Rollover-assisted C(sp2)-C(sp3) bond formation

Article abstract of DOI:10.1002/chem.201304257

Rollover cyclometalation involves bidentate heterocyclic donors, unusually acting as cyclometalated ligands. The resulting products, possessing a free donor atom, react differently from the classical cyclometalated complexes. Taking advantage of a rollover / retro-rollover reaction sequence, a succession of oxidative addition and reductive elimination in a series of platinum(II) complexes [Pt(N,C)(Me)(PR₃)] resulted in a rare C(sp²)-C(sp³) bond formation to give the bidentate nitrogen ligands 3-methyl-2,2′-bipyridine, 3,6-dimethyl-2,2′- bipyridine, and 3-methyl-2-(2′-pyridyl)-quinoline, which were isolated and characterized. The nature of the phosphane PR₃ is essential to the outcome of the reaction. This route constitutes a new method for the activation and functionalization of C-H bond in the C(3) position of bidentate heterocyclic compounds, a position usually difficult to functionalize. Rock and rollover! Rare cases of C(sp²)-C(sp³) bond formation from Pt ^IV cyclometalated complexes are described, through a succession of oxidative addition and reductive elimination reactions, taking advantage of a rollover / retro-rollover sequence (see scheme).

Full text of DOI:10.1002/chem.201304257

DOI: 10.1002/chem.201304257
Full Paper
&
Cyclometalated Complexes
Rollover-Assisted C(sp2)ÀC(sp3) Bond Formation
Antonio Zucca,*[a] Luca Maidich,[a, b] Laura Canu,[a] Giacomo L. Petretto,[a] Sergio Stoccoro,[a]
Maria Agostina Cinellu,[a] Guy J. Clarkson,[b] and Jonathan P. Rourke[b]
In memory of Professor Giovanni Minghetti, October 18th 1937–January 29th 2014
Abstract: Rollover cyclometalation involves bidentate heter-
ocyclic donors, unusually acting as cyclometalated ligands.
The resulting products, possessing a free donor atom, react
differently from the classical cyclometalated complexes.
Taking advantage of a “rollover”/“retro-rollover” reaction se-
quence, a succession of oxidative addition and reductive
elimination in a series of platinum(II) complexes [Pt(N,C)(Me)-
(PR3)] resulted in a rare C(sp2)ÀC(sp3) bond formation to give
the bidentate nitrogen ligands 3-methyl-2,2’-bipyridine, 3,6-
dimethyl-2,2’-bipyridine, and 3-methyl-2-(2’-pyridyl)-quino-
line, which were isolated and characterized. The nature of
the phosphane PR3 is essential to the outcome of the reac-
tion. This route constitutes a new method for the activation
and functionalization of CÀH bond in the C(3) position of bi-
dentate heterocyclic compounds, a position usually difficult
to functionalize.
In the vast field of cyclometalated derivatives,[8] the so-called
“rollover complexes” constitute an emerging class of com-
pounds.[9] In contrast to classical cyclometalated complexes,
obtained from monodentate ligands, this new class of com-
plexes arise from bidentate heteroaromatic ligands, which,
after chelation, may partially decomplex and undergo internal
rotation around a suitable bond and promote the selective ac-
tivation of a CÀH bond previously directed away from the
metal. After cyclometalation, rollover complexes contain a free
donor atom able to contribute to the reactivity of the complex,
this possibility is usually not accessible to classical cyclometa-
lated species.[10] Among these possibilities we can find proto-
nation,[11] retro-rollover reactions,[12] successive cyclometala-
tions,[9c,13] and polymerization.[14] Recently, catalytic cycles have
been designed, based on rollover and retro-rollover reac-
tions,[15] with the ultimate aim of activating internal positions
in heteroaromatic rings. Rollover complexes were also found
to promote CÀC bond formation in the gas phase.[16]
Introduction
Reductive elimination is one of the most fundamental steps in
stoichiometric and catalytic processes leading to CÀC bond
formation, representing the reaction where the final product is
formed.[1] A great attention has been focused on reductive
elimination reactions from PtIV complexes, which often take
place after initial loss of a ligand to generate a more reactive
five-coordinate intermediate.[2] A great number of studies are
concerned with reductive C(sp3)ÀC(sp3) elimination from meth-
ylplatinum(IV) compounds, and analogous studies have been
carried out on arylplatinum(IV) compounds, including studying
the competition between C(sp2)ÀC(sp2) and C(sp2)Àhalide
bond formation,[3] or C(sp3)- and C(sp2)Àhalide elimination.[4]
In contrast, examples of C(sp2)ÀC(sp3) reductive elimination
are very scarce,[5] though they do include a catalytic process in-
volving platinum(IV) compounds.[6]
PtIV complexes with chelated nitrogen ligands (both N,N and
N,C) are often obtained through oxidative addition of CÀX
bonds.[7] In the case of cyclometalated species, the resulting
PtIV complexes [Pt(N,C)(Me)2(X)(L)] (L=neutral ligand, X=
halide) usually display a fac-PtC3 arrangement, due to the natu-
ral tendency of carbon-bonded ligands to avoid mutual trans
coordination (trans-phobia), and are stable at room tempera-
ture both in solution and in the solid state.
Results and Discussion
We have recently reported the synthesis of the rollover com-
plex [Pt(L-H)(Me)(DMSO)], 1, where L is k2-C,N-C10N2H7, a C(3)-
deprotonated, cyclometalated 2,2’-bipyridine. Complex 1 is the
parent compound of a family of complexes [Pt(L-H)(LX)(LN)]
where LX and LN are anionic and neutral ligands, respective-
ly.[12,13]
[a] Dr. A. Zucca, Dr. L. Maidich, L. Canu, Dr. G. L. Petretto, Prof. S. Stoccoro,
Prof. M. A. Cinellu
Dipartimento di Chimica e Farmacia
Universitꢀ degli Studi di Sassari, Via Vienna 2, 07100 Sassari (Italy)
E-mail: zucca@uniss.it
Complex 2a, [Pt(L-H)(Me)(PPh3)], was obtained both from
1 by substitution of the labile DMSO ligand [Eq. (1)], or directly
from [Pt(Me)2(DMSO)2], bipyridine and PPh3, in a “one pot” re-
action.[12] Complex 2a is stable in air, both in solution and in
the solid state, and was characterized in solution by means of
NMR spectroscopy.
[b] Dr. L. Maidich, Dr. G. J. Clarkson, Dr. J. P. Rourke
Department of Chemistry
University of Warwick, CV4 7AL Coventry (UK)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201304257.
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Table 1. Comparison of structural and DFT calculated[12] bond distances
(ꢁ), ESD in parentheses, and angles (degrees) for complex 2a.
Experimental
Theory
Pt–N1
Pt–C8
Pt–C1
Pt–P1
N1–C6
C6–C7
C7–C8
2.130(2)
2.043(3)
2.077(4)
2.2904(7)
1.360(3)
1.468(5)
1.405(4)
2.1940
2.0373
2.0445
2.3492
1.3508
1.4705
1.4134
Our efforts in obtaining an X-ray diffraction structure were
successful and we managed to obtain crystals suitable for X-
ray determination by slow evaporation of an acetone solution.
The structure of 2a consists of the packing of [Pt(L-
N1–Pt–C8
C8–Pt–C1
C1–Pt1–P1
P1–Pt1–N1
N1–Pt1–C1
P1–Pt1–C8
79.61(9)
91.65(11)
91.41(8)
97.91(6)
169.26(1)
173.67(7)
79.47
90.12
92.77
97.68
169.39
176.84
¯
H)(Me)(PPh3)] molecules in the triclinic space group P1. The ge-
ometry around the metal center deduced from NMR spectros-
copy is confirmed: a slightly distorted square planar PtII com-
plex with a k2-N,C-cyclometalated unit, a PPh3, and a methyl
group (Figure 1).
Though the reaction may generate several isomers, due to
trans-phobia,[12] only fac-PtC3 isomers are expected and consid-
ering the cyclometalated N,C restriction, only two isomers may
be reasonably formed, as indicated in Scheme 1, with the
phosphane in “axial” or “equatorial” position (with respect to
the bipyridine plane).
