A luminescent Pt-POCN pincer complex via direct cyclometalation

Article abstract of DOI:10.1016/j.jorganchem.2015.03.002

We report the synthesis of the novel pincer complex κ^P,κ^C,κ^N-[2,6-(iPr₂PO)-(C₆H₃)(CH₂[c-N(CH₂)₄O])]PtBr (Pt(POCN)Br, 1a) by direct cyclometalation of PtBr₂(SEt₂)₂. This represents the first report of a mixed-donor platinum pincer complex synthesized using cyclometalation. The complex is emissive in the solid state and frozen solution. Comparison of the structure of 1a to related pincer complexes provides insights into the effect of pincer arm donor substituents on cyclometalation.

Full text of DOI:10.1016/j.jorganchem.2015.03.002

Journal of Organometallic Chemistry 785 (2015) 100e105
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Note
A luminescent Pt-POCN pincer complex via direct cyclometalation
Kevin B. Lavelle, Liliana Gutierrez, Jeanette A. Krause, William B. Connick*
University of Cincinnati, Department of Chemistry, PO Box 210172, Cincinnati, OH 45221-0172, USA
a r t i c l e i n f o
a b s t r a c t
We report the synthesis of the novel pincer complex kP kC N-[2,6-(iPr2PO)-(C6H3)(CH2[c-N(CH2)4O])]PtBr
, ,k
Article history:
Received 19 December 2014
Received in revised form
1 March 2015
Accepted 3 March 2015
Available online 11 March 2015
(Pt(POCN)Br, 1a) by direct cyclometalation of PtBr2(SEt2)2. This represents the first report of a mixed-
donor platinum pincer complex synthesized using cyclometalation. The complex is emissive in the
solid state and frozen solution. Comparison of the structure of 1a to related pincer complexes provides
insights into the effect of pincer arm donor substituents on cyclometalation.
© 2015 Elsevier B.V. All rights reserved.
Keywords:
Cyclometalation
Platinum
Mixed-donor
Pincer complex
Crystal structure
Emission spectroscopy
Introduction
examined [12,17,32,33]. Such studies have served to deepen un-
derstanding of the influence of ligand architecture on the electronic
For a number of years, chelating symmetric pincer complexes of
platinum group metals have been studied, with emphasis on cat-
alytic applications [1e6]. For example, phosphine and phosphinite
pincer complexes are active catalysts for reactions ranging from
CO2 reduction [7e9], hydrosilylation [10,11], and the Kharasch
addition [12] to organic carbonecarbon coupling reactions [13].
More recently, there has been growing interest in mixed-donor li-
gands, in which either the central carbon donor is replaced with a
heteroatom [14e16] or the two pincer arms each have different
donors [17e20]. These modifications influence the ligand's steric
and donor properties, allowing for fine-tuning of reactivity
(Scheme 1). Less attention has been given to their electronic and
emission spectroscopic properties. Even among symmetric pincer
ligand complexes, although NCN derivatives have been the subject
of a number of such studies [21e31] (e.g., Pt(pip2NCN)Lnþ com-
plexes [29], Scheme 1), phosphorus-containing pincer complexes
have received comparatively little attention. One exception is the
work of Zargarian and coworkers, which focused on a charge-
transfer band observed at approximately 400 nm in both nickel
POCOP and POCN complexes. The effects of modifications to pincer
ligand substituents, metal oxidation state, and trans-ligand were
structures of pincer complexes, which is essential for intelligent
ligand design aimed at accessing multielectron catalytic properties
of pincer complexes using light.
Because of the rich emission spectroscopy, long-lived excited
states and photochemistry of many platinum complexes, there is
particular interest in mixed-donor pincer ligand platinum com-
plexes. Yet surprisingly little precedent exists for the synthesis of
such compounds. The one platinum POCN complex previously re-
ported was synthesized via oxidative addition [19]. However, that
synthetic route requires use of a Pt0 precursor that is prepared in
low yield [34,35]. In a second strategy, lithiation of a brominated
metal precursor has proven a reliable choice for synthesis of NCN
complexes [36]. However, that route is problematic for synthesis of
phosphinite complexes, because attack on the PeO bond by
nucleophilic organolithium reagents competes with the desired
metal-halide substitution chemistry. Another possible strategy is
transcycloplatination, which takes advantage of the relative
strength of PteP bonds vs. PteN bonds to accomplish the formal
exchange of ligands, such as replacement of an NCN ligand with a
PCP ligand [37]. An inconvenience of this approach is the need to
prepare a cyclometalated starting material.
