Syntheses of bispyrazolyl monotriazolyl heteroscorpionate platinum(IV) complexes including an unusual Pt-CH2CH2-Pt bridge

Article abstract of DOI:10.1016/j.molcata.2016.07.002

Scorpionate ligands provide the benefit of hemilability while minimizing complete dissociation of the ligand. Previous investigations into Tp′PtL_nX_m complexes [Tp′?=?hydridotris(3,5-dimethylpyrazolyl)borate] revealed the importance o

Full text of DOI:10.1016/j.molcata.2016.07.002

Accepted Manuscript
Title: Syntheses of Bispyrazolyl Monotriazolyl
Heteroscorpionate Platinum(IV) Complexes Including an
Unusual Pt-CH2CH2-Pt Bridge
Author: Katherine D. Lavoie Bryan E. Frauhiger Peter S.
White Joseph L. Templeton
PII:
S1381-1169(16)30252-7
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.07.002
MOLCAA 9936
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
26-5-2016
29-6-2016
2-7-2016
Please cite this article as: Katherine D.Lavoie, Bryan E.Frauhiger, Peter S.White,
Joseph L.Templeton, Syntheses of Bispyrazolyl Monotriazolyl Heteroscorpionate
Platinum(IV) Complexes Including an Unusual Pt-CH2CH2-Pt Bridge, Journal of
Molecular Catalysis A: Chemical http://dx.doi.org/10.1016/j.molcata.2016.07.002
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Syntheses of Bispyrazolyl Monotriazolyl Heteroscorpionate Platinum(IV)
Complexes Including an Unusual Pt-CH2CH2-Pt Bridge
Katherine D. Lavoie, Bryan E. Frauhiger, Peter S. White, and Joseph L. Templeton*
W. R. Kenan Laboratory, Department of Chemistry, University of North Carolina at Chapel Hill
Chapel Hill, NC 27599-3290
joetemp@unc.edu
This article is dedicated to the prolific and productive platinum chemist Professor Georgiy B.
Shulpin on the event of his 70th birthday.

Graphical Abstract
B(ArF)4
H
C
2
N(pz)
Pt
B(ArF)4
H
C
(tz)N
(pz)N
Me
Me
N(tz)
1) NaOMe
(pz)N
Me
Me
THF, overnight
Pt
2) NaB(ArF)4
(pz)N
N(pz)
Me
Me
Pt
N(tz)
C
N(pz)
Cl
H
Keywords: Scorpionate ligand; C-Cl bond breaking; Pt dimer
2

Highlights
Comparison of 5 scorpionate ligands bound to Pt(IV)
Synthesis of new heteroscorpionate ligand
C-Cl bond breaking of chlorinated solvent

Unprecedented Pt dimer with bridging ethylene ligand
3

Abstract
Scorpionate ligands provide the benefit of hemilability while minimizing complete
dissociation of the ligand. Previous investigations into TpPtLnXm complexes [Tp =
hydridotris(3,5-dimethylpyrazolyl)borate] revealed the importance of hemilability as the Tp
ligand facilitates Pt(II/IV) interconversions. Here we discuss the synthesis and metalation of a
series of asymmetric scorpionate ligands bearing two pyrazolyl rings and one triazolyl ring. In
addition to utilizing triazole donor arms with differing substituents, we also compare octahedral
structures of Pt(IV) complexes with P=O and C-H and B-H caps at the pole of the facial
tridentate umbrella. Oxidation from Pt(II) to Pt(IV) with electrophilic reagents, simple acids and
acid chlorides, leads to isomers in some cases, and the binding properties of the various donor
arms dictate the stereochemistry of the products. Investigations into the reactivity of
heteroscorpionate tridentate ligands bound to platinum(II) led to C-Cl activation reactions with
methylene chloride and 1,2-dichloroethane. Isolation of a dinuclear platinum complex bridged by
an ethylene unit produced an unusual proton NMR AAXX pattern in the 1H NMR spectrum due
to chirality at each platinum center.
1. Introduction
Platinum is known for its paradoxical chemical behaviour, namely its high catalytic
activity, which is often easy to probe due to its chemical robustness. More specifically, platinum
is able to easily cycle between three different oxidation states, Pt(0)/Pt(II)/Pt(IV), allowing it to
catalyse a wide variety of transformations, often involving C-H activation. The homogeneous
Shilov system first reported the organometallic transformation of methane to methanol, and the
4

first step is C-H activation of methane.1 Shulpin et al. have reported on extensive studies of
Caryl-H activation to generate stable Pt(IV) complexes.2,3 More recently, investigations into C-H
bond activation using Pt centers has focused on hemilabile ligands to help promote formation of
stable Pt(IV) complexes that are less prone to reductive elimination.4-7
Hemilabile ligands containing both strong and weak donor arms have been employed in a
wide range of metal-mediated processes.8,9 Such ligands provide the benefit of opening a
coordination site on the metal while preventing complete ligand loss. Bidentate and tridentate
hemilabile representations are sketched in Figure 1, where a strong donor ligand (X) remains
bound to the metal center while a weaker donor (Y) can bind reversibly, thus creating a vacant
coordination site.
Figure 1. Hemilabile representations in (a) bidentate and (b) tridentate ligands.
A plethora of metal-ligand systems have been developed that display reversible binding
of one arm, and pincer ligands are an important class of hemilabile ligands. Milstein and co-
workers have reported a Ru PNN pincer system that converts alcohols to esters, and the
mechanism is believed to involve reversible coordination of a tethered amine.10 Goldberg et al.
have synthesized a PCO tridentate ligand that can adopt κ1, κ2, and κ3 coordination modes.11 Van
der Vlugt and co-workers have reported a simple synthetic route that generates PNN chelates that
5