Figure 1. ORTEP view of the crystal structure of complex 2a. Thermal ellip-
soids are drawn at 50% probability.
The structure is slightly distorted due to the presence of the
bulky PPh3 in the coordination plane and this is reflected by
the angle N1-Pt-CH3, 169.268. In the crystal packing of 2a,
neighboring complexes form discrete dimers interacting via
a p–p overlap between the bipy ligand (N1ÀN12) of one com-
plex and a symmetry related bipy on a neighboring complex.
The interacting p systems are parallel (the interaction lies
across an inversion center) and the closest atomic contact is
C5ÀC7 (3.2965 ꢁ) (see Figure SI1 in the Supporting Informa-
tion). Selected bond distances and angles are given in Table 1
along with the previously DFT calculated data.[12] All the
lengths are in the expected range and no particular deviations
are worth noting.
Scheme 1.
The accepted SN2 mechanism proposed for this reaction re-
quires the trapping of the five-coordinate intermediate formed
by the nucleophilic attack of the PtII on the alkyl halide to form
the kinetic equatorial product which subsequently converts
into the thermodynamic axial product.[7a,18] In the chemistry of
cyclometalated PtIV the isolated product of the reaction is usu-
ally the axial species, and isolation of the equatorial isomer is
extremely rare.
The reaction at room temperature of the electron-rich com-
plex 2a with MeI gives the corresponding PtIV complex [Pt(L-
H)(Me)2(I)(PPh3)], 3a [Eq. (2)], which was isolated in high yields
and characterized. In the absence of an X-ray structural charac-
terization compounds 3a was analytically and spectroscopical-
ly characterized, in particular, through mono and bi-dimension-
al NMR techniques.
One example in which the equatorial isomer was isolated
was with the analogous rollover complex [Pt(L-H)(Me)(PMe3)],
2b, which gives, under the same experimental conditions as
here, the PtIV complex [Pt(L-H)(Me)2(I)(PMe3)], 3b(eq), with the
small PMe3 ligand located in the plane of the cyclometalated
ligand as demonstrated by an X-ray diffraction analysis.[19]
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In the absence of X-ray data, complex 3a was characterized
as the axial isomer, that is, 3a(ax), on the basis of analytical
data and NMR spectroscopy, as detailed below. The 31P NMR
spectrum of 3a(ax) shows PtÀP coupling constant values in
1
agreement with a PtIV species (1JPtÀP =992 Hz), and the H NMR
spectrum shows two different platinum-bonded methyl groups
3
(d=1.12 ppm, 3JPÀH =8 Hz, 2JPtÀH =61 Hz; 1.64 ppm, JPÀH
=
7.5 Hz, 2JPtÀH =71 Hz) as doublets with satellites due to cou-
pling to 195Pt and 31P nuclei. A comparison of 31P, 1H, and
13C NMR data of 3a(ax) and 3b(eq) (see Table 2) suggests co-
ordination of PPh3 in “axial” position in 3a(ax) with respect to
the cyclometalated plane. Accordingly, the 13C NMR spectrum
shows two PtÀCH3 carbon atoms having very different PÀC
Figure 2. NOE contacts for complex 3a.
coupling constants, attributable to an “axial” methyl, coordi-
2
nated trans to phosphorus (CH3(ax): d= +7.06 ppm, JPÀC
=
113 Hz), and an “equatorial” methyl (CH3(eq): d=À6.87 ppm,
2JPÀC =4 Hz) coordinated cis to phosphorus. Furthermore, a 13C-
1H HSQC spectrum showed the following correlations: CH3
trans to P, that is, CH3(ax): dC =7.06 ppm, dH =1.12 ppm, CH3
cis to P that is, CH3(eq): dC =À6.87 ppm, dH =1.64 ppm.
No intermediate species was observed, and the reaction was
then performed with CD3I in an NMR tube, in order to distin-
guish the previously coordinated methyl from that coming
from MeI. Reaction of 2a with CD3I, however, gives from the
beginning an equal distribution of the CD3 group in axial and
equatorial positions. This may be explained assuming that the
first step of the reaction, that is, the addition of Me+, is fol-
lowed by rapid rearrangement of the five-coordinate inter-
mediate before addition of IÀ.
Different geometries between 3a(ax) and 3b(eq) were sup-
31
ported by 195PtÀ P coupling constant values: whereas 2a and
1
1
2b, show similar JPtÀP values (2a: d=33.6 ppm, JPtÀP
=
2229 Hz; 2b: d=À18.6 ppm, 1JPtÀP =2112 Hz), 3a(ax) and
3b(eq) have very different J values (3a(ax): d=À11.2 ppm,
1JPtÀP =992 Hz; 3b(eq): d=À44.6 ppm, 1JPtÀP =1467 Hz). Also
31P chemical shifts do not follow the same trend. Furthermore,
1D-NOE experiments on 3a(ax) (Figure 2) clearly pointed out
the geometry showing, after irradiation at 1.64 ppm (CH3(eq)),
enhancement at 7.05 (H4’), 7.29 (H5’) and 1.12 ppm (CH3(ax)), and
enhancements at 9.58 (H6) and 1.64 ppm (CH3) by irradiation at
It is also worth noting that complex 3a(ax) should be pres-
ent as a racemic mixture, with PPh3 coordinated “above” and
“below” the cyclometalated plane.
In analogy to the analysis carried out in the case of 3b, we
performed DFT calculations on the two most probable isomers
of 3a, the axial and equatorial ones. We decided to concen-
trate only on these two assuming that, due to trans-phobia,
fac PtC3 isomers would be more stable than the other isomers.
Full optimization at PBE0/def2-SVP level led to the conclusion
that the axial is more stable than the equatorial one by
25.4 kJmolÀ1 (DH ZPE corrected in vacuo) showing a very simi-
lar energy difference to that for the two isomers of 3b.[19] This
result is interesting because shows that although the two
7.21 ppm (Hortho, PPh3). Finally, irradiation at 1.12 ppm (CH3(ax)
)
produces enhancement only at 1.64 ppm (CH3(eq)). It follows
that the signal at 1.12 ppm is the axial CH3 while the signal at
1.64 is due to the equatorial one (Table 2).
On the basis of all these data we conclude that 3a does
indeed have a PPh3 coordinated in “axial” position with respect
to cyclometalated plane. The
iodide, coordinated close to the
N-bonded pyridine ring, is re-
sponsible for the downfield shift
Table 2. Selected NMR data.[a]
of the H6 proton (d=9.98 ppm)
as usual.[20] The different behav-
ior of PPh3 and PMe3 derivatives
1H (CH3)
31P
33.6 [2229]
À18.6 [2112]
19.3 [2083]
32.6 [2226]
À11.2 [992]
13C
2a
[Pt(bipy-H)(Me)(PPh3)]
0.74 (7.5) [83]
0.85 (8) [84]
1.00 (5.5) [84]
0.74 (8) [83]
1.12 (8) [61]
1.64 (7.5) [71]
1.40 (7.5) [69]
0.82 (7.5) [69]
1.40 (8.5) [71]
0.76 (8) [56]
1.73 (6.5) [72]
0.96 (7) [57]
1.65 (7.5) [71]
1.19 (7.5) [60]
À12.37 (4.5) [725]
À17.0 (7.0) [n.r.]
2b
[Pt(bipy-H)(Me)(PMe3)]
[Pt(bipy-H)(Me)(PCy3)]
[Pt(L’-H)(Me)(PPh3)]
can be ascribed to different elec-
tronic and steric properties of
the two phosphanes, and in par-
ticular to the larger cone angle
of PPh3, which destabilizes coor-
dination in the same plane of
the cyclometalated bipyridine.
The reaction of 2a with CH3I
was followed by NMR spectros-
copy, in order to identify the
presence of the “kinetic” species
with the phosphane in “planar”
position and its isomerization.
