Hemilability of P(X)N-type ligands (X = O, N-H): Rollover cyclometalation and benzene C-H activation from (P(X)N)PtMe2 complexes

Article abstract of DOI:10.1039/c4dt01143k

The thermolyses of (^tBuP(O)N)PtMe₂ (1, ^tBuP(O)N = (di-tert-butylphosphinito)pyridine) and ( ^tBuP(N-H)N)PtMe₂ (3, ^tBuP(N-H)N = (di-tert-butylphosphino)-2-aminopyridine) in benzene-d₆ were investigated. With (^tBuP(O)N)PtMe₂, the product of a rollover cyclometalation of the pyridyl ring was observed in 80% yield along with formation of CH₄. In contrast, thermolysis of ( ^tBuP(N-H)N)PtMe₂ resulted in competing rollover cyclometalation and intermolecular benzene C-H activation with production of a mixture of CH₄ and CH₃D. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4dt01143k

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Hemilability of P(X)N-type ligands (X = O, N–H):
rollover cyclometalation and benzene C–H
activation from (P(X)N)PtMe2 complexes†‡
Cite this: Dalton Trans., 2014, 43,
12018
Margaret L. Scheuermann,a Kyle A. Grice,§a Matthew J. Ruppel,a
Marta Roselló-Merino,b Werner Kaminskya and Karen I. Goldberg*a
The thermolyses of (tBuP(O)N)PtMe2 (1, tBuP(O)N = (di-tert-butylphosphinito)pyridine) and (tBuP(N–H)N)PtMe2
(3, tBuP(N–H)N = (di-tert-butylphosphino)-2-aminopyridine) in benzene-d6 were investigated. With (tBuP(O)
N)PtMe2, the product of a rollover cyclometalation of the pyridyl ring was observed in 80% yield along with
formation of CH4. In contrast, thermolysis of (tBuP(N–H)N)PtMe2 resulted in competing rollover cyclometala-
tion and intermolecular benzene C–H activation with production of a mixture of CH4 and CH3D.
Received 18th April 2014,
Accepted 11th June 2014
DOI: 10.1039/c4dt01143k
www.rsc.org/dalton
that might otherwise be inaccessible. Here we describe new
Pt complexes with P(X)N-type ligands (X = O, N–H) and report
Introduction
Bidentate ligands bearing two different donor atoms have rollover cyclometalation reactions at (P(X)N)PtMe2 complexes
been widely employed in the field of catalysis.1 They are often (X = O, N–H). Intermolecular C–H activation was also observed
chosen for properties such as hemilability, tunability, and for (P(N–H)N)PtMe2. The reactivity observed for both complexes
ability to support metal centers in multiple oxidation states. can be attributed to the hemilability of the P(X)N ligands.
One such ligand motif that has recently gained prominence is
the P(X)N-type ligand (Fig. 1; X = O, N–R, CH2).2–12 Complexes
supported by such ligands have been shown to be competent
catalysts for Suzuki–Miyaura reactions,5,9,10 alkyl amine
synthesis,2,6 transfer hydrogenation,3 methoxycarbonylation of
olefins,7 and arene coupling.8 As with many bidentate ligands
bearing two different donor atoms, P(X)N-type ligands can be
hemilabile.1 The hemilability of the ligand generates an open
site allowing the metal center to engage in reaction pathways
Results and discussion
Synthesis, characterization, and thermolysis of (tBuP(O)N)-
PtMe2
The 2-(di-tert-butylphosphinito)pyridine (tBuP(O)N) ligand was
prepared using methods for the synthesis of structurally
related molecules.13,14 Unrefined ligand can be mixed directly
15
with [PtMe2(SMe2)]2 in toluene at room temperature to gene-
rate the PtII dimethyl complex, (tBuP(O)N)PtMe2 (1, tBuP(O)N =
(di-tert-butylphosphinito)pyridine, Fig. 2). Complex 1 was
characterized by NMR spectroscopy, elemental analysis, and
1
X-ray crystallography. The H NMR spectrum of 1 in C6D6 con-
tains signals corresponding to two inequivalent Pt–Me groups
at 1.65 ppm (2JPt–H = 90 Hz, JP–H = 6 Hz) and 1.36 ppm
3
(2JPt–H = 64 Hz, JP–H = 7 Hz). The substantial difference in the
Fig. 1 P(X)N ligand motif.
3
2JPt–H values is consistent with the methyl group resonating at
1.65 ppm being situated trans to the pyridine moiety of the
tBuP(O)N ligand and the methyl group resonating at 1.36 ppm
being situated trans to the more strongly σ-donating phos-
phine.16 The 31P{1H} NMR spectrum of 1 contains a singlet
at 184.2 ppm (1JPt–P = 2113 Hz), shifted downfield of the
153.3 ppm value measured for free tBuP(O)N. Crystals of 1 were
obtained from a toluene–ether solution at −35 °C and were
analyzed by X-ray diffraction. The crystals were very small and
of poor quality, causing a high mosaicity but still a molecular
aDepartment of Chemistry, University of Washington, Box 351700, Seattle,
WA 98195, USA. E-mail: goldberg@chem.washington.edu
bDepartamento de Química Inorgánica, CSIC-Universidad de Sevilla, Instituto de
Investigaciones Químicas, Avda. Américo Vespucio 49, 41092 Sevilla, Spain
†Contributed in honor of the 80th birthday of Professor William C. Kaska in rec-
ognition of his tremendous contributions to the field.