reversibly bind to Rh and Ir metal centers.12 A hemilabile bidentate ligand with an oxygen donor
arm bound to Rh(I) developed by Dunbar has been utilized for the reversible uptake of small
molecules.13,14 A hemilabile Ru(II) system reported by Wolf displays reversible uptake of SMe2,
DMSO, and acetonitrile.15 Recently, Miller has synthesized a NCP pincer-crown ether complex
that can reversibly bind oxygen from the crown ether to help stabilize the metal center.16
The past decade has seen significant progress in the development of asymmetrical
hemilabile tridentate facial chelates, including contributions from Rominger17 (NCO, PCO),
Zargarian18 (PCN), Szabó19 (PCS), and Song20 (PCP). The initial syntheses of asymmetric
facially-coordinating ligands suffered from poor yields, especially for tris(3,5-
dimethylpyrazolyl)methane (Tpm) derivatives. 21-24 Canty and co-workers developed an
improved synthetic route to asymmetrical Tpm-derived heteroscorpionates containing two
pyrazolyl rings and a third, unique donor arm using a cobalt-catalyzed condensation.25-30
Deprotonation of free pyrazole followed by the addition of phosgene results in a bispyrazolyl
ketone intermediate in this preparative route. Reger31 et al. advanced relevant synthetic
methodology by using a bispyrazolyl sulfoxide intermediate that was prepared by employing
thionyl chloride in place of phosgene, noteworthy since the thionyl chloride is both safer and
easier to handle than phosgene (eq 1).
O
N
12 mol% CoCl2
THF, reflux
1) 2 NaH
2) SOCl2
O
S
NH
N
C
N
(1)
2
N
N
N
N
THF, 0 oC
N
N
N
N
6

With a goal of facilitating interconversions between Pt(II) and Pt(IV), our lab
investigated the synthesis and reactivity of tris(1-R-1H-1,2,3-triazol-4-yl)phosphine oxide (R =
phenyl, cyclohexyl) (TtR).32-34 The TtR ligand was completely displaced from the Pt(II) center
when nucleophilic reagents were introduced to the solution in attempts to attack the carbonyl
ligand in the Pt-CO unit at the carbon atom. The inability to add nucleophilic reagents to ligands
like CO without displacing the chelating ligand motivated us to synthesize a mixed
pyrazolyl/triazolyl ligand. For Tp'Pt derivatives (Tp' = hydridotris(3,5-
dimethylpyrazolyl)borate), strong donation from the pyrazolyl nitrogen donors keeps the ligand
bound while still allowing the κ2/κ3 interconversion.35-37
Utilizing Reger’s improved method for generating mixed donor ligands, we prepared
similar heteroscorpionates with a C-H backbone that incorporates the triazolyl functionality.
Herein, we report the synthesis and metalation of ligands that contain two pyrazolyl rings and
one triazolyl ring as well as investigations into their reactivity. Structural data from seven
monomeric platinum d6 complexes that exhibit octahedral geometries are included here for
comparison of Pt(IV) bond distances; four of these structures complement the facial tridentate
ligand in the coordination sphere with the PtMe3 fragment. The second section of our report
addresses electrophilic oxidation of Pt(II) isomers to form Pt(IV) products, and the third section
probes chloride displacement by Pt(II) as a nucleophile in reactions leading to Pt(IV) alkyl
complexes. Both C-13 and 1H NMR spectra for [(bpztzPh)Me2PtCH2CH2PtMe2(bpztzPh)] provide
definitive data pointing to the dinuclear skeleton due to the presence of Pt-195 coupling in the C-
13 spectrum and due to chirality at both platinum centers complicating the 1H splitting pattern.
2. Results and Discussion
7

2.1. Scorpionate Ligands: Comparing Tp', Tpm, TtR, POPz3 and Bpztz Geometrical Parameters
in Octahedral Pt(IV) Complexes
In the synthetic route to heteroscorpionate ligands reported by Reger,31 the final
condensation step requires a carbonyl functionality attached to the heterocycle that will
ultimately be the unique donor arm of the heteroscorpionate. We synthesized a 1,2,3-triazole
with an aldehyde at the 4-position. Employing the same conditions as were used for the copper-
catalyzed azide-alkyne cycloaddition “click” reaction that generated TtR,33,34,38 phenyl azide and
propargyl alcohol were combined to produce (1-phenyl-1H-1,2,3-triazol-4-yl)methanol 1-L1 in
66% yield (step 1 in eq 2). The analogous cyclohexyl (cy) derivative (1-L2) was generated
in 69% yield using cyclohexyl azide while the pentafluorophenyl (C6F5) derivative was
generated in 64% yield. Stahl et al. recently reported selective oxidation of primary alcohols to
the corresponding aldehyde using an aerobic Cu(I) triflate/TEMPO system.39 Utilizing similar
reaction conditions here for oxidation of the alcohol produced 1-phenyl-1H-1,2,3-triazole-4-
carbaldehyde 2-L1 (step 2 in eqn 2).
5 mol% Cu(I)OTf
5 mol% TEMPO
5 mol% bpy
1 mol% CuSO4
O
OH
H
sodium ascorbate
10 mol% NMI
OH
(2)
+
RN3
N
N
1:1 H2O: tBuOH
N
CH3CN
N
N
N
R
R
R = Ph
Cy
1-L1
1-L2
1-L3
2-L1
2-L2
2-L3
R = Ph
Cy
C6F5
C6F5
With the aldehyde compounds in hand, a heteroscorpionate ligand containing two 3,5-
dimethyl pyrazolyl donors and a 1,2,3-triazole ring was targeted. Treating a THF solution of
3,5-dimethyl pyrazole with sodium hydride resulted in deprotonation of the pyrazole NH and
evolution of hydrogen gas. The addition of thionyl chloride presumably generated the
8

