Intramolecular C-H activation by air-stable Pt(II) phosphite complexes

Article abstract of DOI:10.1016/j.jorganchem.2015.09.031

Air-stable L₂PtMe₂ complexes ligated by bulky aryl phosphite ligands were synthesized and characterized. The aryl phosphite ligands were prepared from substituted phenols and PCl₃ or P(NMe₂)₃ without

Full text of DOI:10.1016/j.jorganchem.2015.09.031

Accepted Manuscript
Intramolecular C–H Activation by Air-Stable Pt(II) Phosphite Complexes
Kyle A. Grice, Jason A. Kositarut, Anthony E. Lawando, Roger D. Sommer
PII:
S0022-328X(15)30160-1
10.1016/j.jorganchem.2015.09.031
JOM 19246
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 10 August 2015
Revised Date: 23 September 2015
Accepted Date: 24 September 2015
Please cite this article as: K.A. Grice, J.A. Kositarut, A.E. Lawando, R.D. Sommer, Intramolecular C–H
Activation by Air-Stable Pt(II) Phosphite Complexes, Journal of Organometallic Chemistry (2015), doi:
10.1016/j.jorganchem.2015.09.031.
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Intramolecular C–H Activation by Air-Stable Pt(II) Phosphite Complexes
Kyle A. Grice*a, Jason A. Kositaruta, Anthony E. Lawandoa, Roger D. Sommerb
a Department of Chemistry, DePaul University, 1110 West Belden Ave, Chicago, IL USA 60614
b X-ray crystallography facility, North Carolina State University, Raleigh, NC USA 27695
* Corresponding author. E-mail address: kgrice1@depaul.edu
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ABSTRACT: Air-stable L2PtMe2 complexes ligated by bulky aryl phosphite ligands were synthesized
and characterized. The aryl phosphite ligands were prepared from substituted phenols and PCl3 or
P(NMe2)3 without the use of an external base. The steric environment of the phenol controlled the
number of substitutions at P(NMe2)3, resulting in selective formation of P(NMe2)(O-2,6-Me2C6H3)2.
The phosphite ligands and their corresponding platinum(II) complexes were found to be air stable and
robust. X-ray diffraction structures of the platinum(II) complexes were obtained in order to determine
the steric environment around the metal centers. Thermolysis of the L2PtMe2 complexes in C6D6 at
100°C results in loss of methane and formation of one major platinum-containing product in high yield.
The products of thermolysis were found to be a result of cyclometalation by C−H activation of a sp3
C−H bond in the ligand to form a 6-membered ring, as confirmed by X-ray crystallography.
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Keywords Organoplatinum(II) complexes; Phosphite ligands; X-ray crystallography; Cyclometallation
Highlights
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Bulky aryl phosphite ligands were synthesized without an external base
L2PtMe2 complexes of bulkyl phosphites were synthesized and characterized
Thermolysis led to cyclometallation, as confirmed by X-ray crystallography
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1. Introduction
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Platinum complexes are effective catalysts for a variety of reactions, including C−H bond
functionalization [1]. Alkane C−H bond functionalization is particularly useful because it can be used to
transform inert bonds into valuable and versatile function groups such as olefins. Platinum complexes
capable of C−H bond activation are commonly ligated by nitrogen donor ligands [2], and alkane
functionalization reactions such as catalytic alkane transfer dehydrogenation have been reported [3] The
C−H activations that occur at the complexes supported by nitrogen-based ligands generally occur from
the Pt(II) oxidation state. In comparison, C−H activation by phosphine-ligated Pt(II) complexes is
relatively rare and is generally observed at sp2 C−H bonds [4-10].
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There is one report in the literature by Shinoda and coworkers wherein acceptorless cyclooctane
dehydrogenation was observed at phosphite-ligated platinum complexes (Scheme 1) [11]. The reported
complexes were coordinated by trialkyl phosphite ligands and trichlorostannate ligands. These two
types of ligands are uncommon in C−H activation, but have been studied for hydrogenation and
hydroformylation on platinum and other metals [12-14]. In the report, a turnover number (TON) of 10
was observed for (P(OMe)3)3Pt(SnCl3)2. Turnover frequencies were not measured and catalyst
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decomposition was also observed, which clearly limits the utility of the reaction. The decomposition
could be due to Michaelis-Arbuzov decomposition of the alkyl phosphites (Scheme 2) that has been
observed to occur readily at very similar platinum complexes [15, 16].
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Scheme 1. Acceptorless alkane dehydrogenation reported by Shinoda and coworkers.