‡CCDC 996399, 996400, 996401 and 966402. For crystallographic data in CIF or
other electronic format see DOI: 10.1039/c4dt01143k
§Present address: Department of Chemistry, DePaul University, 1110 West
Belden Avenue, Chicago, IL 60614, USA.
12018 | Dalton Trans., 2014, 43, 12018–12025
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hexamethylbenzene internal standard indicated that the new
species, 2-L, formed in ca. 80% yield. The solution remained
clear and colorless throughout the thermolysis and a broad
31P NMR resonance at approximately 159 ppm with coupling
to Pt (1JPt–P ∼ 4400 Hz) indicated that the phosphine was still
bound to Pt, although trans to a more weakly donating ligand
than a Me group.16
To explore the identity of 2-L, the volatiles were removed
in vacuo and the sample was redissolved in pyridine-d5. The
1H NMR spectrum of the sample in pyridine-d5 revealed a
single major species with sharp resonances. A single Pt–Me
Fig. 2 POV-Ray18 rendition of the ORTEP19 of (tBuP(O)N)PtMe2, 1, at the
50% level of probability with hydrogen atoms and full molecule disorder
2
resonance at 0.53 ppm with a JPt–H value of 46 Hz was
omitted for clarity. Selected bond lengths (Å) and angles (°): O1–C1 = observed. This small coupling constant suggests a more
1.36(3), O1–P1 = 1.704(19), N1–Pt1 = 2.088(18), P1–Pt1 = 2.223(7),
strongly donating trans ligand than a pyridine moiety or a
phosphine. Of note, the JPt–P value remains large, 4272 Hz,
Pt1–C14 = 2.05(3), Pt1–C15 = 2.14(3); C1–O1–P1 = 116.9(16), C14–Pt1–
C15 = 86.0(12), N1–Pt1–C15 = 92.9(10), C14–Pt1–P1 = 99.0(9), N1–Pt1–
P1 = 82.1(6).
1
suggesting that the phosphine remains trans to a weakly co-
1
ordinating ligand. Similarly large JPt–P values have been
observed in other PtII species with phosphine and pyridyl moi-
eties trans to one another, albeit in cationic complexes.21
A
structure consistent with the NMR data was obtained (Fig. 2).17
The crystals contained disordered molecules related by a
mirror plane parallel to (0 1 0). The mirror plane is not part of
the space group symmetry but is part of the point group, and
thus, is inconsistent with twinning. A distinctive feature of this
nearly square planar structure is the significant difference in
the Pt–CH3 bond lengths with values of 2.05(3) and 2.14(3) Å
for the Pt–C bonds trans to nitrogen and phosphorus,
respectively.
Upon heating a sample of 1 in C6D6 at 100 °C for four days,
the NMR resonances of 1 disappeared and broad resonances
corresponding to a new species, 2-L, appeared along with a
sharp singlet at 0.16 ppm consistent with the formation of
CH4 (Scheme 1).20 The broad resonances that could be clearly
identified were centered at 0.77 ppm (ν1/2 = ∼90 Hz, Pt–Me),
1.45 ppm (ν1/2 = ∼160 Hz, tBu resonances having the appear-
ance of a virtual triplet), and in the aromatic region centered
at 6.48 and 7.99 ppm. Another aromatic feature appears to be
centered under the benzene residual solvent resonance
(7.16 ppm). Integration of the 1H NMR resonances versus a
COSY experiment in pyridine-d5 confirmed the presence of
three and only three aromatic protons in the molecule. This
observation suggested the possibility that a rollover cyclometa-
lation cleaving an aromatic C–H bond could have occurred.
In benzene-d6, the open site that would be generated by
rollover cyclometalation and methane loss from 1 is likely
filled by a pyridyl moiety in the ligand backbone of another
molecule resulting in oligomeric species22 or possibly by
η2-benzene as was reported by Brookhart, Templeton, and co-
workers for a similar PtII system.23 In contrast, dissolution of
the material in the donor solvent pyridine-d5 leads to coordi-
nation of pyridine solvent forming the distinct monometallic
species, (tBuP(O)C)PtMe(pyr-d5), 2-pyr-d5. Efforts to isolate crys-
talline samples of 2-L and 2-pyr-d5 were not successful.
An isolable species, (tBuP(O)C)PtMe(CNtBu) (2-CNtBu) was
formed in approximately 80% yield based on starting 1
(1H NMR integration) upon addition of tert-butylisonitrile
(ca. 1 equiv.) to a solution of the thermolysis product, 2-L, in
benzene. The 1H NMR spectrum of 2-CNtBu features three
Scheme 1 Thermolysis of (P(O)N)PtMe2, 1, and generation of 2-pyr-d5 and 2-CNtBu.
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Fig. 3 POV-Ray18 rendition of the ORTEP19 of 2-CNtBu at the 50% level
of probability with hydrogen atoms omitted for clarity. Selected bond
lengths (Å) and angles (°): Pt1–C15 = 1.975(6), Pt1–C1 = 2.066(6), Pt1–
C14 = 2.122(6), Pt1–P1 = 2.2363(14), P1–O1 = 1.642(4), O1–C5 = 1.389
(7); C15–Pt1–C1 = 94.6(2), C15–Pt1–C14 = 86.3(3), C1–Pt1–P1 = 80.68
(16), C14–Pt1–P1 = 98.41(19), C5–O1–P1 = 116.0(3).