2c[b]
2d
3a(ax)
[Pt(bipy-H)(Me)2(I)(PPh3)]
À6.87 (4) [632]
7.06 (113) [494]
2.88 (3) [593]
6.45 (4) [n.r.]
3b(eq)
3b(ax)
3c(ax)
3d(ax)
[Pt(bipy-H)(Me)2(I)(PMe3)]
[Pt(bipy-H)(Me)2(I)(PMe3)]
[Pt(bipy-H)(Me)2(I)(PCy3)]
[Pt(L’-H)(Me)2(I)(PPh3)]
À44.6 [1467]
À42.9 [1239]
À17.9 [957]
À9.71 [967]
[a] Chemical shift are given in ppm, coupling constants, in parentheses (coupling with 31P) and square brackets
(coupling with 195Pt), are given in Hz. [b] Data taken from ref. [12].
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phosphanes are quite different they seem to have the same
effect on the stabilization of the axial isomer; whether this
effect is due to steric or electronic factors is hard to say.
This reaction has points of interest: 4a is generated by a re-
ductive elimination reaction involving a less common C(sp3)À
C(sp2) coupling; no ethane evolution, formed through a more
common C(sp3)ÀC(sp3) coupling, is observed. In addition, the
reductive elimination is followed by a retro-rollover reaction,
to give a chelated bipyridine adduct, and it may be that this
assists the overall process. CÀC bond making and breaking by
means of transition metals has been demonstrated;[21] in our
case, however, we do not see any experimental evidence for
a reversible process at room temperature.
Treatment of PtIV complexes with Ag+
Compound 3a(ax) is stable in the solid state and in solution,
in the presence of air and moisture. It is well known, however,
that five-coordinated PtIV species are highly reactive towards
reductive elimination reactions.[2] For this reason we reacted
3a(ax) with silver salts in acetone solution in order to abstract
the iodide ligand and create coordinatively unsaturated spe-
cies. Unexpectedly, the reaction of 3a(ax) with AgBF4 gave the
cationic PtII adduct [Pt(k2-N,N-3-Me-2,2’-bipyridine)(Me)(PPh3)]+,
4a, as indicated by analytical and NMR data, where the ligand
3-Me-2,2’-bipyridine is formed through an uncommon C(sp3)À
C(sp2) coupling. The reaction is unique because, following
a retro-rollover process, the newly formed ligand stabilizes the
complex by chelation, taking advantage of the formerly unco-
ordinated nitrogen.
Our observation is in line with that reported recently by
Butschke and Schwarz,[16a] which proposed a C(sp3)ÀC(sp2) cou-
pling in the gas phase by means of PtII rollover complexes.
At first sight, the elimination of ethane, rather than the ob-
served C(sp3)ÀC(sp2) coupling, might be the expected outcome
of the treatment of 3a(ax) with Ag+. After all, this results in
the formation of a strong CÀC bond in ethane with the addi-
tional favorable entropic contribution to the Gibbs free energy
of the reaction that arises from the release of a molecule of
gas. However, these thermodynamic factors are irrelevant in
this case as they neglect to take into consideration the spatial
requirements of the coupling groups at the five coordinate in-
termediate. If we consider the geometry of 3a(ax) we can see
that removal of the iodide results in a vacancy in the equatori-
al plane defined by the cyclome-
1
The H and 31P NMR spectra of 4a show two species in a 1:1
molar ratio; the analysis of the spectra allows their characteri-
zation as the geometric isomers 4a’ and 4a’’, as indicated
below [Eq. (3)].
talated bipyridine. Orbital con-
siderations are clear: reductive
coupling of groups within this
vacancy-containing plane results
in the population of
a non-
bonding orbital, whereas cou-
pling of groups in a four coordi-
nate plane requires occupation
of a high-energy strongly anti-
In particular, the 31P spectrum of 4a shows two singlets with
satellites at 19.2 and 18.9 ppm with coupling constants
(1JPtÀP =4294 and 4318 Hz respectively) indicative of P trans to
bonding orbital.[22]
Thus the two Me groups occupy a four coordinate plane
(elimination unfavored) whereas one of the Me groups and the
sp2 carbon occupy a three coordinate plane (elimination fa-
vored).
1
N in a platinum(II) complex. Also the H NMR spectra show two
PtÀCH3 groups (4a’: 0.78 ppm, 3JPÀH =3.5 Hz, 2JPtÀH =71 Hz;
3
2
4a’’: 0.80 ppm, JPÀH =3.5 Hz, JPtÀH =72 Hz) in agreement with
methyl groups coordinated in cis to a PPh3 and trans to a nitro-
gen. Two singlets at 2.94 and 2.98 ppm were assigned to
methyls on the bipyridine in the 3-position. A 1H-1H NOESY
spectrum helped in the characterization and assignments
(Scheme 2).
Even though the five coordinate intermediate is likely to be
very fluxional, it is impossible for it to rearrange, and for the
two methyl groups to occupy a three coordinate plane, with-
out bringing the large PPh3 group back into the equatorial
plane defined by the cyclometalated group; sterically this
would be unfavorable for exactly the same reasons that
3a(eq) is not observed.
The free ligand 3-Me-2,2’-bipyridine, 5, was isolated by reac-
tion of 4a with dppe, 1,2-bis-diphenylphosphinoethane. The
chelating diphosphine easily displaces the bipy ligand so that
1
species 5 can be isolated in good yield and analyzed. The H
and 13C NMR spectra clearly show the methyl in position-3 (1H,
2.51 ppm, 13C, 18.89 ppm) and a NOE-1d NMR spectrum
showed a contact between the singlet at 2.51 ppm (CH3-bipy)
and the adjacent H4 proton at 7.62 ppm. The aromatic region
show 7 protons, with 4- and 3-spin systems for the unsubsti-
tuted and substituted pyridine rings, respectively, and a COSY
1
Scheme 2.
HÀH spectrum allowed assignment of all H NMR signals. Fur-
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ther evidence for the characterization of 5 was provided by
a high-resolution mass spectrum, which shows the [M+H]+
peak with an excellent match between the calculated and ex-
perimental values (m/z found: 171.0915, calculated for C11H11N2
[M+H]+: 171.0917).
mild conditions. In contrast, the PCy3 complex 2c behaves as
the PPh3 one, to give the axial complex 3c(ax), [Pt(L-H)(Me)2(I)-
(PCy3)]. The reaction of 2b with MeI at room temperature was
followed through NMR spectroscopy: the reaction rapidly pro-
duces the PtIV complex 3b(eq), which converts very slowly
(ca. 10 days at 608C in [D6]acetone) into the isomer having the
phosphane in axial position, 3b(ax), that is, the thermodynami-
cally stable species (Scheme 4). NMR data of 3b(ax) are com-
The synthesis of 3-methyl-2,2’-bipyridine was previously re-
ported in the literature, through co-cyclotrimerization,[23] Ne-
gishi coupling,[24] or Suzuki–Miyaura reactions.[25] Our new syn-
thetic method, however, could be of general use and might be
extended to other bidentate heterocyclic donors taking ad-
vantage of rollover CÀH bond activation.
Assessing the generality of the coupling
In order to better understand the nature of the process we ex-
tended the study to the phosphanes PMe3 and PCy3, both
better donors than PPh3, but with minor and major steric hin-
drance, respectively.
The general applicability of the method was also checked,
extending the study to two bidentate heterocyclic ligands: 6-
methyl-2,2’-bipyridine, L’, and 2-(2’-pyridyl)-quinoline, L’’
(Scheme 3), both able to give rollover cyclometalation.[26]
Scheme 4.
Scheme 3.
1
parable to those of 3a(ax), in particular H Pt-bonded methyls
and 31PÀ195Pt coupling constant values (see Table 3).
Accordingly, a series of platinum(II) complexes were pre-
pared and characterized: [Pt(L-H)(Me)(PMe3)], 2b, [Pt(L-
H)(Me)(PCy3)], 2c, [Pt(L’-H)(Me)(PPh3)], 2d, [Pt(L’’-H)(Me)(PPh3)],
2e, and [Pt(L’’-H)(Me)(PCy3)], 2 f.