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Scheme 2. Michaelis-Arbuzov decomposition of alkyl phosphite ligands on platinum(II).
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We seek to develop ligands that could be used to stabilize the platinum complexes and thus increase
TON of the acceptorless alkane dehydrogenation reaction. Towards this end, we have explored aryl
phosphites, which would not be capable of Michaelis-Arbuzov dealkylation. In addition, aryl phosphites
are more π-accepting than alkyl phosphites [17], which may have beneficial effects in the activity of
catalytic reactions. Similar bulky phosphite ligands have been used previously in hydroformylation and
other catalytic transformations with rhodium [18]. Triphenylphosphite (P(OPh)3) can be cyclometallated
at Pt(II) [19], and therefore substituents were added to the 2 and 6 positions on the aryl rings to obviate
that unwanted reactivity. Herein we describe the synthesis and characterization of platinum(II)
complexes of sterically bulky aryl phosphite ligands as well as their thermal reactivity.
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2. Results and Discussion
2.1. Ligand Syntheses
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Triaryl phosphite species can be synthesized via reaction of phenols with PCl3, often in the presence of a
base. However, we were curious if the reaction could be simplified or if the toxic and caustic PCl3 could
be avoided. One strategy that has been used in the literature is the reaction of HOR groups with a P–N
bond [20]. Therefore, 2,6-dimethylphenol was reacted with P(NMe2)3, a phosphine source that possesses
internal bases (the formally anionic NMe2 amide groups). One product was obtained by heating
P(NMe2)3 in the presence of excess 2,6-dimethylphenol (>3 equiv.). This product possessed a 1H NMR
spectrum consistent with the di-substituted ligand P(NMe2)(O-2,6-Me2C6H3)2 (L1) (Scheme 3).
Presumably, the steric hindrance around the phosphorus center due to the two substituted phenoxy
groups prevents the third phenoxy group from being installed. The less sterically hindered o-cresol has
been heated with P(NMe2)3 to form P(O-2-MeC6H4)3 [21]. Ligand L1 possesses one signal in the
31P{1H} NMR spectrum at δ = 143.19 ppm. The ligand was also characterized by elemental analysis and
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X-ray crystallography. The crystal structure of L1 is shown in Figure 1 and confirms the assignment
based on spectroscopy. In the structure of L1 the phosphorus is highly pyramidalized, whereas the
nitrogen is nearly trigonal planar (with C−N−C and P−N−C angles of 115, 119, and 123º), consistent
with delocalization of the nitrogen lone pair into an antibonding orbital with P−O character [22].
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Scheme 3. Synthesis of aryl phosphite ligands L1 and L2.
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Fig. 1. Molecular structure of L1 obtained from X-ray crystallography with ellipsoids shown at 50%
probability level and hydrogen atoms omitted for clarity. Selected bond lengths (Å) and bond angles (º):
P1–N1 = 1.6492(11), P1–O1 = 1.6590(7), P1–O2 = 1.6586(7); N1–P1–O1 = 98.20(5), O1–P1–O2 =
94.32(3), O2–P1–N1 = 102.93(5).
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Notably, directly reacting PCl3 and 2,6-dimethylphenol without a base leads to the previously reported
tris-substituted P(O-2,6-Me2C6H3)3 (L2) (Scheme 3). The increased reactivity of the P−Cl group as
compared to the P−NMe2 likely allows the third substitution to happen in this case. Both ligands L1 and
L2 were isolated as crystalline solids that were stable to air over the course of 18 months at room
temperature as judged by NMR spectroscopy.
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Reacting the even more sterically hindered 2,6-diisopropylphenol with PCl3 without a base did not
proceed to the tri-substituted phosphite, but rather yielded PCl(O-2,6-iPr2C6H3)2, as determined by
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31P{1H} NMR spectroscopy (δ = 173.8 ppm, similar to literature data [23]). The known compound
P(O-2,6-iPr2C6H3)3 (L3) was synthesized according to literature methods using triethylamine and PCl3
[24]. Based on these observations, the direct synthesis of phosphite ligands without an external base
from a phenol and PCl3 or P(NMe2)3 is governed by steric hindrance around the phenol and the
phosphorus as the substitutions occur. Further studies are ongoing in our laboratory to use this approach
for the selective modular synthesis of P(OR)3 ligands with different R groups on the same ligand.
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2.2 Syntheses of Platinum Complexes
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With the phosphine ligands in hand, they were combined with soluble platinum(II) starting materials.