Fig. 4 POV-Ray18 rendition of the ORTEP19 of (tBuP(N–H)N)PtMe2, 3, at
the 50% level of probability with C–H hydrogen atoms omitted for
clarity. Selected bond lengths (Å) and angles (°): Pt1–C14 = 2.041(5),
Pt1–C15 = 2.112(5), Pt1–N2 = 2.131(4), Pt1–P1 = 2.2411(13), P1–N1 =
1.705(4), N1–C9 = 1.382(6); C14–Pt1–C15 = 85.5(2), C15–Pt1–N2 =
94.56(18), C14–Pt1–P1 = 97.54(15), N2–Pt1–P1 = 82.44(11), N1–P1–
Pt1 = 101.08(14), C9–N1–P1 = 120.5(3).
aromatic protons and a single Pt–Me resonance at 1.08 ppm
(2JPt–H = 54 Hz). This JPt–H value is consistent with a Pt–Me
2
spectrum of 3 contains a single peak at 105.3 ppm with a 1JPt–P
value of 2080 Hz. X-ray quality crystals of 3 were obtained by
slow evaporation of a toluene solution in an N2 atmosphere
(Fig. 4).
group situated trans to a strongly sigma-donating sp2 aryl
carbon.16,24 The 31P NMR resonance for 2-CNtBu at 155 ppm
1
has a JPt–P value of 3452 Hz, consistent with CNtBu being a
stronger trans donor than pyridine.25
When a benzene-d6 solution of 3 was thermolyzed at
100 °C, complete disappearance of the starting material was
observed over four days. Well-resolved resonances corres-
X-ray quality crystals of 2-CNtBu were obtained by slow
evaporation of a benzene–ether solution in an N2 atmos-
phere.17 The X-ray structure (Fig. 3) obtained confirms the
stereochemical assignment made based on the NMR coupling
constants. The Pt–C bond trans to the aryl ligand has a bond
length of 2.122(6) Å.
The crystal structure of 2-CNtBu clearly illustrates that roll-
over cyclometalation of the pyridine moiety has taken place.
This is similar to the reported rollover cyclometalation
chemistry of various (bpy)PtR2 complexes (bpy = bipyridine or
bipyridine derivative; R = Me or Ph).22,26,27 Notably, when the
thermolysis of (bpy)PtPh2 was performed in the presence of a
suitable trapping ligand, discrete molecular species formed.
However in the absence of such a ligand, oligomeric or poly-
meric species were reported.22 The observation of rollover
cyclometalation from 1 is unusual as there are in general
few examples of C–H activation at phosphine ligated
Pt centers.28,29 Rollover cyclometalation at other metals with a
phosphine ligand present are known.30,31
1
ponding to a new species appeared in the H NMR spectrum,
however spectroscopic yields of this new species were typically
limited to ca. 30% (integration vs. hexamethylbenzene internal
standard). In contrast to the thermolysis of 1 where CH4 was
the only isotopologue of methane observed, in the thermolysis
of 3, both CH4 and CH3D were observed (typically in a ratio of
∼1 : 1), the CH3D being consistent with cleavage of C–D bonds
of the C6D6 solvent. In the aromatic region of the 1H NMR
spectrum of the
(
tBuP(N–H)N)PtMe2 thermolysis product
mixture in benzene-d6, four resonances were observed indicat-
ing that the formation of this product did not involve a rollover
cyclometalation. This product displayed a single Pt–Me reson-
1
ance in the H NMR spectrum, 1.60 (2JPt–H = 88 Hz) and a 31P
resonance at 102.2 (1JPt–P = 1980 Hz). These data suggest a
species with a single Pt–Me group located trans to the weakly
donating pyridyl and a phosphorus donor trans to a strong
ligand. Upon repeating the thermolysis in benzene-h6, fol-
lowed by spectroscopic characterization of the product in
benzene-d6, the product was identified as (tBuP(N–H)N)PtMePh
Synthesis and thermolysis of (tBuP(N–H)N)PtMe2
The chemistry of the related bifunctional P(N–H)N ligand on (4, Scheme 2), wherein a phenyl group is located trans to phos-
platinum was also examined. The tBuP(N–H)N ligand phorus. Thus, rather than rollover cyclometalation with C–H
(
tBuP(N–H)N = (di-tert-butylphosphino)-2-aminopyridine)32 was activation of the bidentate ligand, intermolecular C–H(D) acti-
15
combined with [PtMe2(SMe2)]2 in toluene at room tempera- vation of a benzene solvent molecule takes place to produce
ture to generate (tBuP(N–H)N)PtMe2 (3). Complex 3 was charac- 4(-d5).
terized by NMR spectroscopy, elemental analysis, and X-ray
Upon removal of the benzene solvent from the thermolysis
crystallography. As was seen with the related (tBuP(O)N)PtMe2 product mixture of 3 in vacuo and redissolution of the residue
complex 1, the 1H NMR spectrum of 3 contains two inequi- in pyridine-d5, a 1H NMR spectrum revealed well-resolved reso-
valent Pt–Me resonances with very different 2JPt–H values, 88 Hz nances corresponding to two significant species. A COSY experi-
and 65 Hz for the Pt–Me groups trans to the pyridyl and phos- ment revealed that one of the species contained four coupled
phine moieties respectively. Notably, these values are very aromatic protons, consistent with (tBuP(N–H)N)PtMePh, 4,
close to those of 1 (90 and 64 Hz, respectively). The 31P NMR while the other species had only three aromatic protons
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Scheme 2 Thermolysis of (P(N–H)N)PtMe2 in C6D6.