bispyrazolyl sulfonyl intermediate, 3. Treating 3 with 1,2,3-triazole aldehyde 2-L1 and a
catalytic amount of cobalt chloride40-42 while refluxing overnight produced 4-(bis(3,5-dimethyl-
1H-pyrazol-1-yl)methyl)-1-phenyl-1H-1,2,3-triazole 4-L1 in 87% yield, which we will
abbreviate as bpztzPh (eq 3). The analogous bpztzCy ligand 4-L2 was produced in 56% yield
while the bpztzC6F5 ligand 4-L3 was produced in 72% yield.
O
H
CH3
H
C
N
CH3
N
O
S
H3C
CH3
H3C
N
N
1) 2 NaH
2) SOCl2
R
R
N
N
NH
N
12 mol% CoCl2
THF, reflux
N
N
N
(3)
2
N
THF, 0oC
N
N
N
N
H3C
CH3
H3C
CH3
CH3
bpztzR
R = Ph
Cy
4-L1
4-L2
C6F5 4-L3
Metalation of the bpztzR ligands was achieved by sequentially treating two equiv of
ligand 4 with one equiv of [PtMe2(SMe2)]2 and then an excess of methyl iodide was added (eq
4). The Pt(IV) iodide salt [bpztzRPtMe3][I] 5 was isolated as a pale yellow solid. In the 1H NMR
spectrum of 5-L1, two Pt-Me singlets appear in a 2:1 ratio at 1.4 and 1.3 ppm with platinum
satellites (2JPt-H = 71 Hz and 73 Hz, respectively), reflecting the presence of a molecular mirror
plane. The chemical shift values and coupling constants for these Pt-Me signals are typical for
Pt(IV)-Me complexes.43-46 The triazolyl proton in 5-L1 shifts downfield over 2 ppm (to 10.7
ppm) when the triazole ring binds to the metal center, an unusually large shift when compared to
1H NMR spectra of TtR platinum complexes.33,38 In the C-13 NMR spectrum, the one-bond
platinum-carbon coupling values for the two Pt-Me resonances are 685 Hz (trans to pyrazole)
and 706 Hz (trans to triazole) for 5-L3. These one-bond coupling constants are not good
9

indicators of bond strength; that correlation works well when hydrogen is involved, but for other
NMR active nuclei the correlation is far more complex. Coupling constants for Pt-C bonds in 5-
L1 and 5-L2 were comparable in magnitude.
I-
H
2 equiv bpztzR
xs MeI
Me
Me
C
N(tzR)
S
S
Me
Me
Me
Me
(pz)N
(pz)N
Me
Pt
Me
Pt
Me
(4)
Pt
CH2Cl2, 30 min
Me
Me
5
As a point of departure before looking at Pt-N bond lengths we note that the pKa for
pyrazole is 19.8 whereas the triazole proton is far more acidic with a pKa of 13.9. While binding
a proton six orders of magnitude more tightly may not map directly to binding a metal center,
and occurs at an alternative nitrogen, it’s reasonable to keep the proton dissociation properties of
pyrazoles and triazoles in mind as we look at structures of Pt(IV) compounds. Figure 2 shows the
various ligands that will be discussed throughout this paper.
H
N
N
N
N
pKa = 19.8
pKa = 13.9
+ H+
(5)
(6)
H
N
N
N
N
N
N
+ H+
10

H3C
H3C
H3C
CH3
CH3
CH3
H
B
H
C
O
P
CH3
N
CH3
N
CH3
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
H3C
H3C
H3C
CH3
CH3
CH3
CH3
CH3
CH3
6
c. Tpm
a. Tp'
b. POPz3
CH3
H
C
O
P
CH3
N
R
N
N
R
N
R
N
N
N
N
N
N
N
R
N
N
N
N
N
CH3
CH3
d. TtR
e. bpztzR
R = Ph
Cy
R = Ph
4-L1
4-L2
4-L3
Cy
C6F5
Figure 2. a. hydridotris(3,5-dimethylpyrazolyl)borate. b. tris(3,5-dimethylpyrazolyl)phosphine
oxide. c. tris(3,5-dimethylpyrazolyl)methane. d. tris(1-R-1H-1,2,3-triazol-4-yl)phosphine oxide
(R = phenyl, cyclohexyl). e. 4-(bis(3,5-dimethyl-1H-pyrazol-1-yl)methyl)-1-R-1H-1,2,3-triazole
(R = phenyl, cyclohexyl, pentafluorophenyl).
Figure 3 shows the x-ray structure of 5-L2 and selected bond distances and angles are
presented in Table 1. The Pt-N bond distance to the triazolyl donor (2.135 Å) is the shortest of
the three Pt-N bond lengths. The two pyrazolyl donors, with Pt-N distances of 2.18 and 2.19 Å,
appear to be weaker donors to Pt(IV) than the triazole ring. The PtMe3 unit assures that identical
trans influences will impact the three Pt-N bond lengths. The contrast between the pKa indicators
and the observed Pt-N bond distances is striking.
11

Figure 3. X-ray structure of [bpztzCyPtMe3][I] 5-L2. Ellipsoids are drawn at the 50%
probability level. Hydrogen atoms and counterion are omitted for clarity. Bond lengths (Å): Pt-
N1 2.135(4), Pt-N4 2.178(4), Pt-N6 2.193(4), Pt-C1 2.061(5), Pt-C2 2.042(5), Pt-C3 2.054(4).
Bond angles (deg): N1-Pt-N4 84.85(14), C1-Pt-N1 93.54(17), C2-Pt-C1 84.8(2).
Figure 4 shows the x-ray structure of 5-L3 and selected bond distances and angles are
presented in Table 2. The Pt-N(triazole) bond distance of 2.138 Å is again shorter than the two
Pt-N(pyrazole) distances, both 2.19 Å, reflecting stronger binding from the triazolyl ring
compared to the two pyrazoles. Comparing the Pt-N(triazole) distance to the analogous
[bpztzCyPtMe3]+ structure reveals that the addition of the pentafluorophenyl substituent did not
have a noticeable impact on the donor strength of the triazolyl ring as determined by bond
distance measurements.
12