Reactions with the commonly used chloride starting materials (COD)PtCl2 (COD = 1,8-cyclooctadiene)
or (SEt2)2PtCl2 led to multiple species as determined by 31P{1H} NMR spectra of reaction mixtures, and
purification of the reactions were not attempted. However, reactions of the dimeric starting material
[(SEt2)PtMe2]2 with > 2 equivalents of L1 or L2 at 60 ºC led to one major species, 1 and 2 respectively
(Scheme 4). Complexes 1 and 2 were both obtained as white, air-stable products. The [(SEt2)PtMe2]2
compound was selected as a starting material because it is more thermally robust than the related
[(SMe2)PtMe2]2 [25]. Complexes 1 and 2 were characterized by 1H and 31P{1H} NMR spectroscopy,
elemental analysis, and X-ray crystallography (Figures 2 and 3). The X-ray crystal structure of 1
displays C2 symmetry with the dimethylamino groups anti to each other and verifies a the cis ligand
structure around the square planar Pt(II). The ligand P–O bond lengths have shortened, but the P–N
bond and the angles around the nitrogen in 1 are similar to those in L1. The geometry around the
platinum center in 2 is similar to that of 1, but with a larger P–Pt–P angle, likely due to the increased
steric bulk of L2.
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Scheme 4. Synthesis of L2PtMe2 species 1 and 2.
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Fig. 2. Molecular structure of 1 obtained from X-ray crystallography with ellipsoids shown at 50%
probability level and hydrogen atoms omitted for clarity. The two sides of the molecule are related by a
C2 axis. Selected bond lengths (Å) and bond angles (º): Pt–C1 = 2.098(2), Pt–P1 = 2.2596(5), P1–O1 =
1.6114(14), P1–O2 = 1.6400(15), P1–N1 = 1.646(2); C1–Pt–C1’ = 82.38(14), C1–Pt–P1 = 88.15(7),
P1–Pt–P1’ = 102.06(3).
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P1
Pt
P2
C2
C1
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Fig. 3. Molecular structure of 2 obtained from X-ray crystallography with ellipsoids shown at 50%
probability level and hydrogen atoms omitted for clarity. Selected bond lengths (Å) and bond angles (º):
Pt–P1 = 2.2505(9), Pt–P2 = 2.2471(9), Pt–C1 = 2.106(3), Pt–C2 = 2.099(3); P1–Pt–P2 = 104.40(3), P2–
Pt–C2 = 86.17(10), C2–Pt–C1 = 82.15(13), C1–Pt–P1 = 87.68(10).
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(P(OPh)3)2PtMe2 has previously been synthesized [26], and the NMR spectral data was compared with
the data for 1 and 2. In solution, complexes 1, 2, and (P(OPh)3)2PtMe2 all have 31P{1H} NMR shifts in
similar locations (130 with 1JPt−P = 3054 Hz and 118 ppm with 1JPt−P = 3158 Hz in C6D6 for 1 and 2,
respectively, and 120 ppm with 1JPt−P = 3036 Hz for (P(OPh)3)2PtMe2 in CDCl3). The similar coupling
constants indicate that the P−Pt bonding is similar among all of the complexes. These Pt−P coupling
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constants are higher than those observed at phosphine-based L2PtMe2 complexes because the phosphites
have additional bonding interactions from the back-bonding of the platinum d-orbitals in the P−O
antibonding orbitals [27-29], The proton NMR spectra of complexes 1 and 2 both exhibit similar
features: broad signals for the CH3 groups on the ligands and a triplet with 195Pt satellites for the Pt−CH3
signals. The coupling constants of the Pt−Me groups in 1 and 2 are similar to (P(OPh)3)2PtMe2 (2JPt−H
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70.6 Hz in CDCl3).
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Attempts were made to synthesize the corresponding (L3)2PtMe2 complex. However, upon heating of L3
with [(SEt2)PtMe2]2 in C6D6 at 60 °C for 14 h, only one major platinum-containing species was
observed by 1H NMR spectroscopy. The NMR spectra were consistent with a (L3)(SEt2)PtMe2 species;
a coordinated SEt2 group as well as two different Pt−Me groups were observed in the 1H NMR spectrum
(both were d with 195Pt satellites: one at 0.43 ppm with 2JPt-H = 85.9 Hz and 3JP-H = 8.9 Hz as well as one
at 0.82 ppm with 2JPt-H = 65.6 Hz and 3JP-H = 10.9 Hz) and a Pt−P group was observed in the 31P{1H}
NMR spectrum (114.78 ppm with 195Pt satellites,1JP-Pt = 3296 Hz). Continued heating of this sample
lead to decomposition of the platinum species, as evidenced by black material plating on the sides and
bottom of the reaction vessel. Therefore, it was concluded that L3 was too sterically hindered to allow
formation of a (L3)2PtMe2 complex and the (L3)(SEt2)PtMe2 complex was not thermally stable towards
decomposition at this temperature (Scheme 5).