An attractive alternative route is direct cyclometalation, which
has been employed successfully with platinum phosphine pincer
complexes since the earliest days of pincer complex chemistry [38]
and has been used previously in preparation of platinum
* Corresponding author.
E-mail address: william.connick@uc.edu (W.B. Connick).
http://dx.doi.org/10.1016/j.jorganchem.2015.03.002
0022-328X/© 2015 Elsevier B.V. All rights reserved.

K.B. Lavelle et al. / Journal of Organometallic Chemistry 785 (2015) 100e105
101
purchased from Alfa Aesar, n-hexane was purchased from Tedia,
ethyl acetate was purchased from Pharmco-Aaper, high-purity
spectroscopic anhydrous methanol and ethanol were purchased
from Sigma Aldrich, and all other solvents were purchased from
Fisher. THF was distilled after drying over sodium metal and
benzophenone. Toluene and benzene were dried over calcium hy-
dride and distilled prior to use. Air-sensitive work was performed
under an argon atmosphere using standard Schlenk and glovebox
techniques. Solvents for air-sensitive reactions were degassed using
three freeze-pump-thaw cycles. PtBr2(SEt2)2 was prepared analo-
gously to a literature procedure for the preparation of PtCl2(SEt2)2
[42].
Scheme 1. Types of pincer complexes.
phosphinite pincer complexes [39]. One possible mechanism in-
volves coordination of the pincer arms prior to CeH activation, as
shown in Scheme 2; other mechanisms are possible, including
those in which an amine pincer arm acts as an internal base [37]. In
the case of cyclometalation, care must be taken in choice of metal
precursor in order to circumvent formation of insoluble PtII and PdII
oligomers [38,40,41]. Moreover, direct cyclometalation has a mixed
record with respect to platinum NCN-type pincer complexes and is
typically not a practical synthetic route. For example, this is not an
effective approach for the synthesis of Pt(pip2NCN)Br and related
complexes [29]. Studies suggest that the bonds between the metal
and aliphatic amines of the pincer arms are comparatively weak.
Therefore, it is more difficult to achieve simultaneous coordination
of both pincer arms, which for the mechanism shown in Scheme 2,
is believed to be necessary to promote activation of the relatively
inert central sp2 CeH bond [1]. Herein, we examine the interme-
diate case involving a mixed-donor pincer ligand. Whereas a pre-
vious synthesis of platinum POCN-type pincer complexes has been
accomplished via the oxidative addition route [19], here we report
the preparation of a novel mixed-donor platinum pincer complex,
Pt(POCN)Br, by direct cyclometalation (Scheme 1) of a dihalobis(-
thioether) platinum precursor. Analysis of the conditions under
which this cyclometalation occurs, as well as comparison of the
structure of Pt(POCN)Br with other known pincer complexes, pro-
vides insight into the influence of pincer arm substituents on
cyclometalation.
1H and 31P NMR spectra were recorded at room temperature
using a Bruker AC 400 MHz instrument. Deuterated solvents (CDCl3
and C6D6) were purchased from Cambridge Isotope Laboratories. 1H
NMR spectra are reported relative to TMS, and 31P NMR spectra are
reported relative to 85% H3PO4 at
d
¼ 0. UVꢀvisible absorption
spectra were recorded using a HP8453 UVꢀvisible diode array
spectrometer. Frozen solution emission spectra were recorded us-
ing a SPEX Fluorolog-3 fluorimeter equipped with a double-
emission monochromator and a single excitation monochromator.
77 K Glassy solutions were prepared by inserting a quartz EPR tube
containing a 4:1 ethanol:methanol solution of the respective
complex into a quartz-tipped finger dewar. Emission spectra were
corrected for instrumental response. Solid-state emission spectra
were collected using a Renishaw Raman Microscope using a
442 nm HeCd laser for excitation and a 20x objective lens to
magnify the sample. Mass spectra were obtained by electrospray
ionization (ESI) of acetonitrile solutions using a Micromass Q-TOF-2
instrument.
