pt−P4, 149.086(22). 4: P2−Pt, 2.2606(9); P1−Pt,
2.3025(10); P3−Pt, 2.3123(11); P4−Pt, 2.367(1); P1−Pt−P2, 87.418(51); P2−Pt−P3, 84.453(50); P3−Pt−P4, 96.573(44); P1−Pt−P4,
96.912(44); P1−Pt−P3, 163.388(37); P2−Pt−P4, 154.962(33).
The Pd dimer 1 can be broken into the monomer complex
[(PPP)Pd(PMe3)][PF6] (2) by treatment with PMe3 (Scheme
1). In comparison to 1, the 31P NMR shift of the central
phosphorus atom in 2 is shifted more upfield (235.6 ppm).
Structural characterization of monomer 2 reveals that the
central phosphorus adopts a pyramidal geometry with a bent
angle of 130.9° between the N−P−N plane and the P(NHP)−Pd
bond vector. In addition, the P(NHP)−Pd bond distance is
considerably longer (2.2535(6) Å) than the short P(NHP)−Pd
double bond distance in 1 (2.16316(15) Å), indicative of a Pd−
P single bond in 2. On the basis of literature precedent, the
pyramidal NHP unit in 2 can be described as either an NHP+
phosphenium ligand accepting electron density from a Pd0
center or an NHP− phosphido ligand covalently bound to a PdII
center.14,16,20 Given the nearly square planar geometry of the
Pd center in 2, the latter phosphide description appears to be
more reasonable. Moreover, the Pd−P(NHP) distance in 2 is
essentially identical to the central Pd−P distance in a recently
reported phosphido-diphosphine pincer ligand complex of Pd
(2.2533(9) Å).22 While the PdII center is not rigorously square
planar, this is likely the result of steric factors, and
computational results are consistent with the phosphide
description (vide infra). Thus, NHP+ phosphenium (1) and
NHP− phosphido (2) transition metal binding modes can be
interconverted simply by dissociation or association of an
additional two-electron donor.
The solid state structure of 3 is shown in Figure 2, and, as
expected from the 31P NMR spectrum, the geometry at the
central phosphorus is unequivocally pyramidal, with a bent
angle of 126.2° between the N−P−N plane and the P(NHP)-Pt
bond vector. The P(NHP)-Pt bond distance in 3 (2.2600(7) Å) is
significantly longer compared to reported NHP-Pt0 complexes
(2.1073(9) Å,23 2.116(3) Å24), and in line with the phosphide
PtII−P distances in the diphosphino-phosphide pincer ligand
complexes reported by Mazzeo et al. (2.3091(14) Å,
2.2573(11) Å).25 Similar to 2, the Pt metal center in 3 adopts
a distorted square planar geometry. Thus, the NHP unit in 3 is
best described as an NHP− phosphido ligand, and the Pt center
is best described as divalent PtII. The distorted geometry about
Pt is attributed, like the aforementioned Pd case, to steric
factors. Indeed, there are no examples of square planar PPh3-
coordinated complexes with aryl linked bis(diarylphosphine)
pincer ligands in the literature for comparison. The coordinated
PPh3 in 3 can be replaced by PMe3 to generate [(PPP)Pt-
(PMe3)][PF6] (4), which is isostructural to 2. Because of the
smaller cone angle of PMe3 in comparison to PPh3, the Pt
geometry in 4 is less distorted from ideal square planar than in
3 (Figure 2).
Addition of P-X bonds to M0 Starting Materials. The
oxidative addition of C-X (X = halide) bonds to Pd0 precursors
has proven to be a useful method for the synthesis of NHC-PdII
complexes.26,27 PdII phosphide fragments can also be generated
via oxidative addition of P−Cl bonds to Pd0.28 Jones and co-
workers reported the addition of a bromophosphine NHP
precursor to a Pd0 center to generate the first example of a
structurally characterized Pd0 halide via heterolytic (non-
oxidative) addition of the P(NHP)-Br bond to Pd0.21 Similarly,
our ligand precursors [PPP]-X also react with M0 (Pd/Pt)
starting materials to afford neutral four-coordinate complexes,
(PPP)Pd-X (X = Cl (5), Br (6), I (7)) and (PPP)Pt-X (X = Cl
(8), Br (9), I (10)), via the addition of the P(NHP)−X bond
across the metal center, as shown in Scheme 2. The ease of
P(NHP)−X bond cleavage is consistent with Gudat’s description
of increasing ionic Lewis acid−base pair character of the
P(NHP)−X bond in the order I− > Br− > Cl−.9 The 31P NMR
spectrum for each of the [PPP]-M-X complexes features one
downfield central phosphorus resonance (246−250 ppm for 5−
7, 219−225 ppm for 8−10) and one upfield peak for the
phosphine side arms (6.1−7.5 ppm for 5−7, 9.4−12.1 ppm for
In contrast to the Pd case, treatment of the [PPP]+ ligand
with Pt(PPh3)4 does not lead to the formation of a Pt dimer
similar to 1. Instead, the reaction cleanly generates the four-
coordinate monomer complex [(PPP)Pt(PPh3)][PF6] (3,
Scheme 1). Here, the 31P NMR spectrum features a downfield
signal at 198.8 ppm and two upfield shifts at 29.1 and 13.6 ppm,
which are consistent with a coordinated NHP unit coupled to
two Pt-bound aryl phosphine arms and one ligated
triphenylphosphine ligand, respectively. Interestingly, the
central P(NHP)-Pt coupling constant (1JPt−P = 445 Hz) is
substantially smaller compared to all five reported NHP-Pt0
complexes in the literature (ranging from 6162 to 7354
Hz),23,24 but within the range of bis(phosphine)phosphido-PtII
complexes reported by Mazzeo and co-workers (648 to 2790
Hz).25 This suggests that the central NHP in 3 is adopting a
pyramidal geometry and acting as an NHP− phosphido ligand.
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PtII. This assignment is consistent with the relatively long
P(NHP)−M distances (2.2424(13) Å (5); 2.2446(11) Å (9)),
similar to those in 3 and 4.
Scheme 2
Treatment of the (PPP)Pd-X complexes with TlPF6 at room
temperature generates the halide-abstraction products, dimers
[(PPP)Pd]2[PF6]2 (1) and [(PPP)Pt]2[PF6]2 (11), as shown in
Scheme 2. Complex 11 is isostructural to complex 1 and, thus,
the NHP units in complex 11 feature bridging NHP+
phosphenium character in agreement with the NHP+/M0
assignment (see Supporting Information). In analogy to the
aforementioned reactivity of 1, Pt dimer 11 reacts with PMe3 to
cleanly generate the NHP− phosphido PtII complex [(PPP)Pt-
(PMe3)][PF6]2 (4). Pd monomer [(PPP)Pd(PMe3)][PF6]2
(2) can also be generated via addition of TlPF6 to 5−7 in
the presence of PMe3.