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Scheme 5. Attempted synthesis of (L3)2PtMe2.
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2.3 Thermolysis Experiments of L2PtMe2 species
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With pure samples of 1 and 2 in hand, their thermal stability was explored. When samples of 1
or 2 were heated at 100 ºC in C6D6 in sealed medium-walled J.-Young style NMR tubes, signals for one
new platinum compound (complexes 3 and 4, formed from 1 and 2 respectively) were observed by
NMR. These species were the only major product observed after 30 minutes of heating. The signal for
methane-h4 (CH4) was observed by 1H NMR [30],with no evidence for deuterated methane or other
volatile products. No platinum black or deposited metal was observed to form on the sides of the tubes
after continued heating at 100 ºC for >1 day, indicating that complexes 3 and 4 were very stable at these
temperatures.
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Complex 3 displayed signals for a Pt-Me group (δ = 0.51 ppm, triplet with 195Pt satellites, 2JPt-H = 68.8
Hz, 3JP-H = 8.2 Hz) as well as sharp signals for ligand CH3 groups, in stark contrast to the NMR spectra
for 1. A signal with platinum satellites was observed at 2.85 ppm. Similarly, complex 4 also displayed
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signals for one Pt−Me group (δ = 0.49 ppm, triplet with 195Pt satellites, 2JPt-H = 70.4 Hz, 3JP-H = 9.4) and
sharp signals for the ligand, as well as a signal with platinum satellites at 2.80 ppm, suggesting a
Pt−CH2R group. The yields for 3 and 4 were found to be quantitative after 30 min at 100 ºC, based on
integration against an internal standard (1,4-dioxane), and no other Pt-containing species were observed
by NMR. Interestingly, quantitative yields of 3 and 4 were also observed when 1 or 2 were heated at
100 ºC for 30 min using benchtop C6D6 without excluding air, indicating that this reaction can occur in
the presence of O2 and water. The reaction of 2 in C6D6 in the presence of added D2O (1 uL) yielded 4
and CH4, with no other isotopomers of methane, indicating that the methane does not come from
adventitious water or other proton sources. The 31P{1H} NMR spectra of 3 and 4 exhibited two signals
coupling to each other, consistent of a P2Pt structure with phosphorus ligands in inequivalent
environments. Based on these data, complexes 3 and 4 were believed to have formed cyclometallated
structures (Scheme 6). In order to verify this assignment, crystals of complex 4 were grown and the X-
ray structure is shown in Figure 4. Crystals of 3 suitable for X-ray crystallography were not obtained
despite multiple attempts.
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Scheme 6. Cyclometallation of complexes 1 and 2.
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Fig. 4. Molecular structure of 4 obtained from X-ray crystallography with ellipsoids shown at 50%
confidence. Hydrogen atoms and a disordered co-crystallized solvent molecule are removed for clarity.
Selected bond lengths (Å) and bond angles (º): Pt–C1 = 2.1078(12), Pt–C40 = 2.1110(12), Pt–P1 =
2.2363(3), Pt–P2 = 2.2364(3); C1–Pt–C40 = 85.51(5), C40–Pt–P2 = 85.45(3), P2–Pt–P1 = 97.977(11),
P1–Pt–C1 = 90.79(4).
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The X-ray structure of 4 confirms the NMR assignment of a cyclometallated structure. The six-
membered ring is in a boat-like structure, with a Pt−C bond of 2.11 Å. The two phosphorus ligands are
significantly different in their geometry, consistent with the 31P{1H} NMR spectra. This
Pt−P−O−C−C−C metallacycle is a rare motif, but appears to be very stable [31-33], The mechanism for
this C−H bond activation is not clear at this time, but it is apparent from the products that the platinum
complex did not proceed through Pt(0) to reach 3 or 4. No ethane, the product of C−C reductive
elimination, was observed and a Pt−Me group is still present in the product. This suggests that the C−H
activation occurs through a species of Pt(II) or Pt(IV) character via sigma-bond metathesis or an
oxidative addition mechanism, respectively. Further experiments are planned to elucidate the
mechanism.