Synthesis of Pt(POCN)Br (1a)
The ligand precursor, POC(H)N, was prepared in situ. To a solu-
tion of chlorodiisopropylphosphine (1.3 mmol, 0.21 mL) in 5 mL
toluene was added via cannula a solution of 3-((morpholino)
methyl)phenol (1.3 mmol, 0.3455 g) and triethylamine (1.6 mmol,
195 m
L) in 5 mL toluene at 10 ꢁC and allowed to warm to room
Experimental
temperature while stirring. The mixture was filtered to remove
precipitate, and the solvent was removed to give a pale orange oil.
Next, a 2 mL toluene solution of PtBr2(S(CH2CH3)2)2 (1.3 mmol,
General
0.6796 g) and triethylamine (1.6 mmol, 195
mL) were added to
PtBr2, NiBr2, 3-hydroxybenzaldehyde, chlorodiisopropylphos
phine, and ethyl sulfide were purchased from Sigma Aldrich. All
other reagents were purchased from Fisher. Benzene was
POC(H)N via cannula, and the resulting solution was refluxed for
24 h. After cooling to room temperature, the solvent was removed
under vacuum. The product was purified by chromatography using
a silica column and dichloromethane as the eluent. Yield: 0.1231 g,
17%. 1H NMR (CDCl3,
d
): 1.21-1.26 (dd, 3JPH ¼ 16 Hz, 3JHH ¼ 7 Hz, 6H,
3
3
PCHCH3), 1.32-1.39 (dd, JPH ¼ 19 Hz, JHH ¼ 7 Hz, 6H, PCHCH3),
2.41-2.49 (m, 2H, PCH), 2.92 (d, 3JHH ¼ 13 Hz, 2H, NCH2CH2), 3.88-
3
3.98 (m, 4H, ArCH2N and OCH2CH2), 4.21 (t, JHH ¼ 11 Hz, 2H,
NCH2CH2), 4.49 (t, 3JHH ¼ 12 Hz, OCH2CH2), 6.66 (d, 3JHH ¼ 8 Hz, 1H,
3
3
{Ar}H5), 6.74 (d, JHH ¼ 7 Hz, 1H, {Ar}H3), 6.98 (t, JHH ¼ 8 Hz, 1H,
{Ar}H4). 31P{1H} NMR (CDCl3,
d) 160.56 (JPtP ¼ 4655 Hz). MS-ESI (m/
z). Observed (Calculated): 582.1 (582.1), [Pt(POCN)BrH]þ; 605.1
(605.1), [Pt(POCN)BrNa]þ; 503.2 (503.1) [Pt(POCN)]þ.
Synthesis of Ni(POCN)Br (1b)
The product was prepared by modification of the procedure of
Zargarian et al. [17]; however, the metal precursor was added
directly to the ligand precursor without first isolating the ligand
precursor. A solution of chlorodiisopropylphosphine (2.7 mmol,
0.45 mL) in 7.5 mL THF was added via cannula to a solution of 3-
((morpholino)methyl)phenol (2.6 mmol, 0.5046 g) and triethyl-
Scheme 2. Steps during cyclometalation of pincer complexes.
amine (2.8 mmol, 400
m
L) in 17.5 mL THF at ꢀ10 ꢁC and allowed to

102
K.B. Lavelle et al. / Journal of Organometallic Chemistry 785 (2015) 100e105
warm to room temperature while stirring. The mixture was filtered
to remove precipitate, and the filtrate was evaporated to dryness to
give a pale orange oil. A 20 mL benzene solution of NiBr2 (2.7 mmol,
0.219 g) and triethylamine (2.8 mmol, 400
mL) were added via
cannula, and the resulting solution was refluxed for 3 h. After
cooling to room temperature, the reaction mixture was filtered
through a plug of silica, and the filtrate was dried. Further purifi-
cation was achieved by crystallization via slow diffusion of n-hex-
ane into a saturated benzene solution of the product to give dark
yellow/brown crystals. Yield: 0.7251 g, 63%. 1H NMR (C6D6, ): 1H
d
NMR (C6D6,
d
): 1.16-1.21 (dd, 3JPH ¼ 15 Hz, 3JHH ¼ 7 Hz, 6H, PCHCH3),
1.47-1.53 (dd, 3JPH ¼ 18 Hz, 3JHH ¼ 7 Hz, 6H, PCHCH3), 2.19-2.28 (m,
2H, PCH), 2.34 (d, 3JHH ¼ 13 Hz, 2H, NCH2CH2), 3.17 (t, 3JHH ¼ 12 Hz,
3
2H OCH2CH2), 3.24 (d, JHH ¼ 12 Hz, 2H, NCH2CH2), 3.63 (s, 1H,
ArCH2N), 4.24 (t, 3JHH ¼ 12 Hz, OCH2CH2), 6.45 (d, 3JHH ¼ 7 Hz, 1H,
{Ar}H5), 6.65 (d, JHH ¼ 8 Hz, 1H, {Ar}H3), 6.95 (t, JHH ¼ 8 Hz, 1H,
3
3
{Ar}H4). 31P{1H} NMR (C6D6,
d) 200.67. MS-ESI (m/z). Observed
(Calculated): 445.0 (445.0), [Ni(POCN)Br]þ.
X-ray crystallography
Colorless rod-shaped crystals of 1a were grown by slow diffu-
sion of n-hexane into a benzene solution. Yellow block-shaped
crystals of Ni(POCN)Cl (1c) were grown by slow diffusion of n-
hexane into saturated benzene solution. Air-sensitive colorless
plate-shaped crystals of the chloride salt of the protonated
brominated ligand precursor, [POC(Br)NH]Cl (POC(Br)N), were
grown from diethyl ether (see Appendices for further information).
For X-ray examination, suitable crystals of 1a and 1c were each
mounted in loops with Paratone-N oil and transferred to the
goniostat bathed in a cold stream. Intensity data were collected at
150 K on a standard Bruker SMART6000 CCD diffractometer using
Scheme 3. Synthesis of pincer complexes.
context of the mechanism in Scheme 2 suggests that the combi-
nation of phosphorus and aliphatic amine donor groups is sufficient
for bonding of POC(H)N in a trans-bidentate fashion, which occurs
prior to CeH bond activation. More broadly, application of Ham-
mond's postulate to this case suggests a transition state that re-
sembles the cyclometalation product. Because the PteP bond is
stronger than the PteN bond, it follows that the barrier to CeH
bond activation will be lower in the POCN case than in the NCN case
by approximately the difference between the PteP and PteN bond
energies. This prediction holds for Scheme 2, as well as other
mechanisms in which the PteP bond forms prior to the transition
state, including ones in which the morpholino group acts as an
internal Brønsted base. We believe that this effect is a significant
factor in the efficacy of cyclometalation with POC(H)N.
graphite-monochromated Cu K
a
radiation,
l
¼ 1.54178 Å. The data
frames were processed using the program SAINT. The data were
corrected for decay, Lorentz and polarization effects as well as ab-
sorption and beam corrections based on the multi-scan technique.
The structures were solved by a combination of direct methods in
SHELXTL and the difference Fourier technique and refined by full-
matrix least squares on F2. Non-hydrogen atoms were refined
with anisotropic displacement parameters. The positions of the H-
atoms were calculated and treated with a riding model in subse-
quent refinements.
Interestingly, when the same reaction conditions with
PtCl2(SEt2)2 were employed in the presence of two equivalents of
POC(H)N, very little pincer complex formed. Instead, as indicated
by 1H and 31P{1H} NMR spectra, as well as mass spectrometry data,
the reaction yielded a symmetric bis-pincer complex, Pt(h
1-POC(H)
Results and discussion
N)2Cl2, in which each POC(H)N ligand is bonded to PtII through
phosphorus (Scheme 3). Notably, in CDCl3 solution a single reso-
nance was present in the 31P{1H} NMR spectrum at 124 ppm with
platinum satellites (JPtP ¼ 2666 Hz), confirming bonding of phos-
phorus to the metal. The 1H NMR spectrum showed that the
phosphinite methine proton resonances were shifted downfield, as
expected for coordination to Pt(II), whereas the morpholine and
aromatic resonances appear at chemical shifts more consistent
with the free ligand precursor. The major peaks in the ESI mass
spectrum were consistent with the singly and doubly protonated
Synthesis
The protonated phosphinite ligand precursor, POC(H)N, was
prepared using the method developed by Zargarian et al. [17].
However, because this compound is susceptible to hydrolysis under
atmospheric conditions, we found it convenient to keep its
manipulation to a minimum and proceed immediately to the
cyclometalation step (Scheme 3). Using NiBr2, the cyclometalation
reaction proceeds smoothly in refluxing benzene to give Ni(POCN)
Br (1b) within 4 h. Ni(POCN)Cl (1c) was isolated from the reaction
of 1b with Pt(COD)Cl2. By contrast, we saw no evidence of forma-
tion of a pincer complex under the same conditions using
PtBr2(SEt2)2. Also unsuccessful was an attempt to prepare the
platinum pincer complex by lithiation of the brominated ligand
precursor POC(Br)N using butyllithium followed by addition of the
platinum precursor. On the other hand, refluxing POC(H)N in
toluene with PtBr2(SEt2)2 for a longer period of time (24 h) gave
Pt(POCN)Br (1a) in good yield. This result indicates that, contrary to
the typical NCN case, direct cyclometalation using POC(H)N is a
perfectly practical synthetic route. Interpretation of these results in
bis-pincer complexes, Pt(POCN)(POCNH)Clþ2 and Pt(POCNH)2Cl22þ
,
respectively. In contrast, Pt(
h
1-POCN)2Cl2 is unstable in air and
decomposed within 48 h, most likely via ligand hydrolysis. The bis-
pincer product is reminiscent of insoluble oligomers {-PC(H)P-M-}n
proposed to form during cyclometalation reactions aimed at pre-
paring Pt and Pd PCP complexes [38,40,41]. In the present case,
strong PteP bonds (c.f., PteN bonds) and 2:1 ligand-to-metal ratio
favor formation of the bis-POC(H)N complex (analogous to {-PCP-
M-}n oligomers), rather than a monomeric
h
3-pincer complex.
Moreover, these results indicate that a pendant amine group is
ineffective at displacing a trans-situated P-donor group of another

K.B. Lavelle et al. / Journal of Organometallic Chemistry 785 (2015) 100e105
103
Table 1
POC(H)N ligand, as would be required to occur prior to CeH acti-
vation ultimately leading to 1a (Scheme 2).
Selected bond lengths (Å) and angles (ꢁ) for 1a and 1b.
Measurement
Pt(POCN)Br
Ni(POCN)Br[17]
Ni(POCN)Cl
Crystal structure
M-C(1)
M-P(1)
M-N(1)
M-X
C(1)-M-X
P(1)-M-N(1)
P(1)-M-X
N(1)-M-X
P(1)-M-C(1)
N(1)-M-C(1)
1.966(6)
2.1802(15)
2.192(5)
2.5088(7)
175.77(19)
159.44(14)
99.86(5)
97.48(13)
82.12(19)
79.9(2)
1.853(2)
2.1107(6)
2.0428(18)
2.3319(4)
176.26(7)
162.20(6)
95.08(2)
99.65(5)
81.72(7)
83.85(9)
1.855(3)
2.1140(8)
2.046(2)
2.2300(7)
176.41(9)
161.54(7)
96.00(3)
98.93(7)
81.93(9)
83.67(11)
The structures of 1a and 1c were confirmed by X-ray crystal-
lography (Fig. 1). The distances and angles are normal (Table 1), and
there are no unusual intermolecular interactions. Little difference is
observed between the structure of 1c and that previously reported
for 1b, other than the Ni-halide bond lengths. In the case of the
platinum complex, 1a, the distance (4.302(5) Å) between pincer
arm donor N and P atoms is intermediate between those of Pt(NCN)
Cl (N ¼ piperidine; 4.156(5) Å, 4.174(5) Å) [29] and Pt(POCOP)Cl
(P ¼ diisopropylphosphinite; 4.471(1) Å, 4.458(2) Å) [39], consis-
tent with the relative lengths of PteP and PteN bonds. The PePteN
bond angle (159.44(14)ꢁ) is several degrees smaller than the
PePteP angles of Pt(POCOP)Cl (161.20(4)ꢁ; 160.79(4)ꢁ) [39] and the
NePteN angles of Pt(NCN)Cl (164.29(13)ꢁ, 164.75(14)ꢁ) [29]. Also,
and in keeping with the relative trans influence of the phosphorus
and nitrogen donor groups, the PteN bond is longer than in the
NCN complex, and the PteP bond is shorter than in the POCOP
complex (Table 2).
Table 2
Comparison of selected bond lengths and angles of platinum pincer complexes.
Structure M-C (Å)
Pt(POCN) 1.966(6)
Br
Pt(NCN)Cl 1.910(4);
[29]
Pt(POCOP) 1.984(3);
Cl [39] 1.976(4)
Structure D-M-D (ꢁ)
M-P (Å)
2.1802(15)
M-N (Å)
2.192(5)
e
2.115(4); 2.101(4);
2.099(3); 2.094(3)
e
1.899(5)
2.2688(10), 2.2636(10);
2.2619(11), 2.2592(10)
D-M-C (ꢁ)
D-D (Å)
Interestingly, comparison of the structure of 1a with that of the
nickel analog 1b provides insight into possible reasons for differing
outcomes under cyclometalation conditions. In crystals of 1a, there
is greater deviation from ideal square planar coordination geome-
try as compared to the previously reported nickel analog, 1b [17].
For example, the PePteN bond angle (159.44(14)ꢁ) is significantly
smaller than found for 1b (162.20(6)ꢁ); this strain is attributable to
longer metal-ligand bond lengths in the platinum complex
(Table 1), in accord with the larger size of platinum. Through
application of Hammond's postulate it may be suggested that
increased strain in 1a compared to 1b, as evidenced by these de-
viations from ideal square planar geometry, will contribute to a
higher activation energy for platinum cyclometalation as compared
to nickel.
Pt(POCN) 159.44(14)
Br
Pt(NCN)Cl 164.29(13);
82.12(19) (P); 79.9(2) (N)
4.302(5)
82.0(2), 82.3(2); 82.4(2),
82.5(2)
80.50(11), 80.72(11);
80.61(12), 80.55(12)
4.156(5); 4.174(5)
4.471(1); 4.458(2)
[29]
164.75(14)
Pt(POCOP) 161.20(4);
Cl [39] 160.79(4)
broad, moderately intense absorption bands occur in the spectrum
of 1b near 330 nm (5800 Mꢀ1 cmꢀ1) and 400 nm (1200 Mꢀ1 cmꢀ1).
These apparent charge-transfer bands closely resemble those
observed in absorption spectra of other nickel-phosphinite pincer
complexes; a band corresponding to that observed at 400 nm in 1a
has typically been assigned to a MLCT transition [12,17,32,33]. The
absorption spectrum of 1a features a broad, moderately intense
charge-transfer band at 293 nm (3400 Mꢀ1 cmꢀ1). Both the 293 and
330 nm bands in the spectrum of 1b exhibit mild solvatochromism,
characterized by a blue-shift with increasing polarity. A review of
the literature pertaining to charge-transfer spectra of square planar
d8-electron complexes generally [43e46], and with particular
UVevisible absorption spectroscopy
In an effort to build on existing pincer complex electronic
structure data, electronic spectra were collected for 1a and for its
nickel analog, 1b (Fig. 2). At longer wavelengths (>300 nm), two
Fig. 1. ORTEP drawings of Pt(POCN)Br (1a, left) and Ni(POCN)Cl (1c, right), 50% probability ellipsoids. H- atoms omitted for clarity.

104
K.B. Lavelle et al. / Journal of Organometallic Chemistry 785 (2015) 100e105
through NSF-MRI grant (CHE-0215950. We would like to thank Drs.
Larry Sallans and Stephen Macha for assistance with MS
measurements.
Appendix A. Supplementary material
CCDC-1035784-1035786 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif
Appendix B. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2015.03.002.
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Acknowledgments
This research was supported by the National Science Foundation
(CHE-1152853). Funding for the SMART6000 diffractometer was

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