In contrast to 3a(ax), treatment of 3b(eq) with AgBF4 does
not induce reductive coupling, but produces a solvato com-
plex, identified as the cationic species [Pt(L-H)(Me)(PMe3)-
(H2O)]+, 3w, with a coordinated water in the equatorial posi-
tion and the phosphane in an axial site, as indicated by NOE
measurements. Treatment of 3w with excess iodide does not
return back to the starting complex 3b(eq) but gives the ther-
modynamic PtIV complex 3b(ax). In contrast to complex 3a(ax)
When a substituent is present in the ligand in proximity of
a nitrogen atom, rollover cyclometalation is facilitated. For this
reason complexes 2b–f can be obtained even at room temper-
ature in acetone, following a one pot synthetic method. Com-
plexes 2d and 2 f are reported
here for the first time. The 1H
and 31P NMR spectra of 2d–f are
very similar to those of 2a,
showing, inter alia, a methyl cou-
pled to 195Pt and 31P in the
Table 3. NMR data for PMe3 complexes.[a]
2b
3b(eq)
0.82 (7.5) [70]
1.40 (7.5) [69]
1.84 (9.5) [12]
7.99 [30]
7.24 [10]
8.32
8.51
8.11
7.52
8.91 [10]
3b(ax)
0.76 (8) [56]
1.40 (8.5) [71]
1.14 (10) [12]
7.69 [44]
7.28 [14]
8.41
8.50
8.18
7.60
9.82 [12]
3w
CH3 ax
CH3 eq
PMe3
H4’
H5’
H6’
0.57 (8) [50]
1.35 (8) [66]
1H NMR spectrum (e.g., 2 f: d=
0.85 (8) [84]
1.57 (8) [21]
8.09 [44]
7.16 [15]
8.28
1.16 (10.5) [12]
7.78 [54]
7.31 [15]
8.47
8.61
8.33
7.84
8.99 [9]
7.10
3
1.11 ppm, 2JPtÀH =84 Hz, JPÀH
=
6.5 Hz) and a phosphorus trans
to a carbon in the 31P spectrum
(e.g., 2 f: d=24.8 ppm, 2090 Hz).
The behavior of the PMe3
complex 2b is different from
that of the PPh3 analogue, 2a, as
only the kinetic product 3b(eq),
H3
8.33
H4
8.10
H5
7.44
H6
8.86 [22]
H2O
31P
195Pt
À18.6 [2112]
À4107
À46.4 [1467]
À3429
À42.9 [1239]
À3411
À25.6 [1243]
À2676
[Pt(L-H)(Me)2(I)(PMe3)],
having
[a] Chemical shift are given in ppm, coupling constants in parentheses (coupling with 31P) and square brackets
(coupling with 195Pt), are given in Hz.
the phosphane in “equatorial”
position was obtained under
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the addition of AgBF4 to 3b(ax) does not result in reductive
elimination but leads back to the water complex 3w. These re-
sults indicate a strong component of steric strain in the octa-
hedral complexes with PPh3, relieved by reductive coupling.
The leading effect in both oxidative addition and reductive
elimination processes seems to be steric rather than electronic,
because the analogous PCy3 complexes 2c and 3c, behave as
2a and 3a, with a slight acceleration, likely due to higher
steric congestion.
As for the reductive elimination process we may conclude
that there is not such strain in the PMe3 complexes 3b(eq)
and 3b(ax), and consequently the coupling does not take
place. It is worth noting that the analogous cyclometalated PtIV
complex originating from 2-phenylpyridine, [Pt(N,C)(Me)2I-
(PPh3)], does not show corresponding behavior, giving a com-
plex mixture of unidentified products. NMR spectra did not
give evidence for C(sp3)ÀC(sp2) bond coupling, so this reactivi-
ty seems to be peculiar to rollover cyclometalated compounds.
Other derivatives
Figure 3. 195Pt-1H HMQC spectra of 2e (below) and 3e (above).
In contrast to 2b, reaction of 2d–f with MeI rapidly gave the
corresponding thermodynamic PtIV derivatives [Pt(L’-H)(Me)2(I)-
(PPh3)], 3d(ax), [Pt(L’’-H)(Me)2(I)(PPh3)], 3e(ax), and [Pt(L’’-
1
H)(Me)2(I)(PCy3)], 3 f(ax), with high yields. H and 31P NMR data
hancements at 1.75 ppm (CH3,eq), whereas irradiating at
1.75 ppm (CH3,eq) gives enhancements at 1.22 (CH3,ax), 7.27 (Ho,
PPh3), and 7.43 ppm (H4’). Furthermore, irradiation at 7.44 ppm
(H4’) enhances signals at 1.75 (CH3,eq), 7.27 (Ho, PPh3), and
7.61 ppm (H5), and irradiation at 9.73 ppm (H6’) gives enhance-
ments at 7.27 (Ho, PPh3), and 7.50 ppm (H5’).
of 3d–f are similar to those of 3a(ax), confirming that the PR3
ligand is coordinated in axial position (see the Experimental
Section).
Confirming the generality of the reductive coupling method,
reaction of 3c–f with AgBF4 gave reductive elimination with
C(sp2)ÀC(sp3) coupling, to give the adducts of the correspond-
ing new methylated ligands, 3-Me-2,2’-bipyridine, (5), 3,6-Me2-
2,2’-bipyridine (6) and 2-pyridyl-3-Me-quinoline (7): [Pt(5)(Me)-
(PCy3)]+, 4c, [Pt(6)(Me)(PPh3)]+, 4d, [Pt(7)(Me)(PPh3)]+, 4e, and
[Pt(7)(Me)(PCy3)]+, 4 f.
In contrast to L, L’ and L’’ have a substituent in proximity to
one nitrogen, so that in the N,N’ bidentate adducts 4d–f steric
repulsions may affect the molar ratio between the geometric
isomers possible for these complexes.
Compounds 2e and 3e(ax) were chosen for a thorough
1
comparative NMR characterization (e.g., H, 31P, 195Pt-1H HMQC,
However, 4d’ and 4d’’ are formed in almost 1:1 molar ratio
1
H-H COSY, NOE-1d). 195Pt-1H HMQC experiments showed the
expected correlations of the Pt signal with H6’, H4, and the
methyl protons in both 2e and 3e(ax). As expected,[27] the
195Pt signal is strongly deshielded in the PtIV complex 3e(ax)
(À3345 ppm) when compared to the PtII complex 2e
(À4177 ppm). The same trend
[Eq. (4)], as demonstrated by H NMR integrals; in one of the
isomers (4d“) a Me in position 6 on the bipyridine is strongly
1
shielded in the H NMR spectrum (d=0.47 ppm) with respect
to other one (d=0.95 ppm) likely due to its proximity to the
PPh3 ligand.
was observed for 2b, 3b(eq)
and 3b(ax) (d=À4107, À3429,
and À3411 ppm, respectively,
Figure 3).
A
series of NOE-1d experi-
ments of 3e(ax) is in agreement
with an axial structure: irradiat-
ing at 1.22 ppm (CH3,ax) gives en-
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In the case of the L’’ ligand, the molar ratio of the isomers
4e’ and 4e“ in solution is slightly different, around 4:3, and
a third, unidentified species is also present in solution.
Conclusion
Rollover cyclometalation constitutes an uncommon way to ac-
tivate the C(3) position in 2,2’-bipyridines as well as in other bi-
dentate heteroaromatic ligands. The coupling reaction report-
ed here constitutes a new synthetic method for the prepara-
tion of substituted heterocycles: in principle, a great array of
heterocyclic bidentate donors may follow this reaction taking
advantage of a rollover/retro-rollover reaction pathway. The
C(3) position is often hard to activate: in the case of 2,2’-bipyri-
dines a bibliographic research (www.reaxys.com, September
2013) showed only 57 3-substituted 2,2’-bipyridines, compared
to 2226, 1101 and 462 2,2’-bipyridines monosubstituted in po-
sition 6, 5, and 4, respectively.