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3. Materials and Methods
3.1. General Considerations
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Unless otherwise stated, all chemicals were used as provided by their respective manufacturers. All
NMR spectra were collected using a Bruker Avance 300 MHz NMR spectrometer. 1H and 13C{1H}
NMR spectra were referenced against residual solvent peaks and reported downfield of
tetramethylsilane (δ = 0 ppm). 31P{1H} NMR spectra were referenced against an external 85% H3PO4
standard (δ = 0 ppm). Pentane and tetrahydrofuran (THF) were purified using an Innovative Technology
PureSolv system under N2. (COD)PtCl2 and (SEt2)2PtCl2 were synthesized according to literature
methods [34, 35]. C6D6 was stored under vacuum over activated 3Å molecular sieves and vacuum
transferred to J-Young NMR tubes on a vacuum line. Activated 3Å molecular sieves have been shown
to be very effective at drying organic solvents [36]. Thermolysis reactions were performed with IKA C-
MAG HS7 hotplates equipped with temperature controllers and Sil 180 Silicone oil to maintain constant
temperatures. Elemental analyses were performed by Midwest Microlab, LLC in Indianapolis, IN. X-ray
diffraction analyses were performed at the North Carolina State University Department of Chemistry X-
ray Structural Facility by R. D. S.
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3.2. Synthesis of P(NMe2)(O-2,6-Me2C6H3)2 (L1)
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A round bottom flask was charged with 15 mL of toluene. To this, 3 mL of
tris(dimethylamino)phosphine (16.5 mmol) and 7.01 g (57.3 mmol) of 2,6-dimethylphenol were added.
The solution was refluxed under N2 atmosphere for 48 hours and the progress was tracked by 31P{1H}
NMR. The solvent was reduced under vacuum to produce white, needle-like crystals (identified as
excess phenol starting material) and an oil. The crystals were collected by filtration and washed with
cold pentane and acetone. The filtrate volume was reduced under vacuum to produce pale yellow oil.
The oil was stored under vacuum for 5 days to produce a yellow liquid over large, white, cubic crystals.
The crystals were filtered and washed with 40 mL of H2O and 60 mL of cold pentane before being dried
under vacuum for 30 minutes. The yield was 2.92 g (55.8%). The crystals were found suitable for single
crystal X-ray analysis. 1 H NMR (C6D6): 6.97 (d, 4H), 6.87 (t, 2H), 2.86 (d, 6H, 3 JH-H = 9.2 Hz), 2.18 (s,
12H). 13C{1H} NMR (C6D6): 151.31 (d, JP-C = 2.8 Hz), 131.37 (d, JP-C = 2.9 Hz), 129.60 (d, JP-C = 1.4
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Hz), 124.29 (d, JP-C = 2.0 Hz), 35.29 (d, JP-C = 21.9 Hz), 18.17 (d, JP-C = 5.7 Hz). 31P{1H} NMR (C6D6):
143.19. Elemental analysis – formula: C18H24NO2P, Calculated: C 68.12%, H 7.62%, N 4.41%, Found:
C 68.16%, H 7.60%, N 4.57%.
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3.3. Synthesis of P(O-2,6-Me2C6H3)3 (L2)
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PCl3 (1.6 mL, 20.1 mmol) was added to a solution of 2,6-dimethylphenol (2.52 g, 60.5 mmol) and 1 mL
toluene in a dropwise manner using a syringe under N2. The solution heated to reflux overnight under a
flow of N2. The resulting solution was initially dissolved in 25 mL of toluene, layered with 15 mL of
pentane, and placed in the freezer for several days, but no precipitate formed. The solvent was then
reduced under vacuum and the residue was allowed to cool in the freezer, yielding crystals and a liquid.
The crystals were filtered off and rinsed with cold pentane and dried 2.82 g (36%) of product. 31P{1H}
NMR data matched literature values [37].
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3.4. Synthesis of P(O-2,6-iPr2C6H3)3 (L3)
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PCl3 (0.8 mL, 9.2 mmol) was added dropwise to a solution of 2,6-diisopropylphenol (5.40 g, 30.3
mmol) and 5.6 mL triethylamine in 40.0 mL dichloromethane under N2. The solution was heated to
reflux for 48 hours under a flow of N2. A cloudy white precipitate was formed and the solvent was
reduced under vacuum. The crystals were filtered off and dried to yield 1.40 g (27%) of product from
the first crop. NMR data matched literature values [24].