Platinum complexes 8−10 are very unstable, especially in
moderately polar solvents such as tetrahydrofuran (THF) or
CH2Cl2. THF solutions of 8−10 decompose slowly at room
temperature to generate unusual trinuclear Pt complexes 12−
14 (Scheme 2). In contrast to 8−10, 31P NMR spectra of 12−
14 feature a more upfield shift for the central NHP phosphorus
atom (152.9 (12), 154.9 (13), 162.9 (14) ppm). In addition,
hydride signals with coupling to both 31P and 195Pt can be
8−10) in an integral ratio of 1:2. All three P(NHP)-Pt coupling
constants observed for complexes 8−10 are notably small (486,
607, 663 Hz) in contrast to reported NHP+ phosphenium Pt
complexes,23,24 but consistent with those in complex 3 (445
Hz) and 4 (496 Hz), suggesting a pyramidal NHP geometry
with NHP− phosphide character in complexes 5−10.
As predicted, the solid state structures of 5 and 9 reveal
pyramidal geometries with respect to the central phosphorus
atom (angles between the N−P−N plane and P(NHP)−Pd bond
vector: 119.1° (5); 120.8° (9)) and the metal centers are
rigorously square planar rather than the tetrahedral geometry
that would be expected for d10-M0 complexes (Figure 3). Thus,
the P(NHP)−X addition reactions shown in Scheme 2 are best
described as formal oxidative addition processes, and the NHPs
here are best described as NHP− phosphido ligands bound to
1
detected in H NMR spectra of 12−14 (−17.1(12), −16.2
(13), −13.3 (14) ppm). Consistent with the NMR spectral
data, the solid state structures of 13 and 14 reveal that in these
decomposition products two (PPP)Pt-X units are bridged
symmetrically through a Pt(H)(X) fragment (Figure 3,
Supporting Information). In contrast to complexes 1 and 11,
the NHPs in 13 and 14 are bridging NHP− phosphide ligands
acting as X-type donors to the flanking Pt centers and L-type
donors to the central Pt center. Each of the three Pt centers is
rigorously square planar, agreeing with this assignment of three
PtII centers. The mechanism of decomposition to form 12−14
is unknown, but the products of prolonged exposure of 8−10
in deuterated dichloromethane do not exhibit hydride
1
resonances by H NMR, suggesting that solvent is origin of
the hydrides in 12−14.
Figure 3. Displacement ellipsoid (50%) representation of 5, 9, and 13. For clarity, all hydrogen atoms and solvate molecules have been omitted. In
the case of 13, aryl rings have also been omitted to reveal the core structure. The Pt-hydride ligand in 13 was not located crystallographically, and its
position, therefore, is geometrically inferred. Relevant interatomic distances (Å) and angles (deg): 5: P2−Pd, 2.2424(13); P1−Pd, 2.3151(11); P3−
Pd, 2.3098(11); Cl1−Pd, 2.4530(11); P1−Pd−P2, 90.528(48); P2−Pd−P3, 81.563(48); P1−Pd−Cl1, 94.035(42); P3−Pd−Cl1, 93.020(42); P1−
Pd−P3, 170.959(38); P3−Pd−Cl1, 169.080(51). 9: P2−Pt, 2.2446(11); P1−Pt, 2.2901(19); P3−Pt, 2.2931(19); Br1−Pt, 2.5430(3); N1−Pt−N2,
89.055(168); P1−Pt−P2, 83.197(45); P2−Pt−P3, 90.224(45); P1−Pt−Br1, 93.856(31); P3−Pt−Br1, 93.434(30); P1−Pt−P3, 172.561(50); P2−
Pt−Br1, 163.726(28). 13: P2−Pt1, 2.2111(11); P2−Pt2, 2.2756(11); P1−Pt1, 2.2903(13); P3−Pt1, 2.2906(11), Br1−Pt1, 2.5341(4); Br2−Pt2,
2.5432(7); P1−Pt1−P2, 88.822(44); P2−Pt1−P3, 88.774(41); P1−Pt1−Br1, 94.200(32); P3−Pt1−Br1, 89.599(27); P1−Pt1−P3, 163.935(42);
P2−Pt1−Br1, 174.398(29); P2−Pt2−Br2, 94.352(25).
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Reactivity toward Small Molecule Activation. The
noninnocent nature of NHP ligands in transition metal
complexes suggests the ability to activate σ bonds across the
P(NHP)−M bond. Indeed, treatment of complexes 2, 3, or 4 with
MeI results in the MII iodide complexes [(PPMeP)MI][PF6] (M
= Pd (15), Pt (17)) and [(PPMeP)MI2] (M = Pd (16), Pt (18),
Scheme 3). The methyl group electrophilically attacks the
Scheme 3
Figure 4. Displacement ellipsoid (50%) representation of 16 and 17.
For clarity, all hydrogen atom with the exception of those on the
NHP-bound Me group have been omitted. Solvate molecules and
−
PF6 counterions (17) have also been omitted. Relevant interatomic
distances (Å) and angles (deg): 17: P2−Pt, 2.228(3); Pt−P3,
2.311(4); Pt−P1, 2.322(4); P2−C39, 1.810(12); Pt−I1, 2.5961(8);
I1−Pt−P2, 170.07(10); P3−Pt−P1, 173.45(11); Pt−P2−C39,
117.1(4). 16: P2−Pd, 2.1984(5); Pd−P3, 2.3079(6); Pd−P1,
2.3051(6); P2−C39, 1.800(2); Pd−I1, 2.6617(2); Pd−I2, 3.1464(2);
I1−Pd−P2, 170.590(16); P3−Pd−P1, 163.11(2); I1−Pd−I2,
105.726(7); Pd−P2−C39, 114.16(8).
for complexes 4 and 10 are covalent in nature with similar
constitution, 41.6% Pt/58.4% P and 41.7% Pt/58.3% P
character, respectively. In contrast, the Pt−P bonding in the
three-coordinate complex 11* is composed entirely of a strong
donor−acceptor interaction (stabilization energy (Edel) = 111
kcal/mol) between the electron-rich Pt center and the empty p
orbital on the central NHP phosphorus (Figure 5). This
difference in bonding results from the absence of an additional
two-electron donor ligand to stabilize a higher formal oxidation
state at Pt in 11*. This data is consistent with the assignment of
4 and 10 as PtII/NHP− phosphido complexes. In the absence of
an additional donor ligand, 11* is better assigned as a Pt0/
NHP+ phosphenium complex in which the NHP is bound via a
Pt-to-P donor−acceptor interaction.
stereochemically active lone pair at the central NHP
phosphorus to generate a trisubstituted phosphine ligand (L-
type), providing further evidence for an electron-rich NHP−
phosphide ligand. In a formal sense, MeI has added to 2, 3, or 4
without altering the oxidation state of the metal center, which
illustrates the noninnocent behavior of the NHP unit in these
transition metal complexes.
While the anionic complexes 15 and 17 are the major
products of these MeI addition reactions, appreciable quantities
of the neutral diiodide complexes 16 and 18 are detected. The
likely mechanism by which these byproducts are obtained
involves attack of MeI on dissociated PPh3 to generate
[PR3Me]+[I]−, which can then undergo anion exchange with
either the starting material 2−4 or the MeI addition products
15 or 17 to generate neutral diiodide complexes 16 and 18 (see
Supporting Information). The proposed intermediate [PR3Me]
+[I]− was synthesized independently, and its presence
The hypothetical two-electron oxidation products 4ox, 10ox,
and 11*ox were also calculated using DFT and NBO analysis.