Figure 4. X-ray structure of [bpztzC6F5PtMe3][I] 5-L3. Ellipsoids are drawn at the 50%
probability level. Hydrogen atoms and the iodide counterion are omitted for clarity. Bond
lengths (Å): Pt-N1 2.138(3), Pt-N4 2.188(3), Pt-N6 2.188(3), Pt-C20 2.042(4), Pt-C21 2.050(4),
Pt-C22 2.058(4). Bond angles (deg): N1-Pt-N4 84.95(12), C20-Pt-N1 91.61(15), C20-Pt-C21
89.67(18).
H3C
CH3
O
P
CH3
N
O
P
N
N
N
N
=
N
N
N
N
H3C
CH3
CH3
6
POPz3
+
While the two bpztzRPtMe3 structures were in good agreement with one another, the
triazole bond distances were shorter than in the analogous TtR complex with three triazole arms
13

+
for the scorpionate ligand. The average Pt-N bond length for the TtR ligand in the TtRPtMe3
complex33 was 2.18 Å, longer than the Pt-N bond to the triazole in 5-L2 and 5-L3.
Since the pentafluorophenyl group on the triazolyl arm had no significant impact on the
Pt-N bond length, we turned to an investigation of the influence of the phosphine oxide spine in
the TtR system. Comparing the new mixed tridentate ligand to the triazole based TtR ligand with
a P=O spine, it’s conceivable that the difference in binding strength of the triazole arm arises
from differences in the spine of the umbrella skeletons. Deprotonation of 3,5-dimethylpyrazole
with sodium hydride followed by addition of phosphoryl chloride resulted in the desired tris(3,5-
dimethylpyrazolyl)phosphine oxide (POPz3) 6, in 96% yield. Access to the desired Pt(IV)
complex was provided by switching from MeI to a stronger methylating reagent, methyl
trifluoromethanesulfonate (MeOTf), and extending the reaction time to form the cationic Pt(IV)
complex, [POPz3PtMe3][OTf], 7 (eq 7). Complex 7 displayed C3v symmetry in the 1H NMR
spectrum and the pyrazolyl proton showed a small coupling to phosphorus (1 Hz) while the 31P
NMR signal at -18.3 ppm exhibited 3-bond coupling to platinum (9 Hz).
O
OTf
P
Me
Me
N
2eq POPz3
xs MeOTf
N
N
Me
Me
S
S
Me
Me
Me
Me
Pt
Me
Pt
Me
Pt
(7)
CH2Cl2, 2 hrs
Me
7
Figure 5 shows the x-ray structure of 7 and selected bond distances and angles are
presented in Table 3. The three pyrazolyl Pt-N bond distances of ca. 2.20 Å are slightly
elongated from those of the analogous trimethylPt(IV)Tp' complex, which are near 2.19 Å.47 The
Pt-N distances for these pyrazolyl donors are also close to the bond lengths observed for the
triazole based TtR ligand (2.17-2.19 Å).33
14

Figure 5. X-ray structure of [POPz3PtMe3][OTf] 7. Ellipsoids are drawn at the 50% probability
level. Hydrogen atoms, trifluoromethanesulfonate counterion, and benzene molecule are omitted
for clarity. Bond lengths (Å): Pt-N1 2.201(4), Pt-N3 2.199(4), Pt-N5 2.185(5), Pt-C1 2.041(6),
Pt-C2 2.047(5), Pt-C3 2.052(4). Bond angles (deg): N1-Pt-N3 88.2(2), C1-Pt-N3 92.6(3), C2-Pt-
C1 89.0(2).
H3C
CH3
H
C
CH3
N
H
C
N
N
N
N
=
N
N
N
N
H3C
CH3
CH3
Tpm
The phosphine oxide backbone did not significantly change the Pt-N bond lengths so we
turned to the C-H backbone in the well-known Tpm ligand, just as the C-H moiety is present in
the mixed bpztzR set of ligands. Employing Tpm31 under the same conditions as used for the
15

preparation of POPz3 complex 7 generated the desired cationic Pt(IV) complex,
[TpmPtMe3][OTf], 8, eq 8.
OTf
H
Me Me
S
C
2 equiv Tpm
xs MeOTf
N
Me
Me
Me
Me
N
Me
Me
Pt
Pt
Pt
(8)
S
CH2Cl2, 30 min
N
Me
Me
Me
8
Figure 6 shows the x-ray structure of one of the three independent molecules found in the
unit cell, and selected bond distances and angles are presented in Table 4. The Pt-N bond
distances in this Tpm complex average 2.18 Å, slightly shorter than those observed in trimethyl
Pt(IV) complexes with the other neutral ligands, namely TtR and POPz3, presumably due to the
more electron donating C-H backbone. The anionic Tp' ligand leads to a neutral Pt(IV) complex
while all the other complexes reported here are cationic Pt(IV) compounds. Despite the slightly
shorter bond lengths of the TpmPt complex compared to the mixed ligand system, the Pt-N
bonds found in Tpm complex 8 were about 0.02 Å longer than the Pt-N distances for the triazole
arm in the bpztz hybrid ligand system.
16