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3.5. Synthesis of [(SEt2)PtMe2]2
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This compound was synthesized according to a modification of a literature procedure.[38] (SEt2)2PtCl2
(530 mg, 1.13 mmol) was suspended in 100 mL diethyl ether in a Schlenk flask under N2 cooled with
an ice bath. To this a solution of MeLi (4.1 mL of 1.6 M in diethylether) was added via syringe. After
allowing the reaction to come to room temperature overnight, 2.1 mL of DI H2O was added. The
solution was dried over Na2SO4(s) and extracted twice with 50 mL dichloromethane. The combined
organics were removed under vacuo and 205 mg (0.325 mmol, 57.5% yield) of the product was
obtained after multiple recrystallizations with acetone. NMR spectra matched literature values [38]. A
Barnstead International MEL-TEMP was used to find the decomposition temperature of [(SEt2)PtMe2]2,
which was found to be 113–114 °C. This was significantly higher than the reported decomposition
temperature of [(SMe2)PtMe2]2, 86 °C.
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3.6. Synthesis of Complex 1
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A glass reaction vessel equipped with a Teflon stopcock was charged with 10 mL of THF into which
130 mg (0.206 mmol) of [(SEt2)PtMe2]2 and 315 mg (0.992 mmol) of P[O-2,6-(CH3)2C6H3]2(NMe2)
(L1) were added under N2. The flask was sealed and the solution was heated at 60 ˚C under N2
atmosphere while stirring for 25 minutes. Excess solvent was removed under vacuum to produce a fine
gray powder which was washed with cold pentane before being dried under vacuum to yield 0.162 g
(91.5%) of product. 1H NMR (CDCl3): −0.31 (t with 195Pt satellites, 6H, 2JPt-H = 69.4 Hz, 3JP-H = 7.9
Hz), 2.48 (broad s, 24H), 2.62 (v. broad s, 12H), 6.90 (m, 12H). 13C{1H} NMR (CDCl3): 152.26,
130.66, 128.95, 123.76, 38.57, 19.26, –1.16 (dd w/ 195Pt satellites, JP-C = 124.8, 14.1 Hz, JPt-C = 570 Hz).
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31P{1H} NMR (CDCl3): 130.35 (s w/ 195Pt satellites, 1JP-Pt = 3054 Hz). Elemental analysis – formula: C-
38H54N2O4P2Pt, Calculated: C 53.08%, H 6.33%, N 3.26%, Found: C 53.25%, H 6.41%, N 3.33%.
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3.7. Synthesis of Complex 2
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[(SEt2)PtMe2]2 (24.5 mg, 0.174 mmol) and P[O-2,6-(CH3)2C6H3]3 (L2) (68.8 mg, 0.0388 mmol) along
with 0.5 mL of C6D6 were added to a medium-walled J. Young NMR tube equipped with a Teflon
stopcock. The tube was evacuated with three freeze-pump-thaw cycles and then was heated at 60 oC for
two hours. The solvent was then removed under vacuum and the residue was taken up in 10 mL
pentane, which was allowed to cool in the freezer, yielding 35.4 mg (45%) of product. Evaporation of
the remaining pentane gave single crystals suitable for X-ray analysis. 1H NMR (C6D6): 0.21 (t with
195Pt satellites, 6H, 2JPt-H = 69.5 Hz, 3JP-H = 6 Hz), 6.79 (m, 18H), 2.65 (broad s, 24H), 1.70 (broad s,
12H), 0.21 (t, 6H). 13C{1H} NMR (C6D6): 151.20, 131.70, 192.98, 125.27, 19.64, –1.91 (dd w/ 195Pt
satellites, JP-C = 146.6, 15.5 Hz, JPt-C = 570 Hz). 31P{1H} NMR (C6D6): 118.28 (s w/ 195Pt satellites,1JP-
Pt = 3158 Hz). Elemental analysis – formula: C50H60O6P2Pt, Calculated: C 59.22%, H 5.96%, Found: C
59.30%, H 6.07%.
272
3.8. Reaction of L3 with [(SEt2)PtMe2]2
273
274
275
276
277
278
279
280
[(SEt2)PtMe2]2 (10.0 mg, 0.158 mmol) and P[O-2,6-(CH(CH3)2)2C6H3]3(L3) (35.7 mg, 0.0634 mmol)
along with 0.5 mL of C6D6 were added to a medium-walled J. Young NMR tube equipped with a
Teflon stopcock. The tube was evacuated with three freeze-pump-thaw cycles and then was heated at 60
oC for 14 hours. The reaction did not go to completion and NMR indicated an intermediate complex was
formed, (L3)(SEt2)PtMe2. The species was not isolated and decomposition was observed upon continued
heating. Characteristic NMR data for (L3)(SEt2)PtMe2: 1H NMR (C6D6): 0.43 (d with 195Pt satellites,
3H, 2JPt-H = 85.9 Hz, 3JP-H = 8.9 Hz), 0.82 (d with 195Pt satellites, 3H, 2JPt-H = 65.6 Hz, 3JP-H = 10.9 Hz).