Interestingly, upon optimization the geometry about the NHP
unit in complexes 4ox and 10ox became more planar, although
steric factors prevented a rigorously planar geometry in the
PMe3 complex 4ox. The natural charge on the NHP phosphorus
atom increases substantially upon oxidation of these two
complexes, from 1.07 to 1.56 in 4/4ox and from 1.01 to 1.65 in
10/10ox. In contrast, however, the natural charge on Pt
increases only slightly in 4/4ox (−0.58 to −0.43) and decreases
in the case of 10/10ox (−0.46 to −0.65). NBO analysis of 10ox
reveals a Pt−P NBO that is strongly polarized toward
phosphorus (71.2% P, 28.8% Pt), indicative of dative donation
of the phosphorus lone pair to the divalent Pt center (Figure
5). The Pt−P interaction is further strengthened by a Pt→P π
donor−acceptor interaction (Edel = 18.8 kcal/mol) between a
filled Pt d orbital and the empty p orbital on the central NHP
phosphorus atom. Because of the more electron-rich Pt center
in 4ox, an NBO corresponding to lone pair donation from the
NHP phosphorus to Pt was not detected. The reduced
symmetry in this distorted complex leads to less orbital overlap
and weakening of the Pt→P π donor−acceptor interaction (Edel
= 12.2 kcal/mol). Thus, two-electron oxidation is predicted to lead
to oxidation of the NHP phosphorus center to generate a
phosphenium complex, with no change in formal oxidation state
at Pt. Notably, solution electrochemistry of both the phosphine
adduct 3 and the Pt−Cl species 8 (analogues of 4 and 10,
respectively), reveals irreversible oxidation events at relatively
mild potentials (−0.22 V for 3 and −0.24 V for 4, see
1
confirmed in the MeI reaction mixture by H and 31P NMR
spectroscopy. Alternatively, diiodide complexes 16 and 18 can
be synthesized exclusively in >80% yields by adding excess
phosphine ligand (PPh3 or PMe3) into the reaction of 2−4 and
MeI.
The solid state structures of 17 and 16, shown in Figure 4,
reveal that the square planar geometry at the Pt and Pd center,
respectively, have been maintained upon MeI addition across
the M−P(NHP) bond. Methylation at the NHP phosphorus has a
negligible effect on the M−P(NHP) distance in both cases,
consistent with the maintenance of a M−P single bond. The
second iodide ligand in 16 is bound weakly (Pd−I2 =
3.1464(2) Å) in an apical position, causing only a minor
perturbation in the square planar geometry at Pd.
Theoretical Investigation of Group 10 NHP-diphos-
phine Complexes. To better understand the electronic
structure of Pt and Pd complexes featuring the [PPP]+ NHP-
diphosphine pincer ligand, a computational investigation was
carried out using density functional theory (DFT, B3LYP/
LANL2DZ). Three Pt complexes were chosen for this study,
[(PPP)Pt(PMe3)]+ (4), (PPP)PtI (10), and the three-
coordinate complex (PPP)Pt (11*), meant to represent half
of dimer 11. A geometry optimization and subsequent NBO
calculation were performed on each complex starting from
crystallographic coordinates. The Pt−P natural bond orbitals
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Figure 5. Visual representations of the calculated Pt−P NBOs of complex 4 (41.6% Pt, 58.4% P), 10 (41.7% Pt, 58.3% P), and 10ox (28.8% Pt,
71.2% P) (top). Pt to P donor−acceptor interactions in 11* (Edel= 111 kcal/mol), 4ox (Edel = 12.2 kcal/mol), and 10ox (Edel = 18.8 kcal/mol)
(bottom).
Supporting Information), suggesting that chemical oxidation of
these complexes is possible but may not lead to stable products.
In the case of the three-coordinate complex 11*ox, oxidation
appears to be Pt-centered, with a large increase in natural
charge at Pt (−0.32 to 0.03) and small change in the natural
charge at the NHP phosphorus atom (1.25 to 1.48), since the
NHP unit is already in the oxidized NHP+ phosphenium state
in 11*.
CONCLUSION
■
In summary, incorporation of an NHP unit into a chelating
bis(phosphine) framework has allowed a systematic inves-
tigation of NHP coordination chemistry at Pd and Pt. We have
established that in the case of electron-rich metal fragments
(e.g., group 10 metal with 3 additional L-type donors), the
pyramidal geometry adopted by the central NHP donor ligand
is attributed to a NHP− phosphido description accompanied by
formal two-electron oxidation of the metal center. Indeed,
cyclic voltammetry (CV) of the free phosphenium ligand
[PPP][PF6] reveals multielectron reduction events at reason-
ably mild potentials (−1.6 to −1.8 V vs ferrocene, see
Supporting Information), consistent with the observation that
two-electron transfer can occur upon treatment with reasonably
electron-rich d10 metal precursors. NHPs bound to metal
fragments with fewer electrons (e.g., group 10 metal with only
2 additional L-type donors), however, maintain phosphenium
character and adopt the canonical planar NHP+ geometry. We
have also shown that interconversion between these two
geometries can be affected by either coordination/dissociation
of a two-electron donor ligand or two-electron redox processes.
Several key points bear further mention:
Wiberg bond indices (WBIs) for all six complexes were also
calculated and are tabulated in Table 1. All six N−C WBI values
Table 1. Calculated Wiberg Bond Indices (WBIs) for
Complexes 4, 10, 11*, 4ox, 10ox, and 11*ox
P−Pt
P−N
N−C
4
0.84
0.80
0.80
0.73
0.95
0.88
0.74
0.72
0.81
1.01
0.99
0.96
0.93
0.93
0.92
0.90
0.91
0.90
10
11*
4ox
10ox
11*ox
are shown to depict that a typical single bond is represented by
a WBI of about 0.91. Notably, as each compound is oxidized a
substantial increase in the P−N WBI is observed, implying
stronger donation from the nitrogen lone pairs to the central
NHP phosphorus atom as it gains NHP+ phosphenium
character. A corresponding increase in P−Pt WBI is also
observed upon oxidation in the cases of complexes 10ox and
11*ox, consistent with both σ and π interactions between P and
Pt. The Pt−P WBI decreases upon oxidation of 4 to 4ox, likely
the result of steric constraints restricting orbital overlap.
(1) Herein we have discussed transition metal complexes of
both NHP+ and NHP− ligands and it is noteworthy that
we have elucidated a trend by which these two limiting
descriptions can be differentiated via 31P NMR. In our
work, NHP− phosphenium complexes have 31P NMR
shifts lower than 250 ppm, while bona fide NHP+
phosphenium complexes have shifts downfield of 250
ppm. We note, however, that while this trend in 31P shift
is useful for this particular chelating NHP-diphosphine
ligand, previously reported complexes of Pt and Pd with
monodentate NHP ligands (only NHP+ phosphenium
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complexes have been described) have chemical shifts that
will continue to focus on the coordination chemistry of NHP-
diphosphine ligands and their derivatives.
vary considerably with ancillary ligands.21,23,24 In the case
of Pt-NHP complexes, however, NHP− and NHP+
binding modes can easily be distinguished in all cases
by 1JPt−P, as phosphenium ligands are bound more tightly
to Pt through both σ and π interactions, leading to larger
coupling constants (>2000 Hz).