Figure 6. X-ray structure of [TpmPtMe3][OTf] 8. Ellipsoids are drawn at the 50% probability
level. Hydrogen atoms and trifluoromethanesulfonate counterion are omitted for clarity. Bond
lengths (Å): Pt-N29 2.187(6), Pt-N36 2.189(6), Pt-N43 2.162(6), Pt-C26 2.044(7), Pt-C27
2.072(8), Pt-C28 2.049(8). Bond angles (deg): N29-Pt-N36 83.0(2), C26-Pt-N36 93.3(3), C26-Pt-
C27 88.4(3).
2.2 Electrophilic Addition: Isomer Preferences for d8 Pt(II) and d6 Pt(IV)
Bidentate binding of the bpztzPh ligand to form a square planar dimethylPt(II) complex
could form two isomers. The ligand could bind either with one free pyrazole arm or with both
pyrazole rings bound and with the triazole ring free. If these two isomers were energetically
equivalent and isomerization were easily accessible, a statistical 1:2 ratio favoring the
asymmetrical isomer should result. The addition of two equivalents of bpztzPh to one equivalent
of [PtMe2(SMe2)]2 in tetrahydrofuran initially displayed a 1:1 ratio of the symmetric to
asymmetric isomers in the 1H NMR. After 30 minutes a preference (3:1) for the symmetric (κ2-
17

bpztzPh)Pt(CH3)2 isomer with the triazolyl arm free was evident. This ratio increased to 5:1 after
90 minutes. This result implies that in the square planar Pt(II) case, the triazole arm binds less
tightly than the pyrazole arms, a different conclusion than was drawn above for Pt(IV) based on
solid state Pt-N bond distances. The difference in Pt(II) d8 versus Pt(IV) d6 binding preferences
seemed poised to promote oxidative addition to the metal center: the triazole arm would be non-
coordinated in the Pt(II) state then stabilize the high valent Pt(IV) by donating electron density to
the metal center and completing a six-coordinate geometry.
Mimicking previous results with Tp and TtR, we synthesized a Pt(IV) hydride complex
by protonation of the Pt(II) intermediate. Metalation of the bpztzPh ligand followed by low
temperature addition of tetrafluoroboric acid resulted in the desired complex,
[bpztzPhPtMe2H][BF4], 9 (eq 9). The 1H NMR spectrum of the hydride product displayed two
isomers in a 2:3 ratio with the symmetric complex as the major isomer. The major isomer has the
hydride trans to the triazole arm and displays 1JPt-H = 1424 Hz, while the minor asymmetric
isomer has 1JPt-H = 1480 Hz. Both 1JPt-H values are typical coupling constants for Pt(IV) hydride
complexes. 37,48,49
The more stable symmetrical isomer of the square planar Pt(II) intermediates with both
pyrazole arms bound is presumably trapped by the oxidation of the Pt center as the acid adds to
platinum. This places the hydride trans to the triazolyl ligand. However, the hydride is a stronger
donating ligand than the methyl groups so it would prefer to bind trans to a pyrazole arm.
Isolation of the symmetric Pt(IV) hydride complex demonstrates that once the Pt(IV) complex is
formed, the barrier to isomerization increases significantly relative to the Pt(II) barrier. Under the
conditions we utilized, isomerization of the mixed ligand only occurs at the Pt(II) d8
configuration before the complex is oxidized to Pt(IV).
18

BF4
BF4
H
H
C
C
Me
Me
2 equiv bpztzPh
HBF4
N(pz)
Pt
N(tz)
Pt
S
S
(tz)N
(pz)N
(pz)N
(pz)N
Me
Me
Me
Me
Me
Me
Me
Me
Pt
Me
Pt
Me
(9)
CH2Cl2, 30 min
-78oC
H
H
9
2:3 Ratio
Similar oxidation of Pt(II) to Pt(IV) addition reactions can be observed with other
electrophiles. As shown in equation 10, the addition of acyl chloride to a solution of bpztzPh and
[PtMe2(SMe2)]2 results in a cationic Pt(IV) acyl complex. Anion exchange of chloride to triflate
to generate [bpztzPhPtMe2(C(O)CH3)][OTf], 10, was accomplished by addition of MeOTf. Only
the symmetric isomer with the triazole donor arm trans to the acyl ligand was observed in the 1H
NMR spectrum. We attribute this to the acyl chloride reacting relatively slowly with the Pt(II)
complex. Slow electrophilic addition of the acylchloride would allow isomerization at the Pt(II)
stage and thus the symmetric isomer could be selectively trapped.
OTf
H
1) 2 equiv bpztzPh
C
Me
Me
Acyl chloride
N(tz)
Pt
S
S
(pz)N
(pz)N
CH2Cl2, 3 hours
Me
Me
Me
Me
Me
Me
(10)
Pt
Me
Pt
Me
2) MeOTf
CH2Cl2, 30 min
O
Me
10
An x-ray structure of 10 was obtained to confirm the geometry of the complex. However,
due to disorder between the acyl oxygen and methyl group of the acyl ligand the geometrical
parameters have been relegated to supporting information.
Comparable acyl chemistry was observed with Tpm as the tridentate ligand. The addition
of acyl chloride to a solution of Tpm and [PtMe2(SMe2)]2 followed by exchange of the anion
results in the anticipated [TpmPtMe2(C(O)CH3)][OTf], 11, (eq 11). The acetyl carbon in the 13C
19

NMR spectrum is seen at 191.5 ppm with 1-bond platinum coupling of 894 Hz, slightly smaller
than the coupling seen for complex 10 (ca 912 Hz).
OTf
H
1) 2 equiv Tpm
Acyl chloride
2) MeOTf
Me Me
S
C
N
Me
Me
Me
Me
N
Me
Me
Pt
Pt
Pt
(11)
S
CH2Cl2, 30 min
N
Me
Me
O
Me
11
Figure 7 shows the x-ray structure of a monomeric unit of 11 and selected bond distances
and angles are presented in Table 5.
Figure 7. X-ray structure of [TpmPtMe2C(O)CH3][OTf] 10. Ellipsoids are drawn at the 50%
probability level. Hydrogen atoms and trifluoromethanesulfonate counterion are omitted for
clarity. Bond lengths (Å): Pt-N6 2.188(5), Pt-N13 2.184(4), Pt-N20 2.170(4), Pt-C1 2.047(5), Pt-
C2 2.024(8), Pt-C3 2.075(12), C3-O 1.197(10). Bond angles (deg): N6-Pt-N20 82.52(15), C1-Pt-
N6 92.4(2), C3-Pt-N20 166.6(5), O-C3-Pt 130.3(12).
20