Scheme 3 Thermolysis of 3 in the presence of pyridine.
as would be expected if a rollover cyclometalation had
occurred. This second species which forms competitively with
4 is suggested to be (tBuP(N–H)C)PtMe(pyr-d5), 5-pyr-d5. The
formation of 5-pyr-d5 is in analogy with the chemistry observed
above with formation of the related 2-pyr-d5 from the similar
reaction of 1. Typical yields for 5-pyr-d5 were low (ca. 15%)
which complicated efforts to obtain full characterization.
In addition, with the N–H substituted ligand rather than the
O-substituted ligand, efforts to find a trapping reagent ana-
logous to the isonitrile (vide supra) that allowed for isolation
and thorough characterization of the rollover cyclometalation
product were not successful.
Fig. 5 POV-Ray18 rendition of the ORTEP19 of (tBuP(N)N)PtMe(pyr), 6, at
the 50% level of probability with hydrogen atoms (except those on H2O)
and co-crystallized dichloromethane omitted for clarity. The sites for
H1b and H1o are at 50% occupancy. Selected bond lengths (Å) and
angles (°): Pt1–C1 = 2.061(9), Pt1–N2 = 2.097(6), Pt1–N1 = 2.106(7),
Pt1–P1 = 2.202(2), P1–N3 = 1.680(7), N3–C6 = 1.348(10), C2–C3 = 1.377
(12), C3–C4 = 1.385(11), C4–C5 = 1.381(11), C5–C6 = 1.420(11), N3⋯O1
= 2.880(8), N3⋯H1a = 2.097(16); C1–Pt1–N2 = 84.2(3), N2–Pt1–N1 =
95.3(2), C1–Pt1–P1 = 99.4(2), N1–Pt1–P1 = 81.22(16), C6–N3–P1 =
115.9(5).
water, which lead to the observation of a possible hydrogen
bond interaction. This interaction between H1a and N3 could
serve to stabilize the formally anionic charge through hydro-
gen bonding.
When the thermolysis of 3 was carried out in the presence
of pyridine (5–50 equiv. or neat pyridine-d5) as a trapping
ligand, a high yield (>80%) of a single product was obtained
(Scheme 3). In these reactions CH4 was produced as a single
isotopologue but neither inter- nor intramolecular C–H acti-
vation was observed.
The changes in the ligand bond lengths between 3 and 6
are subtle, all less than 0.04 Å. Kirchner33 previously reported
crystal structures for [(BINOLP(N–H)N(N–H)P)Fe(MeCN)3]2[BF4],34
The 1H NMR spectrum is consistent with a species having a
single Pt–Me group and all four aromatic C–H bonds intact.
The 31P NMR spectrum has a single resonance at 86.4 ppm
(1JPt–H = 3921) suggesting a phosphine with a weakly donating
trans ligand. Most notable however was that no N–H resonance
was observed in the 1H NMR spectrum of the product. Crystals
of this new species were obtained by slow evaporation of a
solution of dichloromethane (in air using benchtop solvent)
and the product was shown to be (tBuP(N)N)PtMe(pyr) by X-ray
crystallography (6, Fig. 5). No pyridinium was detected spectro-
scopically and the crystal structure contained no anions. Dis-
ordered CH2Cl2 was uniquely identified from electron density
maxima, bond-distances, and free site-occupancy fit to test
correctness of element type assignment. A water molecule was
located in the crystal lattice near the formally anionic nitrogen
atom (N3⋯O1 = 2.880(8)). The water is disordered and two of
the hydrogen atom sites on the water, H1b and H1o, are at
fractional occupancies, consist with H2O rather than H3O+.
Water was refined with a rigid model, leaving H–H distances
and O–H distances fixed, but allowing free rotation of the
(
BINOLP(N–H)N(N–H)P = (N,N′-bis(S-dinaphtho[2,1-d:1′2′-f][1,3,2]-
dioxaphosphepine)-2,6-diaminopyridine) and its mono-deproto-
nated analogue [(BINOLP(N)N(N–H)P)Fe(MeCN)3][BF4].35 Notably
the changes in bond lengths upon deprotonation were also
approximately 0.05 Å or less. The differences in bond lengths
observed in the structures of [(PhP(N–H)N)2Pt]2Cl,36 and its
twice deprotonated analogue (PhP(N)N)2Pt,37 were all 0.07 Å or
less.38
The formation of 6 is notable because it represents the
formal coupling of an N–H proton in the ligand backbone with
a methyl group bound to the metal center. The microscopic
reverse of this reaction would be a net C–H activation reaction
that is reminiscent of H2 activation by Noyori type hydro-
genation catalysts with nitrogen on the ligand receiving a
proton.39 Mechanistic investigations of the formation of 6 are
in progress and preliminary results indicate that the added
pyridine is involved in mediating the reaction; formation of 6
was qualitatively faster with increasing pyridine concentration
and fastest in neat pyridine-d5.