31P{1H} NMR (C6D6): 114.78 (s w/ 195Pt satellites,1JP-Pt = 3296 Hz).
281
3.9. Thermolyses of 1 in C6D6
282
283
284
285
286
287
288
289
290
291
292
293
Thermolyses of 1 were performed according to the following procedure. A medium-walled J. Young
NMR tube was charged with complex 1 (8−10 mg) and 400−600 µL of C6D6 was added via vacuum
transfer. For reactions with an internal standard, the tube was opened, 1,4-dioxane (8−10 µL) was added
via syringe, and tube was sealed and subjected to three freeze-pump-thaw cycles. The tube was heated
to 100 ˚C for up to 20 hours with progress tracked by 1H and/or 31P{1H} NMR. Attempts were made to
grow crystals of 3 for X-ray crystallography, but were unsuccessful. Yields were quantitative (100 ±
5%) based on integration against the 1,4-dioxane standard. A similar yield was obtained when benchtop
1
C6D6 was used and the J-Young tube was sealed under air. H NMR for compound 3 (C6D6): 0.51 (t
with 195Pt satellites, 3H, 2JPt-H = 68.8 Hz, 3JP-H = 8.2 Hz), 2.17 (s, 3H), 2.27 (s, 6H), 2.40 (s, 3H), 2.43 (s,
15H), 2.56 (s, 3H), 2.60 (s, 3H), 2.85 (m w/ 195Pt satellites, 2H, 2JPt-H = 84 Hz), 6.7−7.0 (m, 12H).
31P{1H} NMR (C6D6): 126.3 (d w/ 195Pt satellites, 2JPt-P = 3297 Hz, 3JP-P = 7.45 Hz), 147.8 (broad s w/
195Pt satellites, 2JPt-P = 3243 Hz).
10

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294
3.10. Thermolyses of 2 in C6D6
295
296
297
298
299
300
301
302
303
304
305
Thermolyses of 2 were performed according to the following procedure. A medium-walled J. Young
NMR tube was charged with complex 1 (8−10 mg) and 400−600 µL of C6D6 was added via vacuum
transfer. For reactions with an internal standard, the tube was opened, 1,4-dioxane (8−10 µL) was added
via syringe, and tube was sealed and subjected to three freeze-pump-thaw cycles. The tube was heated
to 100 ˚C for up to 20 hours with progress tracked by 1H and/or 31P{1H} NMR. Yields of 4 were
quantitative (100 ± 5%) based on integration against the 1,4-dioxane standard. Crystals of 4 were
obtained from the slow evaporation of a thermolysis sample. 1H NMR for compound 4 (C6D6): 0.49 (t
with 195Pt satellites, 3H, 2JPt-H = 70.4 Hz, 3JP-H = 9.4 Hz), 1.64 (s, 3H), 2.28 (br s, 15H), 2.37 (s, 15H),
2.80 (t w/ 195Pt satellites, 2H, 3JP-H = 13.4 Hz, 2JPt-H = 94.8 Hz), 6.6−7.1 (m, 18H). 31P{1H} NMR
(C6D6): 114.1 (d w/ 195Pt satellites, 2JPt-P = 3455 Hz, 3JP-P = 19.2 Hz), 140.2 (d w/ 195Pt satellites, 2JPt-P
3386 Hz).
=
306
3.11. X-Ray Crystallography
307
308
309
310
311
312
313
314
X-ray crystallographic analyses of L1, 1, 2, and 4 were performed as follows. Single crystals suitable for
structure analysis were selected from the bulk and mounted on a MiTeGen mounts. Data were collected
on a Bruker-Nonius X8 Kappa Apex II diffractometer by ω and φ scans using MoKa radiation (l =
0.71073 Å). Corrections for Lorentz and polarization effects, and absorption were made using SADABS
[39]. All four structures were solved using direct methods, and refined using full-matrix least squares
(on F2) using the SHELX [40] software package. All non-hydrogen atoms were refined anisotropically.
H atoms were added at calculated positions, with coordinates and Uiso values allowed to ride on the
parent atom. Selected crystallographic data are shown in Table 1.