The main species, 4e’, corresponds to the isomer having the
PPh3 ligand in trans to the quinoline nitrogen as may be ex-
pected by steric reasons. This is corroborated by a NOE-1d
spectrum, which shows a NOE contact between the methyl at
0.47 ppm (PtÀMe, 4e’) and the H8 proton, a doublet at
8.28 ppm (Scheme 5). In this case the phosphorus of the two
The reductive elimination step involves a rare C(sp3)ÀC(sp2)
coupling, in place of the more common C(sp3)ÀC(sp3) one, and
whether this reductive elimination takes place or not seems to
be governed principally by steric factors, as indicated by the
behavior of PPh3, PMe3, and PCy3 complexes. The reaction is
unique to rollover complexes; as a comparison, the analogous
phenylpyridine cyclometalated complexes do not exhibit CÀC
coupling when treated with Ag+.
Scheme 5.
isomers are slightly different, 4e’: d=18. 52 ppm, JPtÀP
=
4470 Hz; 4e’’: d=16.69 ppm, JPtÀP =4325 Hz, reflecting the PtÀ
The next steps in this research will be the extension of the
reaction to other bidentate heterocyclic donors and to alkylat-
ing species different from MeI. It will also be useful to under-
stand the factors that favor the process, such as the nature of
the anionic and neutral ligands on the platinum, the presence
of substituents on the cyclometalated ligand, and the proper-
ties of the oxidant species.
1
PPh3 environments. The H and 31P NMR spectra of complex 4 f
show only one a set of broad signals, indicating a fluxional be-
havior in solution in the NMR time scale, likely due to the bulk-
iness of the PCy3 ligand.
From 4d–f the corresponding free ligands 3,6-Me2-2,2’-bipyr-
idine, 6, and 3-methyl-2-(2’-pyridyl)-quinoline, 7 (Scheme 6),
Experimental Section
All the solvents were purified and dried according to standard pro-
cedures.[30] cis-[Pt(Me)2(DMSO)2] was synthesized according to
ref. [31]. Complexes 2a, 2b, 2c, and 2e were obtained as de-
scribed in refs. [12], [19], and [26]. Elemental analyses were per-
formed with a Perkin–Elmer elemental analyzer 240B.
Scheme 6.
were isolated in the solid state and identified through high res-
olution mass spectrometry and NMR spectroscopy.
1H, 13C{1H} and 31P{1H} NMR spectra were recorded with Varian VXR
Also in this case high-resolution mass spectra show the
[M+H]+ peaks with an excellent match with the calculated
values (6: m/z found: 185.1075, calculated for C12H13N2 [M+H]+
: 185.1073; 7: m/z found: 221.1073, calculated for C15H13N2
[M+H]+: 221.1073).
In detail, 1H NMR of the 3,6-Me2-2,2’-bipyridine, 6, clearly
shows six aromatic signals (six protons) and two singlets (3+3
protons) at 2.58 ppm and 2.41 ppm which can be ascribed to
the methyls. Bipyridine 7 shows the expected signals, among
these it is the presence of the singlet attributable to the H4 in
the condensed pyridine ring and the presence of the singlet
integrating for three protons due to the methyl on the C3 that
are diagnostic.
300 or Bruker Avance III 400, 500 or 600 spectrometers. Chemical
1
shifts are given in ppm relative to internal TMS for H and 13C{1H}
and external 85% H3PO4 for 31P{1H}; J values are given in Hz. 1H-
1
1
1H COSY, H-13C HSQC, HMBC, and H NOESY-1d experiments were
performed by means of standard pulse sequences. 195Pt–1H correla-
tion spectra were recorded using a variant of the HMBC pulse se-
quence and 195Pt chemical shifts quoted are taken from here (refer-
enced to external Na2PtCl6). Accurate mass spectra were run on
a Bruker MaXis mass spectrometer.
Single crystals of 2a suitable for X-ray analysis were grown from
an acetone solution. X-ray diffraction data were obtained on an
Oxford Diffraction Gemini four-circle system with a Ruby CCD area
detector. The crystal was held at 150(2) K with the Oxford Cryosys-
tem Cryostream Cobra. Additional material available from the Cam-
bridge Crystallographic Data Centre comprises H-atom coordinates,
thermal parameters and the remaining bond lengths and angles,
deposition number CCDC 963378. Full details are given in the Sup-
porting Information.
To the best of our knowledge there is only one synthesis of
6 in the literature, by means of co-cyclotrimerization of 2-cya-
nopyridine with propyne.[28] Similarly, there is only one synthe-
sis of 7 present in the literature, that of Hoste in 1950.[29]
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Method A: A [D6]acetone solution of 3b was heated to 608C and
the progress of the reaction was monitored by 1H and 31P NMR
spectroscopy. The reaction looks complete after 10 days. Quantita-
tive yield by NMR criteria.
Method B: A [D6]acetone solution of 3b was treated with excess of
AgBF4, removal of the precipitate by filtration and subsequent
treatment with excess KI gave the product. The characterization
was carried out after filtration of the solid in the NMR tube. Quanti-
Synthesis of 2c–d
1
tative yield. H NMR (600 MHz, [D6]acetone, 258C): d=9.82 (d sat,
3
An equimolar amount of the ligand L’ or L’’ was added to a solution
of cis-[Pt(Me)2(DMSO)2] (0.80 mmol) in acetone (40 mL) under nitro-
gen atmosphere. The solution was refluxed for 5 h, then the phos-
phane was added (PPh3 or PCy3, 10% excess). After 1 h the solu-
tion was evaporated to small volume and treated with n-pentane
to give a precipitate which was filtered off, washed with n-pentane
and vacuum-dried to give the analytical sample as a yellow solid.
[Pt(L’-H)(Me)(PPh3)], 2d: Yield 85%; m.p. 200–2018C; 1H NMR
(CDCl3, 258C, TMS): d=8.34 (dd, J(H,H)=8.9, 1.6 Hz, 1H; H3); 8.11
(dd sat, 1H, 3J(H,H)=7.8 Hz, 4J(H,P)=5.5 Hz, 3J(H,Pt)=47 Hz, 1H;
H4’); 7.68–7.80 (m, 6H; Hm PPh3); 7.35–7.45 (m, 11H; H6 +H4 +Ho +
Hp PPh3); 7.10 (dd sat, J(H,H)=7.8, 1.6 Hz, J(H,Pt)=25 Hz, 1H; H5’);
6.63 (td, J(H,H)=7.4, 5.5, 1.6 Hz, 1H; H5); 2.53 (s, 3H; Me-bipy);
0.73 (d sat, 3J(H,P)=8 Hz, 2J(H,Pt)=83 Hz, 3H; Pt-Me); 31P NMR
(CDCl3, 258C, 85% H3PO4): d=33.6 (s sat, J(P,Pt)=2226 Hz, PPh3);
elemental analysis calcd (%) for C30H27N2PPt: C 56.16, H 4.24,
J(H,H)=5.6 Hz, J(H,Pt)=12 Hz, 1H; H6); 8.50 (d br, J(H,H)=8.0 Hz,
1H; H3); 8.41 (dt, J(H,H)=4.4, 1.2 Hz, 1H; H6’); 8.18 (td, J(H,H)=7.8,
1.2 Hz, 1H; H4); 7.69 (dt sat, J(H,H)=7.8, 1.2 Hz, 3J(H,Pt)=44 Hz,
1H; H4’); 7.60 (ddd, J(H,H)=7.3, 5.6, 1.4 Hz, 1H; H5); 7.28 (dd sat,
J(H,H)=7.8, 4.5 Hz, 4J(H,Pt)=14 Hz, 1H; H5’); 1.40 (d sat, 3J(H,P)=
2
2
8.5 Hz, J(H,Pt)=71 Hz, 3H; Pt-Me(eq)); 1.14 (d sat, J(H,P)=10 Hz,
3J(H,Pt)=12 Hz, 9H; PMe3); 0.76 (d sat, 3J(H,P)=8 Hz, 2J(H,Pt)=
56 Hz, 3H; Pt-Me(ax)); 31P{1H} NMR (242.9 MHz, [D6]acetone, 258C):
d=À42.9 (s sat, J(P,Pt)=1239 Hz, PMe3); 195Pt-1H HMQC (600 MHz,
[D6]acetone, 258C): d=À3411 (d, J(Pt,P)=1250 Hz) correlates with
signals at 9.82, 7.69, 1.40, 1.14, 0.76.