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30 mL THF. To this solution was added tetramethylenediamine
(1.5 mL, 10 mmol) and triethylamine (1.0 mL, 11 mmol). The
Summary
New organometallic Pt complexes with bidentate ligands solution was cooled in an ice water bath and di-tert-butylphos-
bearing two different donor atoms were prepared and their phine (2.0 mL, 11 mmol) was added. The flask was fitted with
thermolyses were explored. The hemilability of the ligand a reflux condenser and heated at reflux for four hours, then
appears to play a significant role in the reactivity observed. cooled to room temperature. The volatiles were removed
Dissociation of the pyridyl ring in (tBuP(O)N)PtMe2, 1, leads in vacuo and the residue was extracted with several portions of
primarily to rollover cyclometalation, with the formation of 2-L benzene (∼50 mL total). The benzene extracts were collected
(where L is benzene solvent or the pyridine N of another roll- and filtered. The volatiles were removed in vacuo to give a pale
over product molecule). Complex 2-L is effectively trapped by yellow oil. Yield: 1.9 g, 7.9 mmol, 74%. The crude material was
tert-butylisonitrile allowing isolation and characterization of used without further purification. 1H NMR (C6D6, 500 MHz,
2-CNtBu. In the case of (tBuP(N–H)N)PtMe2, 3, inter- and intra- 298 K) δ 8.12 (m, 1H), 7.07 (m, 1H), 6.73 (d, 1H), 6.43 (m, 1H),
3
molecular C–H activations appear to be competitive with both 1.19 (d, 18H, JP–H = 11 Hz). 13C{1H} NMR (C6D6, 126 MHz,
1
activation of benzene solvent, to yield the methyl phenyl 298 K) δ 148.5, 139.1, 118.1, 112.6, 35.9 (d, JP–C = 29 Hz), 28.1
2
derivative 4 and rollover cyclometalation to yield a tBuP(N–H)C (d, JP–C = 16 Hz). 31P{1H} NMR (C6D6, 202 MHz, 298 K)
analog of 2-L. Thus the subtle change from an O atom linker δ 153.3.
to a NH linker in the backbone of the bidentate ligand signifi-
Synthesis of (tBuP(O)N)PtMe2 (1). A solution of unrefined
cantly changes the selectivity of the C–H activation reaction. In tBuP(O)N (130 mg, 0.54 mmol) in 5 mL toluene was added to
addition, thermolysis of the (tBuP(N–H)N) ligated 3 in the pres- [PtMe2(SMe2)]2 (ca. 150 mg, 0.26 mmol). The reaction mixture
ence of pyridine allowed for the isolation of complex 6, the was stirred for 15 minutes at room temperature and then the
product of an unusual coupling of the proton on the N of the volatiles were removed in vacuo. The residue was dissolved in
ligand backbone with a Pt–Me group. Studies of the effects of minimal toluene (approx. 3 mL), filtered through a 0.2 μm
varying the linker group (atoms and linker lengths), the substi- PTFE syringe filter, and recrystallized from a toluene–ether
tuents on the nitrogen, and the substituents on the phos- mixture at −35 °C yielding crystalline
(
tBuP(O)N)PtMe2
1
phorus in these P(X)N ligands are ongoing in our laboratory in (113 mg, 0.24 mmol, 46%). H NMR (C6D6, 500 MHz, 298 K)
order to provide insight for the development of catalytic δ 8.84 (d, 1H, 3JPt–H < 24 Hz),41 6.88 (t, 1H), 6.47 (d, 1H), 6.07 (m,
systems involving C–H bond activation.
1H), 1.65 (d, 3H, 3JP–H = 6 Hz, 2JPt–H = 90 Hz), 1.36 (d, 3H, 3JP–H
=
2
2
7 Hz, JPt–H = 64 Hz), 1.21 (d, 18H, JP–H = 14 Hz). 13C{1H}
2
2
NMR (C6D6, 126 MHz, 298 K) δ 166.8 (d, JP–C = 4 Hz, JPt–C
15 Hz), 146.4 (d, 3JP–C = 3 Hz, 2JPt–C = 16 Hz), 139.4, 118.6 (4JPt–C
=
=
=
=
Experimental
General conditions
3
1
2
14 Hz), 110.6 (d, JP–C = 4 Hz), 40.2 (d, JP–C = 9 Hz, JPt–C
45 Hz), 27.7 (d, JP–C = 6 Hz, JPt–C < 19 Hz),41 17.0 (d, JP–C
2
3
2
1
2
1
Unless otherwise specified, all manipulations were carried out
in a nitrogen-filled glovebox, using a Schlenk line, or using a
high vacuum manifold. Glassware was oven-dried prior to use.
Dichloromethane and THF were dried by passage through acti-
vated alumina columns under argon. Benzene, toluene, and
pentane were dried by passage through columns containing
activated alumina and Q5 reactant, a supported copper oxygen
scavenger, under argon.40 C6D6 was vacuum transferred from
sodium/benzophenone ketyl. Pyridine-d5 was vacuum trans-
ferred from CaH2. Tetramethylenediamine and triethylamine
were dried over CaH2 and distilled prior to use. [PtMe2(SMe2)]2
was prepared according to a literature procedure.15 All other
reagents were obtained from commercial vendors and used as
received. NMR spectra were recorded on Bruker AV300,
DRX500, AV500, AV700, or AV800 spectrometers and were refer-
enced to the residual protiated solvent signal. Data from
HMQC and HMBC experiments were used in the assignment
of 13C NMR resonances. Abbreviations: s = singlet, d = doublet,
dd = doublet of doublets, m = multiplet, br = broad.
115 Hz, JPt–C = 665 Hz), −22.8 (d, JP–C = 4 Hz, JPt–C = 760 Hz).
31P{1H} NMR (C6D6, 202 MHz, 298 K) δ 184.2 (1JPt–P = 2113 Hz).
Anal. Calcd For C15H28NOPPt: C, 38.79; H, 6.08: N, 3.02. Found:
C, 38.89; H, 6.09; N, 3.04.