315
316
Table 1. X-ray crystallization data for L1, 1, 2, and 4
Compound
chemical formula
formula weight
temperature /K
crystal system
space group
a /Å
L1
1
2
4
C18H24NO2P
317.35
C38H54N2O4P2Pt
859.86
C50H60O6P2Pt
1014.01
110(2)
C49H56O6P2Pt
1040.04
110(2)
· 0.5C6D6
110(2)
110(2)
monoclinic
P21
monoclinic
C2/c
monoclinic
P21/c
triclinic
P-1
7.9544(2)
8.4891(2)
13.3798(4)
19.7508(14)
11.3662(8)
16.9566(13)
11.1065(4)
18.6506(6)
22.3667(8)
11.5915(3)
14.9899(4)
15.2035(4)
b /Å
c /Å
11

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90
90
90
112.0250(7)
104.9300(9)
92.8200(8)
2334.67(11)
2
α
β
γ
/°
/°
/°
103.3140(10)
90
100.298(4)
90
103.3230(10)
90
V Å3
879.20(4)
2
3745.3(5)
4
4508.4(3)
4
Z
1.199 g/cm3
340
1.525 g/cm3
1744
1.494 g/cm3
2064
1.479 g/cm3
1054
ρcalc g/cm3
F(000)
2.63 to 35.78
28024
2.08 to 32.57
46059
1.87 to 28.34
69532
2.01 to 35.11
133346
θ
range /°
reflections collected
data / restr. / parms.
Rint
7393 / 1 / 205
0.0178
1.044
6821 / 0 / 220
0.0605
1.029
11217 / 0 / 546
0.0813
20698 / 138 / 553
0.0280
1.004
1.044
Goodness-of-fit
a
0.0278
0.0754
0.0269
0.0328
0.0172
R1 (I>2
σ
(I))
(I))
b
0.0523
0.0549
0.0435
wR2 (I>2
σ
2
2 2
½
2
a R = Σ(||Fo| - |Fc||) / Σ|Fo|, b wR = [Σ ( w( Fo – Fc2 )2) / Σ (w(Fo ) ] ; w=1/[σ2(Fo )+(aP)2+bP],
1
2
½
GOF= {
Σ
[ w(Fo2 – Fc2 )2] / (#reflns - #parms )}
317
4. Conclusions and Future Directions
318
319
320
321
322
323
324
325
326
327
328
Three air-stable phosphite ligands were synthesized by reaction of phenols with PCl3 or P(NMe2)3. A
new phosphite ligand, L1, was synthesized from P(NMe2)3 and 2,6-dimethylphenol and characterized by
NMR spectroscopy, elemental analysis and X-ray crystallography. The air-stable bulky aryl phosphite
ligands were reacted with platinum starting materials to yield L2PtMe2 complexes. These species were
also characterized by NMR spectroscopy, elemental analysis, and X-ray crystallography. While the
species were stable on the bench at room temperature, thermolysis in C6D6 resulted in intramolecular
C−H activation of sp3 bonds on the ligands, resulting in stable metallacycles. One of these complexes
was characterized by X-ray crystallography, verifying the presence of a six-membered ring. Future work
will explore ligand structures to facilitate intermolecular sp3 C−H activation instead of intramolecular
sp3 C−H activation. In addition, the reactivity of complexes 1−4 with electrophiles and oxidants will be
examined to determine if C−H bond functionalization can be achieved.
329
5. Acknowledgments
330
331
332
The College of Science and Health (CSH) at DePaul University is acknowledged for startup funds to
support this project. We thank the National Science Foundation CCLI A&I Program (Grant No. DUE-
0310624) for support in purchasing the Bruker 300 MHz NMR spectrometer used in these studies.
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333
Supporting Information
334
335
Cif files for L1, 1, 2, and 4 have been deposited at the CCDC (CCDC structure numbers 1413718-
1413721).
336
References
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Graphical Abstract and Synopsis
Air-stable ligands
Air-stable platinum complexes
O
X
P
L
100 °C
C6D6
L
L
Me
Me
CH2
Me
+ CH4
ArO
Pt
Pt
X = OAr or NMe2
P
P
L =
Intramolecular sp3 C-H activation
or
O
O
Me2N
3
2
Air-stable phosphite ligands were synthesized without added external base. Organoplatinum(II) complexes of the
air-stable bulky phosphite ligands have been synthesized and characterized. These compounds are very robust and
can be stored on the bench without decomposition. Upon thermolysis at 100 °C, these complexes cyclometallate
via intramolecular sp3 C-H activation.
15