3
[Pt(L-H)(Me)2(PMe3)(H2O)] (3w): A [D6]acetone solution of 3b was
treated with excess of AgBF4. The characterization was carried out
after filtration of the solid in the NMR tube. Quantitative yield. The
same synthesis works starting from the complex 3b’. 1H NMR
(600 MHz, [D6]acetone, 258C): d=8.99 (d br sat, J(H,H)=5.0 Hz,
3J(H,Pt)=9 Hz, 1H; H6); 8.61 (d br, J(H,H)=7.8 Hz, 1H; H3); 8.47 (d
br, J(H,H)=4.2 Hz, 1H; H6’); 8.33 (t br, J(H,H)=7.8 Hz, 1H; H4); 7.84
(ddd, J(H,H)=7.4, 5.6, 1.5 Hz, 1H; H5); 7.78 (d sat, J(H,H)=7.9 Hz,
1
N 4.37; found: C 55.94, H 3.99, N 4.21.
[Pt(L’’-H)(Me)(PCy3)], 2 f: Yield 92%; m.p. 2258C; 1H NMR (CDCl3,
258C, TMS): d=8.85 (d sat, J(H,H)=5.4 Hz, J(H,Pt)=14 Hz, 1H; H6);
3
4
3J(H,Pt)=54 Hz, 1H; H4’); 7.31 (dd sat, J(H,H)=7.8, 4.8 Hz, J(H,Pt)=
4
3
8.69 (dd, J(H,H)=7.9 Hz, 1H; H3); 8.59 (d, J(H,H)=5.5 Hz, J(H,Pt)=
50 Hz, 1H; H4’); 7.97–8.01 (m, 2H; H4 + H8’); 7.79 (d, J(H,H)=8.2 Hz,
1H; H5’); 7.55 (dd, J(H,H)=7.2, 6.9 Hz, 1H; H7’ or H6’); 7.41 (dd,
J(H,H)=8.1, 5.5 Hz, 1H; H6’ or H7’); 7.30 (dd, J(H,H)=5.8 Hz, 1H;
H5); 2.46–1.18 (m, 33H; PCy3); 1.11 (d sat, J(H,P)=6.5 Hz, J(H,Pt)=
84 Hz, 3H; Pt-Me); 31P NMR (CDCl3, 258C, 85% H3PO4): d=24.8 (s
sat, 1J(P,Pt)=2090 Hz, PCy3); elemental analysis calcd (%) for
C33H45N2PPt: C 56.97, H 6.52, N 4.03; found: C 56.95, H 6.94, N 4.04.
15 Hz, 1H; H5’); 7.10 (d, 3J(H,P)=1.5, 2H; Pt-OH2); 1.35 (d sat,
2
2
3J(H,P)=8 Hz, J(H,Pt)=66 Hz, 3H; Pt-Me(eq)); 1.16 (d sat, J(H,P)=
10.5 Hz, 3J(H,Pt)=12 Hz, 9H; PMe3); 0.57 (d sat, 3J(H,P)=8 Hz,
2J(H,Pt)=50 Hz, 3H; Pt-Me(ax)); 31P{1H} NMR (242.9 MHz,
[D6]acetone, 258C): d=À25.6 (s sat, J(P,Pt)=1243 Hz, PMe3); 195Pt-
1H HMQC (600 MHz, [D6]acetone, 258C): d=À2676 (d, J(Pt,P)=
1511 Hz) correlates with signals at 8.99, 7.78, 7.31, 7.10, 1.35, 1.16,
0.57; 31P-1H HMQC (600 MHz, [D6]acetone, 258C): d=À25.6 corre-
lates with 8.99, 7.78, 7.10, 1.35, 1.16, 0.57.
3
2
General Procedure for Preparation of Compounds 3
[Pt(L-H)(Me)2(I)(PCy3)] (3c(ax)): Yield: 56%; m.p. 203–2048C;
1H NMR (CDCl3, 258C, TMS): d=9.98 (d, J(H,H)=5.2 Hz, JPtÀH
=
MeI (3.00 mmol) was added to a solution of 2 (0.50 mmol) in ace-
tone (15 mL) under nitrogen atmosphere. The solution was stirred
for 2 h at room temperature, then Et2O was added to give a precipi-
tate which was filtered, washed with Et2O and vacuum-dried to
give the analytical sample as a pale yellow solid.
[Pt(L-H)(Me)2(I)(PPh3)] (3a(ax)): Yield: 64%; m.p. 1698C; 1H NMR
(CDCl3, 258C, TMS): d=9.58 (d sat, J(H,H)=5.5 Hz, J(H,Pt)=10 Hz,
12 Hz, 1H; H6), 8.41 (m, 2H; H3 +H6’), 7.94 (td, J(H,H)=7.9, 6.8 Hz,
3
1H; H4), 7.77 (d sat, JPtÀH =47 Hz, J(H,H)=7.8 Hz, 1H; H4’), 7.40 (t,
J(H,H)=6.2, 6.0 Hz, 1H; H5’), 7.21 (ddd, J(H,H)=7.6, 4.6 Hz, 1H; H5),
2.0–0.90 (m, 33H; PCy3), 1.73 (d sat, 2J(H,Pt)=72 Hz, 3J(H,P)=
3
6.5 Hz, 3H; Pt-Me), 0.96 (d sat, 2J(H,Pt)=57 Hz, J(H,P)=7 Hz, 3H;
Pt-Me); 31P NMR (CDCl3, 258C, 85% H3PO4): d=À17.9 (s sat,
1J(P,Pt)=957 Hz, PCy3); elemental analysis calcd (%) for
C30H46N2PIPt: C 45.75, H 5.89, N 3.56; found: C 46.47, H 5.71, N 3.62.
3
1H; H6), 8.25 (d, J(H,H)=4.1 Hz, 1H; H6’), 8.20 (d, J(H,H)=7.4 Hz,
1H; H3), 7.53 (t, J(H,H)=7.5 Hz, 1H; H4), 7.26–7.11 (m, 16H; HPPh3
+
H5), 7.05 (d sat, J(H,H)=7.9 Hz, 3J(H,Pt)=47 Hz, 1H; H4’), 6.82 (dd
sat, J(H,H)=7.9, 4.6 Hz, 3J(H,Pt)=15 Hz, 1H; H5’), 1.64 (d sat,
3J(H,P)=8 Hz, 2J(H,Pt)=71 Hz, 3H; Pt-Me), 1.12 (d sat, 3J(H,P)=
7.5 Hz, 2J(H,Pt)=60 Hz, 3H; Pt-Me); 13C NMR (CDCl3, 258C, TMS):
d=160.2 (s sat, J(C,Pt)=38 Hz); 152.8 (s sat, J(C,Pt)=6 Hz); 145.8
(d sat, J(C,P)=2 Hz, J(C,Pt)=4 Hz); 145.2 (d, J(C,P)=5 Hz); 139.5 (s);
135.9 (d, J(C,P)=2 Hz); 134.8 (d sat, J(C,P)=8.5 Hz, J(C,Pt)=3 Hz);
131.0 (s sat, J(C,Pt)=n.r.); 130.9 (s); 130.6 (s sat, J(C,Pt)=11 Hz);
128.8 (d, J(C,P)=9 Hz); 126.7 (s sat, J(C,Pt)=10 Hz); 126.4 (d sat,
J(C,P)=2 Hz, J(C,Pt)=25 Hz); 122.8 (s sat, J(C,Pt)=16 Hz); 7.04 (d
sat, J(C,P)=113 Hz, J(C,Pt)=494 Hz); À6.89 (d sat, J(C,P)=4 Hz,
J(C,Pt)=632 Hz); 31P NMR (CDCl3, 258C, 85% H3PO4): d=À9.3 (s
sat, 1J(P,Pt)=970 Hz, PPh3); elemental analysis calcd (%) for
C30H28N2PIPt: C 46.82, H 3.67, N 3.64; found: C 46.95, H 3.73, N 3.66.