Synthesis of tBuP(N–H)N. The preparation of tBuP(N–H)N is
based on the reported synthesis of a related molecule.32
A
Schlenk flask was charged with 2-aminopyridine (1.56 g,
16.6 mmol) in THF (40 mL). The solution was cooled in a dry
ice/acetone bath and n-butyllithium (∼2.5 M hexanes, 7.8 mL,
20 mmol) was added by syringe. The solution was stirred
for one hour then chloro-di-tert-butylphosphine (3.5 mL,
18 mmol) was added and the mixture was allowed to warm to
room temperature. After 45 minutes the volatiles were removed
in vacuo. The residue was extracted with pentane and the
extracts were filtered through a 0.2 μm PTFE syringe filter and
again the solvent was removed in vacuo. The resulting material
was suspended in acetonitrile and stirred overnight. The
resulting white solid tBuP(N–H)N was collected on a medium
frit (1.55 g, 6.5 mmol, 46%). 1H NMR (CDCl3, 500 MHz, 298 K)
δ 8.02 (d, 1H), 7.44 (t(d), 1H), 7.15 (d(d), 1H), 6.62 (t, 1H), 5.02
Synthetic procedures
3
(s-br, 1H), 1.14 (d, 18H, JP–H = 12 Hz). 13C{1H} NMR (CDCl3,
Synthesis of tBuP(O)N. The preparation of tBuP(O)N is based 126 MHz, 298 K) δ 161.3 (d, JP–C = 21 Hz), 148.1, 137.7, 114.4,
2
on the reported synthesis of a related molecule.14 In a Schlenk 109.2 (d, JP–C = 18 Hz), 34.2 (d, JP–C = 19), 28.3 (d, JP–C = 16).
3
1
2
flask, hydroxypyridine (1.11 g, 11.7 mmol) was dissolved in 31P{1H} NMR (CDCl3, 202 MHz, 298 K) δ 59.7.
12022 | Dalton Trans., 2014, 43, 12018–12025
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3
Synthesis of (tBuP(N–H)N)PtMe2 (3). A flask was charged with 1H NMR (C6D6, 500 MHz, 298 K) δ 8.42 (d(d), 1H, JPt–H
=
=
3
tBuP(N–H)N (251 mg, 1.05 mmol) and crude [PtMe2(SMe2)]2 38 Hz), 8.34 (m, 1H), 6.82 (d(d), 1H), 1.29 (d, 18H, JP–H
3
2
(290 mg, 0.51 mmol). Toluene (5 mL) was added and the reac- 15 Hz), 1.08 (d, 3H, JP–H = 4 Hz, JPt–H = 54 Hz), 0.84 (s, 9H).
tion mixture was stirred for approximately an hour during 13C{1H} NMR (C6D6, 126 MHz, 298 K) δ 176.3 (d, 2JP–C = 13 Hz),
3
2
3
which time the solution became pale yellow and a white solid 148.5 (d, JP–C = 3 Hz, JPt–C = 38 Hz), 144.2, 142.6 (d, JP–C
=
formed. The reaction mixture was then diluted with toluene 10 Hz), 132.1, 119.4 (4JPt–C = 29 Hz), 57.4, 40.1 (d, 1JP–C = 28 Hz,
(approx. 10 mL) until the white solid dissolved and was sub- 2JPt–C = 68 Hz), 29.9, 28.3 (d, 2JP–C = 5 Hz, 3JPt–C = 18 Hz), −17.2
sequently filtered through a 0.2 μm PTFE syringe filter. The (d, 2JP–C = 8 Hz, 1JPt–C = 406 Hz). 31P{1H} NMR (C6D6, 202 MHz,
solution was layered with pentane and stored at −35 °C result- 298 K) δ 155 (1JPt–P = 3452 Hz). Anal. Calcd For C19H33N2OPPt:
ing in crystalline (tBuP(N–H)N)PtMe2 (191 mg, 0.41 mmol, C, 42.93; H, 6.26: N, 5.27. Found: C, 42.82; H, 6.14; N, 5.21.
1
2
40%). H NMR (C6D6, 500 MHz, 298 K) δ 8.96 (d, 1H, JPt–H
<
Thermolysis of (tBuP(N–H)N)PtMe2 (3) in C6D6 (or C6H6). In
25 Hz),41 6.88 (t, 1H), 6.00 (t, 1H), 5.95 (d, 1H), 4.58 (s-br, 1H), a typical experiment, a medium-walled NMR tube fitted with
3
2
3
1.68 (d, 3H, JP–H = 7 Hz, JPt–H = 88 Hz), 1.41 (d, 3H, JP–H
=
a ground glass joint was charged with 3 (1.0–10.0 mg,
7 Hz, 2JPt–H = 65 Hz), 1.12 (d, 18H, 2JP–H = 14 Hz). 13C{1H} NMR 0.0021–0.021 mmol) and hexamethylbenzene internal stan-
2
(C6D6, 176 MHz, 298 K) δ 162.9 (d, JP–C = 12 Hz), 147.2 (d, dard. C6D6 (or C6H6) (∼0.4 mL) was added by vacuum transfer,
3JP–C = 2 Hz, JPt–C < 14 Hz),41 137.4, 115.4 (s, JPt–C = 14 Hz), the tube was flame sealed, and an initial 1H NMR spectrum
2
4
3
1
2
109.2 (d, JP–C = 5 Hz), 38.2 (d, JP–C = 12 Hz, JPt–C = 37 Hz), was recorded. The tube was then submerged in an oil bath at
28.4 (d, JP–C = 5 Hz, JPt–C < 19 Hz),41 16.9 (d, JP–C = 113 Hz, 100 °C for four days and was examined daily by H NMR spec-
1JPt–C = 683 Hz), −23.4 (d, 2JP–C = 4 Hz, 1JPt–C = 748 Hz). 31P{1H} troscopy. During heating the solution changed from colorless
NMR (C6D6, 202 MHz, 298 K) δ 105.3 (1JPt–P = 2080 Hz). Anal. to pale yellow.
2
3
2
1
Calcd For C15H29N2PPt: C, 38.87; H, 6.31: N, 6.04. Found: C,
38.81; H, 6.33; N, 6.10.
Spectroscopic data for (tBuP(N–H)N)PtMePh (4). 1H NMR
(C6D6, 300 MHz, 298 K) δ 8.46 (dd, 1H, 3JPt–H = 21 Hz), 8.10 (m,
2H), 7.47 (m, 2H), 7.23 (t, 1H), 6.76 (t, 1H), 5.87 (d, 1H), 5.72
Thermolyses and characterization of thermolyses products
3
2
(m, 1H), 4.59 (br), 1.60 (d, 3H, JP–H = 6 Hz, JPt–H = 88 Hz),
Thermolysis of (tBuP(O)N)PtMe2 (1) in C6D6 to generate 1.13 (d, 18H, JP–H = 14 Hz). 13C chemical shifts obtained by
3
1
(
tBuP(O)C)PtMe(L) (2-L). In a typical experiment, a medium- HMQC (C6D6, H: 500 MHz, 13C: 126 MHz, 298 K) δ 150, 138,
walled NMR tube fitted with a ground glass joint was charged 137, 128, 123, 115, 109, 28, −23. 31P{1H} NMR (C6D6, 121 MHz,
with (P(O)N)PtMe2 (2.0–20.0 mg, 0.0043–0.043 mmol) and hexa- 298 K) δ 102.2 (1JPt–P = 1980 Hz). 1H NMR (pyridine-d5,
methylbenzene internal standard. C6D6 (∼0.4 mL) was added 500 MHz, 298 K) δ 8.54 (d, 1H), 7.43 (t, 1H), 7.06 (d, 1H), 6.24
3
by vacuum transfer, the tube was flame sealed, and an initial (t, 1H), 4.97, 1.45 (d, 18H, JP–H = 14 Hz), Pt–Me not observed
1H NMR spectrum was recorded. The tube was then sub- in pyridine-d5. 31P{1H} NMR (pyridine-d5, 121 MHz, 298 K)
merged in an oil bath at 100 °C for four days, monitored daily δ 104.2 (1JPt–P = 2018 Hz).
by 1H NMR spectroscopy. There was no visual change in the
Generation and characterization of (tBuP(N–H)C)PtMe(pyr-
appearance of the solution during heating and the rollover d5) (5-pyr-d5). Following the thermolysis of 3 (as described
cyclometalation product 2-L formed in approximately 80% above) the volatiles were removed in vacuo and pyridine-d5 was
yield (1H NMR integration vs. hexamethylbenzene internal added by vacuum transfer. 1H NMR (pyridine-d5, 500 MHz,
standard). 1H NMR (C6D6, 500 MHz, 298 K) δ 7.99 (br), 6.48 298 K) δ 7.22 (observed by COSY), 7.01 (d, 1H), 5.85 (t(d), 1H),
2
(br), 1.45 (m, br), 0.77 (br), 0.16 (s, CH4). 31P{1H} NMR (C6D6, 4.97, 1.52 (d, 18H, 3JP–H = 14 Hz), 0.87 (s, 3H, JPt–H = 69 Hz).42
202 MHz, 298 K) δ 159 (br, 1JPt–P = ∼4400 Hz).
31P{1H} NMR (pyridine-d5, 121 MHz, 298 K) δ 87.1 (1JPt–P
=
Generation of (tBuP(O)C)PtMe(pyr-d5) (2-pyr-d5). The vola- 3907 Hz).
tiles from a solution of 2-L in C6D6 (prepared as described
Synthesis of (tBuP(N)N)PtMe(pyr) (6). A 5.0 mL standard
above) were removed in vacuo and pyridine-d5 was added by solution was prepared containing 3 (39.8 mg, 0.0861 mmol),
vacuum transfer. 1H NMR (pyridine-d5, 500 MHz, 298 K) δ 8.22 pyridine (34.6 μL, 0.433 mmol, 5.0 equivalents compared to 3),
3
(d, 1H), 7.07 (d, 1H), 6.76 (t, 1H), 1.44 (d, 18H, JP–H = 15 Hz), and dioxane as an internal standard (3.7 μL, 0.043 mmol) in
2
0.53 (s, JP–H = 46 Hz).42 31P{1H} NMR (C6D6, 202 MHz, 298 K) C6D6. Four medium-walled NMR tubes fitted with Teflon stop-
δ 147.5 (1JPt–P = 4272 Hz).
cocks were each charged with 0.5 mL of this solution and
Synthesis of (tBuP(O)C)PtMe(CNtBu) (2-CNtBu). A solution additional pyridine (3.4 μL, 13.8 μL, and 31.1 μL) was added to
of 2-L in C6D6 (prepared as described above) was treated with three of the tubes (yielding 10, 25, and 50 equivalents of pyri-
1 equiv. tert-butylisonitrile to generate (2-CNtBu) in approxi- dine compared to 3). Initial spectra were taken, showing shifts
mately 80% yield from 1 (1H NMR integration vs. hexamethyl- in the resonances for 3, the most pronounced being a down-
benzene internal standard). Analytically pure material was field shift for the N–H proton. The tubes were heated in a
obtained by allowing the solvent to evaporate from an NMR 100 °C oil bath for 3 days, yielding 6 (86–96% yield by 1H NMR
1
sample then adding pentane to the residue and vigorously stir- integration versus the dioxane standard). H NMR for 6: (C6D6,
ring the suspension before carefully removing the supernatant 300 MHz) δ 8.16 (dt, 6.64, 2H), 7.29 (d, 1H), 6.97 (q, 1H), 6.78
by pipette. X-ray quality crystals were obtained by slow evapora- (dt, 1H), 6.63 (tt, 1H), 6.32 (dd, 2H), 5.67 (td, 1H), 1.61 (d,
3
2
tion of a benzene–pentane solution of (tBuP(O)C)PtMe(CNtBu). 3JP–H = 13.4 Hz, 18H), 0.95 (d, JP–H = 0.7 Hz, JPt–H = 69.9 Hz,
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Table 1 Crystallographic data for 1, 2-CNtBu, 3, and 6
Chemical reference
1
2-CNtBu
3
6
Chemical formula
Formula mass
Temperature
Wavelength
Crystal system
Space group
C
15H28NOPPt
C19H33N2OPPt
531.53
100(2) K
0.71073 Å
Monoclinic
P21/n
C15H29N2PPt
463.46
130(2) K
0.71073 Å
Monoclinic
P21/c
C39H66Cl2N6O2P2Pt2
1174.00
130(2) K
0.71073 Å
Monoclinic
C2/c
464.44
100(2)
0.71073
Monoclinic
P21/c
a/Å
b/Å
c/Å
α/°
14.183(4)
8.830(3)
13.514(4)
90
8.6571(12)
13.566(2)
18.381(3)
90
14.4311(4)
8.6437(2)
13.5536(3)
90
30.661(16)
9.818(12)
17.841(8)
90
β/°
γ/°
95.220(13)
90
1685.4(8)
4
8.414
29 750
5412
0.1628
0.1115
0.2314
0.1469
0.2456
1.191
98.684(8)
90
2133.9(5)
4
6.658
98 592
95.8300(10)
90
1681.91(7)
4
8.428
6896
122.137(13)
90
4548(6)
4
6.372
7882
4116
0.0720
0.0473
0.0870
0.0743
0.0942
1.037
996402
Unit cell volume/Å3
No. of formula units per unit cell, Z
Absorption coefficient, μ/mm−1
No. of reflections measured
No. of independent reflections
Rint
5412
4063
0.0796
0.0360
0.0869
0.0553
0.0957
1.093
0.0517
0.0384
0.0788
0.0537
0.0836
0.998
Final R1 values (I > 2σ(I))
Final wR(F2) values (I > 2σ(I))
Final R1 values (all data)
Final wR(F2) values (all data)
Goodness of fit on F2
CCDC number
996399
996400
996401
3H). 31P NMR (C6D6, 121 MHz) δ 86.4 (s, JP–Pt = 3921 Hz).
1
Acknowledgements
HMQC data (C6D6, 1H: 500 MHz, 13C: 125.7 MHz): δ 8.16,
150.8; 7.28, 115.8; 6.97, 136.0; 6.78, 143.9; 6.62, 136.7; This work was supported by the National Science Foundation
6.29, 125.5; 5.97, 105.7; 1.60, 28.7; 0.94, −21.3 (PtCH3 trans under grant no. CHE-1012045 and DGE-0718124. M.J.R., was
to N).
supported a Mary Gates Undergraduate Research Scholarship
and a Washington NASA Space Grant Consortium Scholarship.
M.R.-M. thanks the Spanish MECD and CSIC for grants to
support her study abroad. We thank Rodney D. Swartz II for
crystallographic assistance.
Crystallographic details
Parameters related to the structures for 1, 2-CNtBu, 3, and 6
are described in Table 1. The data for 1 and 2-CNtBu was
integrated and scaled using SAINT, SADABS within the APEX2
software package by Bruker.43 The data for 3 and 6 was inte-
grated and scaled using DENZO-HKL Scalepack.44 Solution by
direct methods (SHELXS, SIR9745) produced a complete heavy
atom phasing model consistent with the proposed structures.
The structures were completed by difference Fourier synthesis
with SHELXL97.46,47 Scattering factors are from Waasmair and
Kirfel.48 Hydrogen atoms were placed in geometrically ideal-
ized positions and constrained to ride on their parent atoms
with C–H distances in the range 0.95–1.00 Å. Isotropic thermal
parameters Ueq were fixed such that they were 1.2Ueq of their
parent atom Ueq for CH’s and 1.5Ueq of their parent atom Ueq
in case of methyl groups. All non-hydrogen atoms were refined
anisotropically by full-matrix least-squares.
The structure of 1 was highly disordered. To test the dis-
order, a solution in P1 was attempted, which showed the same
disorder with a Flack parameter of close to 0.5. The data was
tested for twinning using Cell_Now49 and by attempting to add
a twin card to the shelex refinement. Some statistical improve-
ment of the structure was achieved by leaving out a larger
section of strongly disagreeing data, however, the remaining
data is still 96.6% complete to 25 degrees in theta.
Notes and references
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