EXPERIMENTAL SECTION
■
General Considerations. All syntheses reported were carried out
using standard glovebox and Schlenk techniques in the absence of
water and dioxygen, unless otherwise noted. Benzene, n-pentane,
tetrahydrofuran, toluene, diethyl ether, and dichloromethane were
degassed and dried by sparging with ultra high purity argon gas
followed by passage through a series of drying columns using a Seca
Solvent System by Glass Contour. All solvents were stored over 3-Å
molecular sieves. Deuterated solvents were purchased from Cambridge
Isotope Laboratories, Inc., degassed via repeated freeze−pump−thaw
cycles, and dried over 3-Å molecular sieves. Solvents were frequently
tested using a standard solution of sodium benzophenone ketyl in
tetrahydrofuran to confirm the absence of oxygen and moisture.
Ligand precursor [PPP]-X,19 ligand [PPP][PF6],19 Pd(PPh3)4,29 and
(2) It is worthy of mention that previously reported Group
10 NHP complexes have, without exception, featured
planar NHP+ geometries indicative of a phosphenium
cation bound to a low valent metal center.21,23,24 Thus, a
question arises as to why incorporation into a chelating
framework would impart such a drastic change on metal
binding preferences. One might argue that the (NHP)-
ML2 (L = NHC or phosphine) complexes reported in
the literature maintain a planar phosphenium geometry
as a result of the absence of a third L-type ligand, similar
to the dimeric complexes 1 and 11.23,24 However, the
(NHP)M(PMe3)3 complexes of Jones and co-workers
are also reported to be NHP+ phosphenium complexes
30
Pt(PPh3)4 were synthesized using literature procedures. Methyl
iodide, triphenylphosphine, and trimethylphosphine were purchased
from commercial vendors and used without further purification. NMR
spectra were recorded at ambient temperature unless otherwise stated
1
on Varian Inova 400 MHz instrument. H and 13C NMR chemical
1
based on 31P NMR data (large JPt−P and equivalence of
shifts were referenced to residual solvent and are reported in ppm. 31P
NMR chemical shifts (in ppm) were referenced to 85% H3PO4 (0
ppm). 19F NMR chemical shifts (in ppm) were referenced to 1%
trifluoroacetic acid (−76.5 ppm). Elemental microanalyses were
performed by Complete Analysis Laboratories, Inc., Parsippany, NJ.
[(PPP)Pd]2[PF6]2 (1). Ligand [PPP][PF6] (105 mg, 0.145 mmol)
was suspended in 10 mL of THF; to this yellow suspension was added
Pd(PPh3)4 (161 mg, 0.139 mmol). The mixture became dark brown
within 5 min but was allowed to stir at room temperature (rt) further
to ensure completion. After 1 h, volatiles were removed from the
resulting solution in vacuo, and the resulting brown residue was
washed with diethyl ether (3 × 5 mL). Crystallization of the crude
brown product via vapor diffusion of diethyl ether into a concentrated
THF solution resulted in analytically pure product as dark red crystals
PMe3 ligands suggest a tetrahedral metal geometry).23
The clearest difference between the [(PPP)M-L]+
complexes reported herein and the aforementioned
four-coordinate complexes is the planar geometry
enforced by the chelating nature of the [PPP]+ ligand.
NHP (and NHC) ligands with N-aryl substitutents
typically adopt a geometry in which the aryl group
orients perpendicular to the plane of the heterocycle, but
in the case of a pincer ligand, these aryl groups are forced
to orient in the same plane as the heterocycle. As a
consequence, it seems feasible that the lone pair of
electrons on the nitrogen atoms can delocalize
throughout the appended aromatic rings in the latter
case, withdrawing electron density that would otherwise
be involved in donation to the empty orbital on the
central phosphorus to stabilize the cationic phosphenium
center (Chart 2). A consequence of this phenomenon
1
suitable for X-ray diffraction (73.1 mg, 60.8%). H NMR (400 MHz,
CD2Cl2): δ 7.61 (m, 12H, Ar-H), 7.55 (m, 4H, Ar-H), 7.47 (t, 8H, Ar-
H), 7.32 (m, 4H, Ar-H), 7.26 (m, 8H, Ar-H), 7.05 (t, 4H, Ar-H), 6.86
(br, 12H, Ar-H), 6.56 (br, 4H, Ar-H), 3.36 (m, 4H, CH2), 3.05 (m,
4H, CH2). 31P NMR (161.8 MHz, CD2Cl2): δ 288.3 (br s, 2P), 14.1
(br s, 4P), −143.8 (sept, 1P, 1JP−F = 710 Hz). 13C NMR (100.5 MHz,
CD2Cl2): δ 133.9, 133.2, 132.6, 132.2, 129.7, 129.6, 129.5, 126.9,
120.0. 118.9, 51.2. Anal. Calcd. for C76H64N4F12P8Pd2: C, 53.01; H,
3.75; N, 3.25. Found: C, 53.03; H, 3.68; N, 3.26.
Chart 2
[(PPP)Pd(PMe3)][PF6] (2). Compound 1 (35.0 mg, 0.0203 mmol)
was dissolved in 10 mL of CH2Cl2 and to this dark red solution was
added trimethylphosphine (8.4 μL, 0.081 mmol). The mixture was
allowed to stir for 12 h to ensure complete reaction. Subsequent
removal of the volatiles from the resulting brown solution yielded
analytically pure product as a yellow solid (35.1 mg, 92.2%). Crystals
suitable for X-ray crystallography were grown via slow evaporation of a
concentrated THF solution of the product in the freezer (−35 °C). 1H
NMR (400 MHz, CD2Cl2): δ 7.53 (m, 14H, Ar-H), 7.35 (m, 8H, Ar-
H), 7.26 (m, 2H, Ar-H), 7.22 (m, 2H, Ar-H), 7.06 (t, 2H, Ar-H), 3.79
would be relative stabilization of the triplet state of the
NHP, thus favoring the anionic phosphide binding mode.
Moreover, NHPs with pendant coplanar aryl rings that
continue to adopt a planar NHP+ binding mode would
be expected to be stronger π-acceptors. This principle
may, in fact, hold true for all pincer ligands containing
both NHPs and NHCs.
2
(m, 2H, CH2), 3.06 (m, 2H, CH2), 0.57 (d, 9H, −CH3, JP−H = 6.8
Hz). 31P NMR (161.8 MHz, CD2Cl2): δ 235.6 (t, 1P, 2JP−P = 24.3 Hz),
2
2
2
11.5 (dd, 2P, JP−P = 24.3 Hz, JP−P = 57.0 Hz), −34.1 (t, 1P, JP−P
=
57.0 Hz), −143.9 (sept, 1P, JP−F = 710 Hz). 13C NMR (100.5 MHz,
CD2Cl2): δ 134.6, 134.5, 134.0, 132.4, 131.9, 131.7, 129.6, 129.5,
122.1, 121.0, 50.2, 21.4. Anal. Calcd for C41H41N2F6P5Pd: C, 52.55; H,
4.41; N, 2.99. Found: C, 52.64; H, 4.57; N, 3.89.