[Pt(L’-H)(Me)2(I)(PPh3)], 3d(ax): (3 h, RT, ppt with n-hexane). Yield
1
70%; m.p. 165–1708C; H NMR (CDCl3, 258C, TMS): d=9.55 (dd sat,
3
J(H,H)=5.6, 1.6, 0.8 Hz, J(H,Pt)=17 Hz, 1H; H6), 8.22 (dd, J(H,H)=
8.0, 0.8 Hz, 1H; H3), 7.82–7.69 (m, 2H; H4 +H5), 7.05–7.26 (m, 15H;
3
4
PPh3), 6.88 (dd sat, J(H,H)=8 Hz, J(H,Pt)=45 Hz, J(H,P)=1 Hz, 1H;
4
H4’), 6.69 (d sat, J(H,H)=8 Hz, J(H,Pt)=14 Hz, 1H; H5’), 2.51 (s, 3H;
Me-bipy), 1.65 (d sat, 2J(H,Pt)=71 Hz, 3J(H,P)=7.5 Hz, 3H; Pt-Me),
1.19 (d sat, 2J(H,Pt)=60 Hz, 3J(H,P)=7.5 Hz, 3H; Pt-Me); 31P NMR
1
(CDCl3, 258C, 85% H3PO4): d=À9.71 (s sat, J(P,Pt)=967 Hz, PPh3);
elemental analysis calcd (%) for C31H30IN2PPt: C 47.52, H 3.86, N
3.58; found: C 47.37, H 3.43, N 3.30.
[Pt(L’’-H)(Me)2(I)(PPh3)], 3e(ax): (5 h, 358C, ppt with n-pentane).
1
Yield 74%; m.p. 150–1558C; H NMR (CDCl3, 258C, TMS): d=9.72 (d
3
sat, J(H,H)=5.5 Hz, J(H,Pt)=10 Hz, 1H; H6); 8.50 (d, J(H,H)=7.9 Hz,
[Pt(L-H)(Me)2(I)(PMe3)] (3b(ax)):
1H; H3); 8.02 (d, J(H,H)=8.2 Hz, 1H; H8’); 7.82 (t, J(H,H)=7.9,
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7.5 Hz, 1H; H4); 7.62 (dd, J(H,H)=7.8, 6.4 Hz, 1H; H7’ or H4’); 7.47–
7.38 (m, 2H; H6’ + H5’); 7.29–6.95 (m, 17H; PPh3 + H5 + H7’ or H4’);
[Pt(k2-N,N-3-Me-2-(2’-pyridyl)quinoline)(Me)(PPh3)][BF4] (4e-BF4):
1
Yield 70%. NMR selected signals: H NMR (CDCl3, 258C, TMS): d=
3
2
3
1.75 (d sat, J(H,P)=7.5 Hz, J(H,Pt)=71 Hz, 3H; Pt-Me); 1.29 (d sat,
9.05 (d, 1H), 8.97 (m sat, J(H,Pt)=n.r., 1H; H6), 8.85–6.75 (m, aro-
2
3J(H,P)=7.5 Hz, J(H,Pt)=60 Hz, 3H; Pt-Me); 31P NMR (CDCl3, 258C,
matics), 3.17 (s, 3H; Me-pyquinoline, species B), 3.16 (s, 3H; Me-py-
quinoline, species A), 0.95 (d sat, 2J(H,Pt)=73 Hz, 3H; Pt-Me species
B), 0.47 (d sat, 2J(H,Pt)=72 Hz, 3H; Pt-Me species A); 31P NMR
85% H3PO4): d=À10.42 (s sat, 1J(P,Pt)=956 Hz, PPh3); elemental
analysis calcd (%) for C34H30N2PIPt.2H2O: C 49.83, H 3.69, N 3.42;
found: C 47.78, H 3.67, N 2.96.
1
(CDCl3, 258C, 85% H3PO4): d=18.52 (s sat, J(P,Pt)=4470 Hz, spe-
cies A, PPh3); 16.69 (s sat, 1J(P,Pt)=4325 Hz, species B, PPh3).
31P NMR spectrum shows the presence of an unidentified third spe-
cies (40% ca.) which has a chemical shift of 31.9 ppm with
1J(P,Pt)=3144 Hz. NOE-1d: irradiating at 0.47 ppm (Pt-Me, species
A) there is enhancement of the doublet at 8.28 ppm attributable
to the H8 of the species A.
[Pt(L’’-H)(Me)2(I)(PCy3)], 3 f(ax): (1 h, 358C, ppt with n-pentane).
Yield 63%; m.p. 185–1908C; H NMR (CDCl3, 258C, TMS): d=10.06
1
(d sat, J(H,H)=5.4 Hz, 3J(H,Pt)=17 Hz, 1H; H6), 8.73 (d, J(H,H)=
7.1 Hz, 1H; H3), 8.16–8.00 (m, 3H; H4 + H4’ +H5’ or H8’), 7.79 (d,
J(H,H)=7.8 Hz, 1H; H8’or H5’), 7.67 (dd, J(H,H)=7.4, 6.7 Hz, 1H, H6’
or H7’), 7.61–7.46 (m, 2H; H7’or H6’ + H5), 2.00–0.82 (m, 33H; PCy3),
1.85 (d sat (overlapping), 3J(H,P)=6 Hz, 2J(H,Pt)=72 Hz, 3H; Pt-
3
2
[Pt(k2-N,N-3-Me-2-(2’-pyridyl)quinoline)(Me)(PCy3)][BF4] (4 f-BF4):
(3 h, 358C) Yield 68%; 1H NMR (CDCl3, 258C, TMS): d=9.00–7.76
(m, 9H; broad overlapping signals); 3.11 (s, 3H; Me-bipy); 2.45–
0.75 (m, 33H; PCy3), 0.54 (s br sat, 2J(H,Pt)=74 Hz, 3H; Pt-Me);
31P NMR (CDCl3, 258C, 85% H3PO4): 15.34 (s br sat, 1J(P,Pt)=
4157 Hz, PCy3).
Me); 1.01 (d sat, J(H,P)=7 Hz, J(H,Pt)=57 Hz, 3H; Pt-Me); 31P NMR
1
(CDCl3, 258C, 85% H3PO4): d=À17.60 (s sat, J(P,Pt)=945 Hz, PCy3);
elemental analysis calcd (%) for C35H51N2PIPt: C 49.30, H 6.03,
N 3.29; found: C 49.36, H 6.40, N 2.97.
3-Methyl-2,2’-bipyridine (5): To a solution of [Pt(k2-N,N-3-Me-2,2’-
bipyridine)(Me)(PCy3)][BF4] (71.1 mg; 0.0948 mmol) in CH2Cl2
(15 mL) dppe was added under stirring (38.1 mg, 0.0956 mmol).
The solution was stirred for 15 min at room temperature. The solu-
tion immediately became colorless. The solution was concentrated
to small volume, treated with Et2O and filtered. The desired prod-
uct was obtained by flash chromatography using Et2O as eluent.
Synthesis of complexes 4-BF4
A 10% excess of AgBF4 (0.22 mmol) was added to a solution of 3
(0.20 mmol) in acetone (15 mL) under stirring. The mixture was
stirred for 3 h at room temperature, then filtered through Celite to
remove the AgI formed and washed with acetone. The filtered so-
lution was concentrated to small volume and treated with Et2O.
The precipitate formed was filtered, washed with Et2O, and
vacuum-dried to give the analytical sample as a pale yellow solid.