2.3 Pt(II) Reactions with alkyl chloride solvents
In the absence of electrophilic reagents, a solution of bpztzPh and [PtMe2(SMe2)]2 in
dichloromethane results in cleavage of a C-Cl bond of the solvent to form cationic complex
[bpztzPhPtMe2(CH2Cl)]+, 12, with Cl- as the leaving group. The addition of tetrafluoroboric acid
at low temperature effected anion exchange to produce [bpztzPhPtMe2CH2Cl][BF4], equation 12.
In the 1H NMR spectrum of 12, a single resonance for both the Pt-CH2Cl protons at 4.36 ppm
(2JPt-H = 50 Hz) along with one resonance for the pyrazolyl-protons at 6.06 ppm, indicate Cs
symmetry. A 2D NMR HSQC spectrum displays crosspeaks between the protons at 4.36 ppm
and a carbon at 20.3 ppm in the 13C NMR. The peak at 20.3 ppm in the 13C NMR displays 841
Hz coupling to Pt, indicative of a one-bond platinum carbon bond, confirming a direct Pt-C bond
as opposed to the solvent binding through a chlorine atom of dichloromethane. 50-52
BF4
H
1) 2 equiv bpztzPh
C
Me
Me
N(tz)
Pt
CH2Cl2
4 hrs
S
S
(pz)N
(pz)N
Me
Me
Me
Me
Me
Me
Pt
Me
Pt
Me
(12)
2) HBF4
-78oC
Cl
30 mins
12
Figure 8 shows the x-ray structure of a single unit of 12, and selected bond distances and
angles are presented in Table 6. The metal center adopts a slightly distorted octahedral geometry.
The Pt-C1-Cl angle through the chloromethyl ligand is 114.8°, surely distorted from the ideal
tetrahedral bond angle (109.5°) due to steric effects. The carbon metal bond to the methyl
chloride ligand (Pt-C1) is the shortest of the Pt-C bonds, at 2.026 Å, consistent with expectations
for an electron withdrawing group on the methyl ligand.
21

Figure 8. X-ray structure of [bpztzPhPtMe2CH2Cl][BF4] 12. Ellipsoids are drawn at the 50%
probability level. Hydrogen atoms and counterion are omitted for clarity. Bond lengths (Å): Pt-
N5 2.110(4), Pt-N16 2.196(4), Pt-N23 2.187(4), Pt-C1 2.026(5), Pt-C2 2.056(5), Pt-C3 2.048(5),
C1-Cl 1.775(6). Bond angles (deg): N5-Pt-N16 83.67(13), C1-Pt-N16 95.64(19), C1-Pt-C2 87.9(2),
Cl-C1-Pt 114.8(3).
Intrigued by the proclivity of Pt(II) to break C-Cl bonds, we investigated the possibility
of the other tridentate ligands facilitating similar reactivity. 53-55 No reactivity was observed with
the POPz3 ligand after a week of stirring in dichloromethane. Due to the low solubility of KTp'
in dichloromethane, we first formed the κ2Pt(II) complex [K][κ2Tp'PtMe2] in tetrahydrofuran,
then added dichloromethane to the reaction mixture. Again, no oxidative addition of methylene
chloride was observed. In contrast, the reaction of Tpm and [PtMe2(SMe2)]2 in dichloromethane
resulted in an analogous complex, [TpmPtMe2(CH2Cl)][Cl] 13, after 24 hours, equation 13.
22

Complex 13 shows a peak in the 1H NMR spectrum at 4.34 with 49 Hz platinum coupling for the
chloromethyl ligand.
Cl
H
C
Me
Me
N
S
S
N
N
Me
Me
Me
Me
Me
2 equiv Tpm
Pt
Pt
Me
Pt
Me
(13)
Me
CH2Cl2
24 hrs
Cl
13
Figure 9 shows the x-ray structure of a monomeric unit of 13 and selected bond distances
and angles are presented in Table 7. The Pt-CH2Cl carbon bond length is again shorter than the
Pt-C distances to the two methyl ligands, but it is elongated by 0.013 Å compared to the distance
in the bpztzPh complex, 12.
23

Figure 9. X-ray structure of [TpmPtMe2CH2Cl][Cl] 13. Ellipsoids are drawn at the 50%
probability level. Hydrogen atoms and counterion are omitted for clarity. Bond lengths (Å): Pt-
N1 2.147(3), Pt-N3 2.109(4), Pt-N5 2.208(4), Pt-C1 2.039(7), Pt-C2 2.046(6), Pt-C3 2.060(7),
C1-Cl 1.775(6). Bond angles (deg): N1-Pt-N5 86.23(14), C1-Pt-N1 93.7(2), C1-Pt-C2 88.9(3), Cl-
C1-Pt 114.8(3).
We next attempted the reaction of 1,2-dichloroethane with a Pt(II) precursor. Stirring a
solution of bpztzPh and [PtMe2(SMe2)]2 in 1,2-dichloroethane for three days results in the
formation of a PtCH2CH2Cl entity. Addition of MeOTf to the complex exchanged the anion to
the noncoordinating triflate. A secondary anion exchange was accomplished through the addition
of sodium tetrakis[(3,5-trifluoromethyl)phenyl]borate (NaB(ArF)4) and led to the isolation of
[bpztzPhPtMe2CH2CH2Cl][B(ArF)4], 14, (eq 14). In the 1H NMR spectrum of 14, the major
isomer is the symmetric product, with the triazole donor trans to the chloroethyl ligand. The two
protons bound to the carbon alpha to the metal center appear as a distinctive 8-line pattern at 2.81
ppm. Further downfield at 3.38 ppm is a similar splitting pattern corresponding to the protons
bound to the beta carbon of the PtCH2CH2Cl fragment, reflecting the second order AAXX
pattern typical of XCH2CH2Y entities.
B(ArF)4
H
1) 2 equiv bpztzPh
C
Me
Me
N(tz)
Pt
ClCH2CH2Cl
3 days
S
S
(pz)N
(pz)N
Me
Me
Me
Me
Me
Me
Pt
Me
Pt
Me
(14)
2) MeOTf
3) NaB(ArF)4
Cl
14
We attempted analogous chemistry with 1,2-dichloroethane and other homoscorpionate
ligands to no avail. The ability to activate the C-Cl bond at one end of 1,2-dichloroethane
demonstrates the unusual reactivity of this asymmetrical bpztz tridentate ligand.
24

In an effort to promote elimination from the Pt-CH2CH2Cl fragment of complex 14 we
added a base, sodium methoxide. We anticipated the methoxide would deprotonate the carbon
alpha to Pt and perhaps lead to elimination to form a Pt-vinyl ligand. Alternatively methoxide
could simply displace the chloride to form a Pt-CH2CH2OCH3 ligand. The addition of sodium
methoxide to cationic complex 14 in THF resulted in the precipitation of a white solid containing
platinum. The solid, sparingly soluble in dichloromethane, reacted with NaB(ArF)4 and led to the
isolation of a dicationic, dinuclear product, [(bpztzPh)Me2PtCH2CH2PtMe2(bpztzPh)][B(ArF)4]2,
15, eq 15.
B(ArF)4
H
C
2
N(pz)
Pt
B(ArF)4
H
(tz)N
(pz)N
Me
Me
C
N(tz)
Pt
1) NaOMe
(pz)N
(pz)N
Me
Me
THF, overnight
(15)
2) NaB(ArF)4
N(pz)
Me
Me
Pt
N(tz)
C
N(pz)
Cl
H
14
15
The 1H NMR spectrum of 15 was complex due to second order effects so we begin by
describing the definitive C-13 NMR spectrum which led to unambiguous identification of the
PtCH2CH2Pt unit. The 13C NMR spectrum indicates the presence of a dinuclear complex
containing a –CH2CH2– bridge: a single peak centered at 20 ppm exhibits two sets of Pt
satellites, one having 1JPt-C = 622 Hz and one having 2JPt-C = 31 Hz. Note that complex 15 proved
challenging to identify by 1H NMR since “apparent” quartets at 2.92 and 2.06 ppm were
misleading. During the formation of 15, one of the tridentate ligands isomerized, resulting in a
pyrazole arm trans to the alkyl bridge, making each of the two Pt centers chiral. The bridging
ethylene ligand then becomes an AXAX system, but instead of the AA hydrogens residing on
the same carbon as in the XCH2CH2Y case, the A and A hydrogens are on Cα and Cβ,
25

respectively, as are X and X. These pairwise diastereotopic protons resemble overlapping
doublets that are roofed, appearing roughly as quartets. The Pt satellites are an additional
complication and there are observable satellite peaks due to the proton seeing a spin active
platinum center both 2 and 3 bonds away (although not in the same molecule, figure 10a).
Simulations supported our analysis, with a geminal proton coupling of -9.5 Hz and vicinal
couplings of 5.0 and 9.0 Hz. We were also able to replicate the platinum satellites through
simulations of an AXAXM system, calculating 2JPt-H and 3JPt-H couplings of 70 and 52 Hz
respectively. Overlaying the simulated spectrum allows for easy comparison to the NMR spectra
(figure 10b).
a. 1H NMR spectra
b. simulation of the bridging dimer
protons
Figure 10. Observed and simulated spectra of the bridging ligand for complex 15. Calculated
with a geminal coupling constant of -9.5 Hz, vicinal coupling constants of 5.0 and 9.0 Hz, two
bond platinum proton coupling constant of 70 Hz and three bond coupling constant of 52 Hz.
26

Preparation of a complex with two bridging methylene units between platinum centers is
rare, and we probed the importance of the base by adding sodium amide to complex 14. The
result was the same product, indicating that the identity of the base is unimportant in forming the
dinuclear product. We propose that the base reacts with the Pt(IV) starting material to
reductively eliminate the ethylchloride ligand, perhaps by simply attacking the alpha carbon to
form MeOCH2CH2Cl and the reduced Pt complex. This κ2-bpztzPhPtMe2 reagent could then react
with the C-Cl bond of a second molecule of 15. Indeed the addition of Pt(IV) complex 14 to a
solution of κ2-bpztzPhPt(II)Me2 in THF resulted in precipitation of 15, eq 16.
B(ArF)4
H
C
2
N(pz)
Pt
B(ArF)4
H
H
(tz)N
(pz)N
Me
Me
C
C
N(tz)
Pt
N(tz)
Pt
(pz)N
(pz)N
(pz)N
(pz)N
Me
Me
Me
Me
1) THF, overnight
2) NaB(ArF)4
(16)
N(pz)
Me
Me
Pt
N(tz)
C
N(pz)
Cl
H
14
15
3. Conclusion
A series of asymmetrical scorpionate ligands bearing two pyrazolyl rings and one
triazolyl ring has been synthesized using a cobalt-catalyzed condensation reaction. Metalation of
these ligands to generate neutral Pt(II) complexes followed by subsequent methylation results in
stable Pt(IV) complexes. Solid-state structural data reveal that the platinum-nitrogen distance to
the triazolyl moiety is shorter than the two platinum-nitrogen pyrazolyl distances, indicating that
the triazolyl ring is a stronger donor to Pt(IV). An electron withdrawing substituent on the
triazole arm does not influence the ligand donor strength as assessed by Pt-N distances. While
27

Pt(IV) in cationic complexes binds the triazole arm more tightly than the pyrazole arms, the
dimethylPt(II) fragment favors binding only the pyrazole arms rather than the triazole arm. The
(bpztzPh)PtMe2 d8 Pt(II) complex displaces chloride from methylene chloride rather than
promoting C-H activation as was reported for various Tp complexes.56 The ability to displace
chloride from chlorinated solvents led to the synthesis of an unusual dicationic Pt(IV) dimer
bridged by an ethylene ligand.57-59
4. Experimental Section
Materials and Methods. All reactions were performed under an atmosphere of dry
nitrogen using standard Schlenk and drybox techniques. Nitrogen was purified by passage
through columns of BASF R3-11 catalyst and 4 Å molecule sieves. Dichloromethane, dimethyl
ether, tetrahydrofuran (THF), and pentane were purified under an argon atmosphere and passed
through a column packed with activated alumina. All other chemicals were used as received
without further purification. Silica column chromatography was conducted with 230-400 mesh
silica gel and alumina column chromatography was conducted with 80-200 mesh alumina.
1H, 19F, 31P, and 13C NMR spectra were recorded on Bruker DRX400, AVANCE500, or
ULTRA SHIELD CRYOQNP600 spectrometers. 1H NMR and 13C NMR chemical shifts were
referenced to residual 1H and 13C signals of the deuterated solvents. Elemental analyses were
performed by Robertson Microlit Laboratories of Madison, NJ.
[bpztzPhPtMe3][I] 5-L1. A 50-mL Schlenk was charged with 0.030 g (0.05 mmol) of
[PtMe2(SMe2)]2 and 0.036 g (0.10 mmol) of 4-L1. The flask was purged with nitrogen and
CH2Cl2 (10 mL) was added through the septum. The yellow solution was treated with 20 uL
(0.30 mmol) of methyl iodide and stirred for 30 min at room temperature. The solvent was
removed via rotary evaporation and the resulting oil was triturated with pentane to afford 5-L1 as
28

1
a pale yellow solid in quantitative yield. H NMR (δ, CD2Cl2, 298 K): 10.67 (s, 1H, tz H), 8.85
(s, 1H, HC(pz)2(tz)), 7.84 (d, 2H, o-Ph), 7.55 (m, 3H, m-Ph and p-Ph), 6.00 (s, 2H, pz H), 2.83,
2.32 (s, 3H each, pz CH3), 1.42 (s, 6H, 2JPt-H = 71 Hz, Pt-CH3), 1.32 (s, 3H, 2JPt-H = 73 Hz, Pt-
CH3) . 13C NMR (δ, CD2Cl2, 298 K): 152.8, 142.8 (pz C-CH3), 139.4 (tz 4-position), 136.4
(ipso-Ph), 130.5, (p-Ph), 130.4 (m-Ph), 126.2 (tz 5-position), 121.1 (o-Ph), 108.8 (pz CH), 61.4
(HC(pz)2(tz)), 13.2, 13.1 (pz CH3), -8.0 (1JPt-C = 688 Hz, Pt-CH3), -10.7 (1JPt-C = 701 Hz, Pt-
CH3). Anal Calcd for C22H30N7IPt: C, 36.82; H, 4.23; N, 13.72. Found: C, 35.97; H, 4.50; N,
13.39.
[bpztzCyPtMe3][I] 5-L2. A 50-mL Schlenk was charged with 0.030 g (0.05 mmol) of
[PtMe2(SMe2)]2 and 0.037 g (0.10 mmol) of 4-L2. The flask was purged with nitrogen and
CH2Cl2 (10 mL) was added through the septum. The yellow solution was treated with 20 μL
(0.30 mmol) of methyl iodide and stirred for 30 min at room temperature. The solvent was
removed via rotary evaporation and the resulting oil was triturated with pentane to afford 5-L1 as
a pale yellow solid in quantitative yield. Crystal Data. C22H36IN7Pt, M =720.57, triclinic, a =
10.0223(4) Å, b = 10.4114(4) Å, c = 13.6904(5) Å, α = 94.156(2)°, β = 109.859(2)°, γ =
107.026(2)°, V = 1261.00(8) Å3, T = 100, space group P-1 (no. 2), Z = 2, μ(CuKα) = 20.192,
21695 reflections measured, 4338 unique (Rint = 0.0471) which were used in all calculations. The
final wR2 was 0.0721 (all data) and R1 was 0.0286 (I>2σ(I)). 1H NMR (δ, CD2Cl2, 298 K): 10.10
(s, 1H, tz H), 8.73 (HC(pz)2(tz)), 5.98 (s, 2H, pz H), 4.50 (tt, 1H, CA-H), 2.83, 2.29 (s, 3H each,
pz CH3), 2.33-1.25 (11H, cyclohexyl), 1.34 (s, 6H, 2JPt-H = 72 Hz, Pt-CH3), 1.25 (s, 3H, 2JPt-H
=
70 Hz, Pt-CH3). 13C NMR (δ, CD2Cl2, 298 K): 152.7, 142.6 (pz C-CH3), 138.6 (tz 4-position),
126.2 (tz 5-position), 108.7 (pz CH), 62.6 (CA), 61.7 (HC(pz)2(tz)), 33.4 (CB), 25.2 (CC and CD),
29