1
The strong analogy between NHPs and nitrosyls has been
substantiated by this study, revealing both electronic and
structural (in terms of dual binding modes) similarities. For this
reason, NHPs have great potential as sterically and electroni-
cally tunable noninnocent ligands in coordination chemistry
and catalysis. Moreover, incorporation of NHPs into a chelating
framework provides a method to impart stability to transition
metal NHP complexes in various redox states. Future studies
[(PPP)Pt(PPh3)][PF6] (3). Ligand [PPP][PF6] (92.0 mg, 0.122
mmol) was suspended in 10 mL of THF and to this yellow suspension
was added Pt(PPh3)4 (152 mg, 0.122 mmol). The resulting mixture
was allowed to stir at rt further to ensure complete reaction. After 1 h,
volatiles were removed from the red solution in vacuo, and the
remaining residue was washed with diethyl ether (3 × 5 mL).
Crystallization of the crude product via vapor diffusion of diethyl ether
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into a concentrated THF solution resulted in product as red crystals
suitable for X-ray diffraction (115 mg, 77.8%). H NMR (400 MHz,
CDCl3): δ 7.56 (t, 2H, Ar-H), 7.54 (t, 2H, Ar-H), 7.38 (t, 4H, Ar-H),
7.32−6.89 (m, 31H, Ar-H), 6.26 (m, 4H, Ar-H), 3.76 (br, 2H, CH2),
2.67 (br, 2H, CH2). 31P NMR (161.8 MHz, CDCl3): δ 198.8 (ddt, 1P,
(PPP)Pt−Cl (8). Ligand precusor [PPP]-Cl (97.0 mg, 0.150 mmol)
was dissolved in 10 mL of toluene and to this stirring light yellow
solution was added Pt(PPh3)4 (187 mg, 0.150 mmol). The reaction
mixture became deep red immediately. After stirring for 10 min, the
resulting red solution was filtered and concentrated in vacuo to about
5 mL. The concentrated toluene solution was stored in the freezer for
12 h (−35 °C), affording 8 as yellow crystals which were collected,
washed with toluene, and further dried in vacuo (98.0 mg, 77.6%). 1H
NMR (400 MHz, CD2Cl2): δ 7.68 (m, 4H, Ar-H), 7.47 (m, 10H, Ar-
H), 7.37 (m, 8H, Ar-H), 7.05 (d, 2H, Ar-H), 6.86 (q, 2H, Ar-H), 6.74
(t, 2H, Ar-H), 3.76(m, 2H, CH2), 3.38 (m, 2H, CH2). 31P NMR
1
2
2
1JPt−P = 445 Hz, JP−P(trans) = 52.1 Hz, JP−P(cis) = 9.2 Hz), 29.1 (ddt,
1
2
2
1P, JPt−P = 2443 Hz, JP−P(trans) = 52.1 Hz, JP−P(cis) = 22.0 Hz), 13.6
1
2
2
(ddd, 2P, JPt−P = 3446 Hz, JP−P(NHP) = 9.2 Hz, JP‑(PPh3) = 22.0 Hz),
−143.9 (sept, 1P, 1JP−F = 710 Hz). 13C NMR (100.5 MHz, CDCl3): δ
146.7, 134.6, 134.5, 134.1, 134.0, 132.7, 132.3, 132.1, 131.5, 131.3,
128.9, 128.8, 121.8, 119.2, 48.2. Anal. Calcd for C56H47N2F6P5Pt: C,
55.60; H, 3.91; N, 2.31. Found: C, 55.45; H, 4.01; N, 2.26.
1
1
(161.8 MHz, CD2Cl2): δ 224.8 (dt, JPt−P = 663 Hz, JP−P = 6.1 Hz),
1
1
12.1 (dd, JPt−P = 3304 Hz, JP−P = 6.1 Hz). 13C NMR (100.5 MHz,
CD2Cl2): δ 135.9, 134.8, 134.2, 132.2, 130.8 130.7, 128.3, 128.2, 119.2,
118.6, 48.6. Anal. Calcd for C38H32N2ClP3Pt: C, 54.33; H, 3.84; N,
3.33. Found: C, 54.28; H, 3.91; N, 3.16.
[(PPP)Pt(PMe3)][PF6] (4). Compound 3 (31.8 mg, 0.0262 mmol)
was dissolved in 10 mL of dichloromethane and to this red solution
was added trimethylphosphine (5.4 μL, 0.052 mmol). The reaction
mixture was allowed to stir further to ensure complete reaction. After 1
h, removal of all volatiles from the resulting yellow solution yielded
analytically pure product as yellow solid (25.7 mg, 95.6%). Crystals
suitable for X-ray crystallography were grown via vapor diffusion of n-
(PPP)Pt−Br (9). An identical procedure to that of compound 8 was
followed, using [PPP]-Br (84.5 mg, 0.123 mmol) and Pt(PPh3)4
1
(152.5 mg, 0.123 mmol) to yield 9 (76.5 mg, 70.6%). H NMR (400
MHz, CD2Cl2): δ 7.67 (m, 4H, Ar-H), 7.46−7.34 (m, 18H, Ar-H),
7.06 (m, 2H, Ar-H), 6.85 (q, 2H, Ar-H), 6.74 (t, 2H, Ar-H), 3.77(m,
2H, CH2), 3.34 (m, 2H, CH2). 31P NMR (161.8 MHz, CD2Cl2): δ
222.0 (dt, 1JPt−P = 607 Hz, 1JP−P = 6.1 Hz), 11.2 (dd, 1JPt−P = 3337 Hz,
1JP−P = 6.1 Hz). 13C NMR (100.5 MHz, CD2Cl2): 136.1, 134.8, 134.3,
132.1, 130.8 130.7, 128.4, 128.2, 119.4, 119.0, 48.8. Anal. Calcd for
C38H32N2BrP3Pt: C, 51.60; H, 3.65; N, 3.17. Found: C, 51.43; H, 3.72;
N, 3.08.
1
pentane into a concentrated dichloromethane solution of 4. H NMR
(400 MHz, CDCl3): δ 7.47 (m, 12H, Ar-H), 7.27 (m, 8H, Ar-H), 7.16
(m, 2H, Ar-H), 6.96 (m, 4H, Ar-H), 6.82 (t, 2H, Ar-H), 3.58 (m, 2H,
CH2), 3.18 (m, 2H, CH2), 0.65 (d, 9H, CH3). 31P NMR (161.8 MHz,
1
2
CDCl3): δ 205.5 (ddt, 1P, JPt−P = 496 Hz, JP−P(trans) = 54.9 Hz,
2JP−P(cis) = 9.1 Hz), 11.1 (ddd, 2P, JPt−P = 3149 Hz, JP−P(NHP) = 9.1
1
2
Hz, 2JP‑PMe3 = 26.5 Hz), −18.7 (ddt, 1P, 1JPt−P = 2221 Hz, 2JP−P(NHP)
=
2
1
54.9 Hz, JP‑(cis) = 26.5 Hz), −143.9 (sept, 1P, JP−F = 710 Hz). 13C
NMR (100.5 MHz, CDCl3): δ 134.7, 134.6, 134.4, 132.3, 132.0, 131.8,
129.7, 129.6, 121.5, 121.1, 49.5, 20.8. Anal. Calcd for C41H41N2F6P5Pt:
C, 48.01; H, 4.03; N, 2.73. Found: C, 47.83; H, 4.02; N, 2.63.
(PPP)Pd−Cl (5). Ligand precursor [PPP]-Cl (128 mg, 0.198
mmol) was dissolved in 10 mL of benzene; to this stirring pale yellow
solution was added Pd(PPh3)4 (229 mg, 0.198 mmol). The reaction
mixture became dark red immediately, and after 5 min, the dark red
solution was filtered and concentrated to about 2 mL. Upon standing
for 12 h at rt, yellow crystals formed and were washed with benzene
and further dried in vacuo to yield pure product (79.7 mg, 49.4%). 1H
NMR (400 MHz, CD2Cl2): δ 7.68 (m, 4H, Ar-H), 7.52 (m, 4H, Ar-
H), 7.48−7.41 (m, 10H, Ar-H), 7.36 (m, 4H, Ar-H), 7.19 (m, 2H, Ar-
(PPP)Pt−I (10). An identical procedure to that of compound 8 was
followed, using [PPP]-I (123.4 mg, 0.168 mmol) and Pt(PPh3)4
1
(208.5 mg, 0.168 mmol) to yield 10 (72.5 mg, 46.4%). H NMR
(400 MHz, CD2Cl2): δ 7.65 (m, 4H, Ar-H), 7.45−7.35 (m, 18H, Ar-
H), 7.07 (m, 2H, Ar-H), 6.90 (m, 2H, Ar-H), 6.75 (t, 2H, Ar-H), 3.79
(m, 2H, CH2), 3.29 (m, 2H, CH2). 31P NMR (161.8 MHz, CD2Cl2): δ
219.5 (dt, 1JPt−P = 486 Hz, 1JP−P = 6.4 Hz), 9.4 (dd, 1JPt−P = 3350 Hz,
1JP−P = 6.4 Hz). 13C NMR (100.5 MHz, CD2Cl2): δ 136.3, 134.9,
134.4, 132.0, 130.6, 130.7, 128.4, 128.1, 119.5, 119.2, 48.9. Anal. Calcd
for C38H32N2IP3Pt: C, 48.99; H, ; 3.46N, 3.01. Found: C, 48.83; H,
3.51; N, 2.94.
[(PPP)Pt]2[PF6]2 (11). Compound 8 (47.4 mg, 0.0564 mmol) was
dissolved in 10 mL of THF and to this yellow solution was added
TlPF6 (19.7 mg, 0.0564 mmol). The mixture was allowed to stir at rt
for 12 h to ensure complete reaction, and the resulting red solution
was filtered through a pad of Celite to remove TlCl. Removal of the
volatiles from the filtrate in vacuo afforded the crude product as a dark
solid. Crystallization of the crude product via vapor diffusion of diethyl
ether into a concentrated THF solution yielded product as dark red
crystals suitable for X-ray diffraction. 1H NMR (400 MHz, CD2Cl2): δ
7.58 (m, 6H, Ar-H), 7.49 (m, 2H, Ar-H), 7.31 (m, 10H, Ar-H), 7.07
(m, 4H, Ar-H), 6.99 (t, 2H, Ar-H), 6.80 (br, 2H, Ar-H), 6.61 (br, 2H,
Ar-H), 3.43 (m, 2H, CH2), 3.06 (m, 2H, CH2). 31P NMR (161.8 MHz,
CD2Cl2): δ 257.5 (dd, 1JPt−P = 2161 Hz, 2JP−P = 42 Hz), 5.4 (br, 2JP−P
H), 6.97 (m, 2H, Ar-H), 6.88 (t, 2H, Ar-H), 3.83 (m, 2H, CH2), 3.36
1
(m, 2H, CH2). 31P NMR (161.8 MHz, CD2Cl2): δ 248.6 (t, JP−P
=
28.3 Hz), 6.2 (d, 1JP−P = 28.3 Hz). 13C NMR (100.5 MHz, CD2Cl2): δ
136.1, 134.8, 134.3, 131.8, 130.5, 130.4, 128.5, 128.4, 121.7, 121.2,
51.6.Anal. Calcd for C38H32N2ClP3Pd: C, 60.74; H, 4.29; N, 3.73.
Found: C, 60.58; H, 4.37; N, 3.59.
(PPP)Pd−Br (6). An identical procedure to that of compound 5
was followed, using [PPP]-Br (53.7 mg, 0.078 mmol) and Pd(PPh3)4
1
(90.0 mg, 0.078 mmol) to yield 6 (40.0 mg, 64.5%). H NMR (400
MHz, CD2Cl2): δ 7.68 (m, 4H, Ar-H), 7.53 (m, 4H, Ar-H), 7.51−7.43
(m, 10H, Ar-H), 7.37 (m, 4H, Ar-H), 7.21 (m, 2H, Ar-H), 6.98 (m,
2H, Ar-H), 6.89 (t, 2H, Ar-H), 3.84(m, 2H, CH2), 3.36 (m, 2H, CH2).
31P NMR (161.8 MHz, CD2Cl2): δ 246.1 (t, 1JP−P = 25.2 Hz), 6.0 (d,
1JP−P = 25.2 Hz). 13C NMR (100.5 MHz, CD2Cl2): δ 136.3, 134.7,
134.4, 131.8, 130.5, 130.4, 128.4, 128.3, 121.9, 121.4, 51.9. Repeated
attempts to obtain satisfactory combustion analysis data for 6 were
unsuccesful as a result of an unidentified minor impurity that
cocrystallized with 6.
1
= 42 Hz), −143.9 (sept, JP−F = 710 Hz). Attempts to obtain
satisfactory 13C NMR and combustion analysis data for 11 were
unsuccesful as a result of an unidentified minor impurity that
cocrystallized with 11. For this reason, an isolated yield has also not
been reported.
[(PPP)Pt−Cl]2Pt(H)(Cl) (12). Compound 8 (67.7 mg, 0.0806
mmol) was loaded into a 20 mL vial and stirred in 10 mL of THF. The
resulting yellow mixture was allowed to stir vigorously for 12 h, and
the volatiles were subsequently removed to afford a yellow solid.
Crystallization of the crude product via vapor diffusion of diethyl ether
into a concentrated THF solution afforded analytically pure product as
(PPP)Pd−I (7). An identical procedure to that of compound 5 was
followed, using [PPP]-I (46.6 mg, 0.063 mmol) and Pd(PPh3)4 (73.1
1
mg, 0.063 mmol) to yield 7 (23.3 mg, 43.7%). H NMR (400 MHz,
1
CD2Cl2): δ 7.69 (m, 4H, Ar-H), 7.52 (m, 4H, Ar-H), 7.48−7.43 (m,
yellow crystals (23.2 mg, 45.2%). H NMR (400 MHz, CD2Cl2): δ
10H, Ar-H), 7.36 (m, 4H, Ar-H), 7.21 (m, 2H, Ar-H), 6.98 (m, 2H,
7.51 (m, 16H, Ar-H), 7.38−7.23 (m, 28H, Ar-H), 6.79 (m, 4H, Ar-H),
Ar-H), 6.86 (t, 2H, Ar-H), 3.83(m, 2H, CH2), 3.35 (m, 2H, CH2). 31
P
6.71 (m, 4H, Ar-H), 6.60 (m, 4H, Ar-H), 3.62 (m, 4H, CH2), 3.19 (m,
NMR (161.8 MHz, CD2Cl2): δ 242.2 (t, 1JP−P = 25.0 Hz), 5.7 (d, 1JP−P
= 25.0 Hz) 13C NMR (100.5 MHz, CDCl3): δ 136.1, 134.9, 134.4,
131.7, 130.4, 130.3, 128.5, 128.2, 121.9, 121.3, 52.0. Anal. Calcd for
C38H32N2IP3Pd: C, 54.15; H, 3.83; N, 3.32. Found: C, 54.26; H, 3.89;
N, 3.34.
4H, CH2), −17.1 (dt, 1H, Pt-H, 1JPt−H = 1300 Hz, 1JP−H = 21 Hz). 31
P
NMR (161.8 MHz, CD2Cl2): δ 152.9 (1JPt−P(side
= 529 Hz,
Pt)
1JPt−P(central Pt) = 2377 Hz, 2JP−P = 30 Hz), 4.1 (1JPt−P = 2586 Hz, 2JP−P
=
30 Hz). 13C NMR (100.5 MHz, CD2Cl2): δ 150.5, 135.8, 135.0, 134.1,
132.4, 130.5, 128.3, 128.2, 119.4, 118.5, 47.0. Anal. Calcd for
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C76H65N4Cl3P6Pt3: C, 47.75; H, 3.43; N, 2.93. Found: C, 47.80; H,
3.37; N, 2.89.
(dd, 2P, 1JPt−P = 2220 Hz, 2JP−P = 26.7), −143.9 (sept, 1P, 1JP−F = 710
Hz). 13C NMR (100.5 MHz, CD2Cl2): δ 146.0, 137.6, 134.9, 134.4,
132.6, 132.1, 129.3, 128.9, 123.3, 120.3, 46.9, 19.7. Anal. Calcd for
C39H35N2F6IP4Pt: C, 42.91; H, 3.23; N, 2.57. Found: C, 42.85; H,
3.22; N, 2.50.
[(PPP)Pt−Br]2Pt(H)(Br) (13). An identical procedure to that of
compound 12 was followed, using 9 (54.7 mg, 0.0793 mmol) to yield
13 (19.7, 36.4%). 1H NMR (400 MHz, CD2Cl2): δ 7.55 (m, 16H, Ar-
H), 7.40−7.22 (m, 28H, Ar-H), 6.78 (m, 4H, Ar-H), 6.68 (m, 4H, Ar-
H), 6.59 (m, 4H, Ar-H), 3.60 (m, 4H, CH2), 5.15 (m, 4H, CH2),
[(PP(Me)P)PtI2] (18). Compound 4 (76.5 mg, 0.0746 mmol) was
dissolved in 10 mL of dichloromethane. To this yellow solution was
added PMe3 (38.6 μL, 0.373 mmol) and MeI (4.6 μL, 0.075 mmol).
The mixture was allowed to stir for 12 h to ensure complete reaction.
The yellow precipitate was collected via filtration to afford analytically
pure product (65.1 mg, 81.3%). 1H NMR (400 MHz, CD2Cl2): δ 7.68
(t, 2H, Ar-H), 7.60−7.48 (m, 14H, Ar-H), 7.43 (m, 8H, Ar-H), 7.07 (t,
2H, Ar-H), 7.01 (m, 2H, Ar-H), 4.68 (m, 2H, CH2), 3.62 (m, 2H,
1
1
−16.2 (dt, 1H, Pt-H, JPt−H = 1425 Hz, JP−H = 17 Hz). 31P NMR
(161.8 MHz, CD2Cl2): δ 154.9 (1JPt−P(side Pt) = 660 Hz, 1JPt−P(central Pt)
=
2750 Hz, JP−P = 25 Hz), 4.1 (1JPt−P = 2586 Hz, JP−P = 25 Hz). 13C
NMR (100.5 MHz, CD2Cl2): δ 150.4, 135.8, 135.0, 134.2, 132.4,
130.4, 128.1, 128.0, 120.0, 119.1, 46.8. Anal. Calcd for
C76H65N4Br3P6Pt3: C, 44.63; H, 3.20; N, 2.74. Found: C, 44.62; H,
3.10; N, 2.71.
2
2
2
3
CH2), 1.35 (dd, 3H, CH3, JP−H = 9.2 Hz, JPt−H = 19.2 Hz)). 31P
1
2
NMR (161.8 MHz, CD2Cl2): δ 81.2 (dt, JPt−P = 2259.1 Hz, JP−P
=
[(PPP)Pt−I]2Pt(H)(I) (14). An identical procedure to that of
compound 12 was followed, using 10 (58.8 mg, 0.0798 mmol) to
yield 14 (29.7 mg, 51.1%). 1H NMR (400 MHz, CD2Cl2): δ 7.62 (m,
16H, Ar-H), 7.43−7.23 (m, 28H, Ar-H), 6.83 (m, 8H, Ar-H), 6.52 (m,
4H, Ar-H), 3.57 (m, 4H, CH2), 2.95 (m, 4H, CH2), −13.3 (dt, 1H, Pt-
H, 1JPt−H = 1487 Hz, 1JP−H = 12 Hz). 31P NMR (161.8 MHz, CD2Cl2):
1
2
26.7 Hz), −2.8 (dd, JPt−P = 2259.1 Hz, JP−P = 26.7 Hz). 13C NMR
(100.5 MHz, CD2Cl2): δ 146.1, 137.4, 134.8, 134.4, 132.5, 132.1,
129.3, 128.8, 123.1, 120.8, 47.4, 19.4. Anal. Calcd for C39H35N2I2P3Pt:
C, 43.63; H, 3.29; N, 2.61. Found: C, 43.58; H, 3.26; N, 2.59.
X-ray Crystallography Procedures. All operations were
performed on a Bruker-Nonius Kappa Apex2 diffractometer, using
graphite-monochromated MoKα radiation. All diffractometer manip-
ulations, including data collection, integration, scaling, and absorption
corrections were carried out using the Bruker Apex2 software.31
Preliminary cell constants were obtained from three sets of 12 frames.
Crystallographic parameters are provided in Supporting Information,
Tables S1, S2, and S3, and further experimental crystallographic details
are described for each compound in the Supporting Information.
Electrochemistry. Cyclic voltammetry measurements were carried
out in a glovebox under a dinitrogen atmosphere in a one-
compartment cell using a CH Instruments electrochemical analyzer.
A glassy carbon electrode and platinum wire were used as the working
and auxiliary electrodes, respectively. The reference electrode was Ag/
AgNO3 in THF. Solutions (THF) of electrolyte (0.40 M [nBu4N]-
[PF6]) and analyte (ca. 2 mM) were also prepared in the glovebox.
Computational Details. All calculations were performed using
Gaussian0932 for the Linux operating system. Density functional
theory calculations were carried out using the B3LYP hybrid
functional, including Becke’s parameter exchange functional (B3),33
δ 162.9 (1JPt−P(side Pt) = 681 Hz, 1JPt−P(central Pt) = 2436 Hz), 0.1 (1JPt−P
=
3048 Hz). 13C NMR (100.5 MHz, CD2Cl2): δ 150.5, 135.5, 135.2,
134.8, 132.4, 130.5, 128.0, 127.9, 121.1, 119.7, 46.7. Anal. Calcd for
C76H65N4Cl3P6Pt3: C, 41.75; H, 3.00; N, 2.56. Found: C, 41.68; H,
3.02; N, 2.59.
[(PP(Me)P)PdI][PF6] (15). Compound 2 (62.6 mg, 0.0668 mmol)
was dissolved in 10 mL of dichloromethane. To this yellow solution
was added MeI (4.2 μL, 0.067 mmol) and the mixture was allowed to
stir for 12 h to ensure complete reaction. Removal of the volatiles
afforded the crude product as a yellow residue. Crystallization of the
crude product via vapor diffusion of pentane into a concentrated
CH2Cl2 solution yielded analytically pure product as yellow crystals
1
(32.4 mg, 48.3%). H NMR (400 MHz, CD2Cl2): δ 7.68 (t, 2H, Ar-
H), 7.62 (m, 6H, Ar-H), 7.54 (m, 6H, Ar-H), 7.42 (m, 10H, Ar-H),
7.09 (t, 2H, Ar-H), 7.03 (t, 2H, Ar-H), 4.36 (m, 2H, CH2), 3.67 (m,
2H, CH2), 1.11 (d, 2H, CH3). 31P NMR (161.8 MHz, CD2Cl2): δ
108.2 (t, 2JP−P = 23.6 Hz), −1.3 (d, 2JP−P = 23.6 Hz), −143.9 (sept, 1P,
1JP−F = 710 Hz). 13C NMR (100.5 MHz, CD2Cl2): δ 144.6, 137.5,
134.9, 134.2, 132.6, 132.0, 129.5, 129.0, 123.4, 120.5, 47.3, 20.7. Anal.
Calcd for C39H35N2IF6P4Pd: C, 46.71; H, 3.52; N, 2.79. Found: C,
46.76; H, 3.55; N, 2.80.
and the correlation functional of Lee, Yang, and Parr (LYP).34
A
mixed-basis set was employed, using the LANL2DZ(p,d) double-ζ
basis set with effective core potentials for phosphorus, iodine, and
platinum35−37 and Gaussian09’s internal LANL2DZ basis set
(equivalent to D95 V38) for carbon, nitrogen, and hydrogen. Using
crystallographically determined geometries as a starting point, the
geometries were optimized to a minimum, followed by analytical
frequency calculations to confirm that no imaginary frequencies were
present. NBO39 calculations were then performed on the optimized
geometries of 4, 10, 11* and their oxidized counterparts. Deletion
energies (Edel) represent the change in energy upon zeroing the matrix
elements corresponding to the lp(Pt)→p(P) donor−acceptor
interactions.40 XYZ coordinates of the optimized geometries of all
six complexes are provided in the Supporting Information.
[(PP(Me)P)PdI2] (16). Compound 2 (83.0 mg, 0.0886 mmol) was
dissolved in 10 mL of dichloromethane. To this yellow solution was
added PMe3 (43.6 μL, 0.421 mmol) and MeI (5.2 μL, 0.084 mmol).
The resulting mixture was allowed to stir for 12 h to ensure complete
reaction and the volatiles were subsequently removed to afford crude
product as a yellow solid. Crystallization of the crude product via vapor
diffusion of pentane into a concentrated CH2Cl2 solution yielded
analytically pure product as yellow crystals (90.0 mg, 83.1%). 1H NMR
(400 MHz, CD2Cl2): δ 7.64 (m, 6H, Ar-H), 7.57 (m, 2H, Ar-H), 7.49
(m, 12H, Ar-H), 7.42 (m, 4H, Ar-H), 7.05 (m, 4H, Ar-H), 4.57 (m,
2H, CH2), 3.78 (m, 2H, CH2), 1.14 (d, 2H, CH3). 31P NMR (161.8
2
2
MHz, CD2Cl2): δ 104.4 (t, JP−P = 21.2 Hz), −2.9 (d, JP−P = 21.2
Hz). 13C NMR (100.5 MHz, CD2Cl2): δ 146.5, 137.5, 135.0, 134.2,
132.3, 131.7, 129.4, 128.8, 122.9, 120.7, 48.1, 19.9 Anal. Calcd for
C39H35N2I2P3Pd: C, 47.56; H, 3.58; N, 2.84. Found: C, 47.48; H, 3.57;
N, 2.90.
ASSOCIATED CONTENT
■
S
* Supporting Information
Computational details, a scheme describing the formation of 16
and 18, cyclic voltammetry data, and crystallographic data
collection and refinement details and data in .cif format for 1−
5, 9, 11, 13, 14, 16, and 17. This material is available free of
charge via the Internet at http://pubs.acs.org.
[(PP(Me)P)PtI][PF6] (17). Compound 3 (73.2 mg, 0.0604 mmol)
was dissolved in 10 mL of THF and to this stirring red solution was
added MeI (3.8 μL, 0.060 mmol). The mixture was stirred at rt for 12
h to ensure complete reaction and the volatiles were subsequently
removed in vacuo from the yellow solution to afford crude product as
a light yellow solid. Crystallization of the crude product via vapor
diffusion of diethyl ether into a concentrated THF solution yielded
AUTHOR INFORMATION
1
analytically pure product as light yellow crystals (37.9 mg, 57.5%). H
■
NMR (400 MHz, CD2Cl2): δ 7.68 (t, 2H, Ar-H), 7.60−7.55 (m, 10H,
Ar-H), 7.51 (t, 2H, Ar-H), 7.46−7.36 (m, 10H, Ar-H), 7.09 (t, 2H, Ar-
H), 7.03 (t, 2H, Ar-H), 4.36 (m, 2H, CH2), 3.64 (m, 2H, CH2), 1.22
Corresponding Author
*E-mail: thomasc@brandeis.edu.
2
3
(dd, 3H, CH3, JP−H = 9.2 Hz, JPt−H = 20.8 Hz). 31P NMR (161.8
Notes
1
2
MHz, CD2Cl2): δ 81.5 (dt, 1P, JPt−P = 3411 Hz, JP−P = 26.7), −2.5
The authors declare no competing financial interest.
4178
dx.doi.org/10.1021/ic202581v | Inorg. Chem. 2012, 51, 4170−4179

Inorganic Chemistry
Article
(33) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652.
ACKNOWLEDGMENTS
■
(34) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785−789.
(35) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299−310.
(36) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270−283.
(37) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284−298.
(38) Dunning, T. H.; Hay, P. J. In Modern Theoretical Chemistry;
Schaefer, H. F., Ed.; Plenum: New York, 1976; Vol. 3, pp 1−28.
(39) Glendening, E. D.; Reed, A. E.; Carpenter, J. E.; Weinhold, F.
NBO, Version 3.1.
The authors are grateful for startup funds provided by Brandeis
University, and C.M.T. is additionally grateful for a 2011 Sloan
Research Fellowship.
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