1H NMR (CDCl3, 258C, TMS): d=8.69 (ddd, J(H,H)=4.8, 1.6, 0.9 Hz,
1H; H6), 8.54 (d, J(H,H)=4.8 Hz, 1H; H6’), 7.83 (td, 1H; H4), 7.78 (dd,
1H; H3), 7.62 (d, J(H,H)=7.8 Hz, 1H; H4’), 7.30 (ddd, J(H,H)=7.6, 4.8,
1.8 Hz, 1H; H5); 7.25 (dd, J(H,H)=7.7, 4.8 Hz, 1H; H5’); 2.51 (d, 3H;
Me-bipy); 13C NMR (CDCl3): 158.91, 156.31, 148.47, 146.69, 139.19,
136.54, 132.26, 124.16, 123.01, 122.64, 19.89 (Me-bipy); MS: m/z
found 171.0915, calculated for C11H11N2 [M+H]+, 171.0917.
[Pt(k2-N,N-3-Me-2,2’-bipyridine)(Me)(PPh3)][BF4] (4a-BF4): Yield
70%. Selected 1H NMR data (CDCl3, 258C, TMS), 4a’: d=9.07 (m,
1H; H6), 8.59 (d, 1H; H3’), 8.43 (d, 1H; H4), 7.93 (dd, 1H; H3), 7.88
(m, 6H; Hortho PPh3), 2.98 (s, 3H; Me-bipy), 0.78 (d sat, 3J(H,P)=
2
3.5 Hz, J(H,Pt)=71 Hz, 3H; Pt-Me). 4a’’: d=9.19 (m, 1H; H6’), 8.69
(dd, 1H; H3), 8.58 (dd, 1H; H4’), 8.17 (dd, 1H; H4), 8.03 (m, 1H; H5’),
3
7.88 (m, 6H; Hortho PPh3), 2.94 (s, 3H; Me-bipy), 0.81 (d sat, J(H,P)=
3,6-Dimethyl-2,2’-bipyridine (6): To a solution of [Pt(k2-N,N-3,6-
Me2-2,2’-bipyridine)(Me)(PPh3)][BF4] (51 mg; 0.0659 mmol) in CH2Cl2
(15 mL) dppe was added under stirring (25.8 mg, 0.0647 mmol).
The solution was stirred for 15 min at room temperature. The solu-
tion immediately became colorless. The solution was concentrated
to small volume, treated with Et2O and filtered. The desired prod-
uct was obtained by flash chromatography using Et2O as eluent.
3.5 Hz, 2J(H,Pt)=72 Hz, 3H; Pt-Me); 31P NMR (CDCl3, 258C, 85%
H3PO4): d=19.2 ppm (s sat, 1J(P,Pt)=4294), 18.9 ppm (s sat,
1J(P,Pt)=4318 Hz).
[Pt(k2-N,N-3-Me-2,2’-bipyridine)(Me)(PCy3)][BF4] (4c-BF4): Yield:
98%; m.p. 160–1658C; 1H NMR (CDCl3, 258C, TMS): d=8.95 (d,
J(H,H)=6.2 Hz, 1H; H6), 8.80 (d, J(H,H)=5.0 Hz, 1H; H6’), 8.49–8.36
(m, 2H; H3 +H4), 8.16 (t, J(H,H)=8.6, 8.5 Hz, 1H; H4’), 7.85–7.68 (m,
2H; H5 +H5’), 2.90 (d, 3H; Me-bipy), 2.45–1.19 (m, 33H; PCy3), 1.01
(d sat, 3J(H,P)=2 Hz, 2J(H,Pt)=72 Hz, 3H; Pt-Me), 0.94 (d sat,
3J(H,P)=2 Hz, 2J(H,Pt)=53 Hz, 3H; Pt-Me); 31P NMR (CDCl3, 258C,
85% H3PO4): d=16.8 (s sat, 1J(P,Pt)=4074 Hz, PCy3); 16.1 (s sat,
1J(P,Pt)=4034 Hz, PCy3); IR (Nujol): n=1080 cmÀ1 (BF4À); LM (ace-
tone, 5 10À4 m)=120 cm2 ohmÀ1 molÀ1; elemental analysis calcd (%)
for C30H46BF4N2PPt: C 48.20, H 6.20, N 3.75; found: C 43.70, H 5.61,
N 3.43.
1H NMR (CDCl3, 258C, TMS): d=8.69 (d, J(H,H)=4.8 Hz, 1H; H6),
7.81 (ddd, J(H,H)=7.8, 7.5 Hz, 1H; H4), 7.73 (d, J(H,H)=7.8 Hz, 1H;
H3), 7.51 (d, J(H,H)=7.9 Hz, 1H; H4’), 7.10 (d, J(H,H)=7.9 Hz, 1H;
H5’), 2.58 (s, 3H; Me-bipy), 2.41 (s, 3H; Me-bipy); MS: m/z found
185.1075, calculated for C12H13N2 [M+H]+, 185.1073.
3-Methyl-2-(2’-pyridyl)quinoline (7): To a solution of [Pt(k2-N,N-3-
Me-2-(2’-pyridyl)quinoline)(Me)(PPh3)][BF4] (35 mg; 0.0440 mmol) in
CH2Cl2 (15 mL) dppe was added under stirring (20 mg,
0.0502 mmol). The solution was stirred for 15 min at room temper-
ature. The solution immediately became colorless. The solution
was concentrated to small volume, treated with Et2O and filtered.
The desired product was obtained by flash chromatography using
Et2O as eluent.
[Pt(k2-N,N-3,6-Me2-2,2’-bipyridine)(Me)(PPh3)][BF4] (4d-BF4): (3 h,
RT) Yield 98%; m.p. 160–1658C; 1H NMR (CDCl3, 258C, TMS): d=
8.85 (m, 3J(H,Pt)=38 Hz, 1H; H6 cis-Pt-Me), 8.50–7.20 (m, aromat-
ics), 6.90 (m, 2H), 2.95 (s, 3H; Me-bipy), 2.92 (s, 3H; Me-bipy), 2.84
(s, 3H; Me-bipy), 1.94 (s, 3H; Me-bipy (cis-Pt-PPh3)), 0.81 (d sat,
3J(H,P)=3.5 Hz, 2J(H,Pt)=70 Hz, 3H; Pt-Me); 0.46 (d sat, 3J(H,P)=
4.5 Hz, 2J(H,Pt)=70 Hz, 3H; Pt-Me); 31P NMR (CDCl3, 258C, 85%
H3PO4): d=18.22 (s sat, 1J(P,Pt)=4463 Hz, PPh3); 15.27 (s sat,
1H NMR (CDCl3, 258C, TMS): d=8.72 (ddd, J(H,H)=4.8 Hz, 1H; H6),
8.13 (d, J(H,H)=8.5 Hz, 1H; H8’), 8.05 (s, 1H; H4’), 7.89–7.84 (m, 2H;
H3 +H4), 7.80 (d, J(H,H)=8.2 Hz, 1H; H5’); 7.67 (m, 1H; H6’ or H7’);
7.54 (m, 1H; H6’ or H7’); 7.30 (m, 1H; H5); 2.62 (s, 3H; Me); MS: m/z
found 221.1073, calculated for C15H13N2 [M+H]+, 221.1073.
1J(P,Pt)=4463 Hz, PPh3). IR (Nujol): n=1060 cmÀ1 BF4 (broad);
À
696 cmÀ1 Ph (PPh3); elemental analysis calcd (%) for
C31H30BF4N2PPt: C 50.08, H 4.07, N 3.77; found: C 49.57, H 4.32,
N 3.36.
Chem. Eur. J. 2014, 20, 5501 – 5510
www.chemeurj.org
5509
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
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Acknowledgements
Financial support from Universitꢂ di Sassari (FAR) is gratefully
acknowledged. L.M. gratefully acknowledges a PhD fund, fi-
nanced on POR/FSE 2007–2013, from Regione Autonoma della
Sardegna. We are grateful for support from Advantage West
Midlands (AWM) (partially funded by the European Regional
Development Fund) for the purchase of a high-resolution mass
spectrometer and the XRD system that was used to solve the
crystal structure of 2a.
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Keywords: bond coupling · NMR spectroscopy · platinum ·
reductive elimination · rollover cyclometalation
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Received: October 31, 2013
Revised: January 17, 2014
Published online on March 28, 2014
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Chem. Eur. J. 2014, 20, 5501 – 5510
www.chemeurj.org
5510
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim