Experimental and Theoretical Investigation for the Level of Conjugation in Carbazole-Based Precursors and Their Mono-, Di-, and Polynuclear Pt(II) Complexes

Article

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Experimental and Theoretical Investigation for the Level of

Conjugation in Carbazole-Based Precursors and Their Mono‑, Di‑,

and Polynuclear Pt(II) Complexes

Rayya A. Al-Balushi,^†Ashanul Haque,^†Maharaja Jayapal,^†Mohammed K. Al-Suti,^†John Husband,^†

Muhammad S. Khan,*^,†Olivia F. Koentjoro,^§Kieran C. Molloy,^§Jonathan M. Skelton,*^,§

and Paul R. Raithby*^,§

^†Department of Chemistry, Sultan Qaboos University, P.O. Box 36, Al Khod 123, Muscat, Sultanate of Oman

^§Department of Chemistry, University of Bath, Claverton Down, Bath, Avon BA2 7AY, United Kingdom

S

* Supporting Information

ABSTRACT: A series of trimethylsilyl-protected monoalkynes

(Me₃SiCC−R) and bis-alkynes (Me₃SiCC−R−C

CSiMe₃) incorporating carbazole spacer groups (R =

carbazole-2-yl, carbazole-3-yl, carbazole-2,7-diyl, N-(2-

ethylhexyl)carbazole-2,7-diyl, carbazole-3,6-diyl, N-(2-

ethylhexyl)carbazole-3,6-diyl), together with the corresponding

terminal monoalkynes (H−CC−R) and bis-alkynes (H−

CC−R−CC−H), have been synthesized and character-

ized. The CuI-catalyzed dehydrohalogenation reaction between

trans-[(Ph)(Et₃P)₂PtCl], trans-[(Et₃P)₂PtCl₂], and trans-

i

[(PⁿBu₃)₂PtCl₂] and the terminal alkynes in Pr₂NH/CH₂Cl₂

aﬀords a series of Pt(II) mono- and diynes, while the

dehydrohalogenation polycondensation reactions with trans-[(PⁿBu₃)₂PtCl₂] under similar reaction conditions yields four

Pt(II) poly-ynes of the form trans-[(PⁿBu₃)₂Pt−CC−R−CC−]_n. The acetylide-functionalized carbazole ligands and the

mono-, di-, and polynuclear Pt(II) σ-acetylide complexes have been characterized spectroscopically, with a subset analyzed using

single-crystal X-ray diﬀraction. The Pt(II) mono-, di-, and poly-ynes incorporating the carbazole spacers are soluble in common

organic solvents, and solution absorption spectra show a consistent red-shift between the 2- and 2,7- as well as 3- and 3,6-

carbazole complexes. Computational modeling is used to explain the observed spectral shifts, which are related to the enhanced

electronic delocalization in the latter systems. These results also indicate that the inclusion of carbazole-2,7-diyl units into rigid-

rod organometallic polymers should enhance electronic transport along the chains.

the T₁excited states by intersystem crossing, allowing for light

emission from the decay of these states. Synthetic ﬂexibility and

compatibility with diﬀerent conjugated spacers thus together

allow ﬁne control over the photophysical properties.^4b−e,7

Among the more electron-rich of the reported spacers used

in Pt(II) poly-ynes is the carbazole unit (Chart 1). This moiety

provides a rigid planar biphenyl unit within the polymer

backbone and facile functionalization via the N atom, oﬀering

INTRODUCTION

■

The last few decades have seen a huge research eﬀort in the

pursuit of developing novel materials with enhanced optoelec-

tronic properties.¹Metal−organic frameworks (MOFs),

especially conjugated organometallic compounds, have at-

tracted considerable attention due to their potential applica-

tions in the modern materials industry (e.g., photovoltaic cells,

ﬁeld-eﬀect transistors, light-emitting diodes, and nonlinear

optics).^1c,d,2One of the most promising systems is the rigid-rod

type Pt(II) poly-ynes, trans-[−Pt(PⁿBu₃)₂−CC−R−C

C−]_n, where R is one of a number of aromatic, heteroaromatic,

or mixed-heterocycle spacer groups.³A particularly attractive

feature of the Pt(II) poly-ynes is the ability to ﬁne-tune their

optoelectronic properties by variation of the spacer groups.⁴

Whereas the spacer plays an important role in determining the

physicochemical properties of the materials, the Pt(II) ion plays

a crucial role in controlling the photophysical properties.⁵It is

well-known that triplet (T_x) states play an important role in the

optoelectronic processes of conjugated polymers,⁶and the

incorporation of Pt(II) into the polymer backbone populates

Chart 1. Carbazole Spacer Unit, with the Numbering

Scheme Adopted in This Work Indicated

Received: March 1, 2016

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Scheme 1. Comparison of Synthetic Routes to Alkylated Carbazoles Using NaH and KO^tBu

Scheme 2. Syntheses of Mono- and Bis-Acetylide-Functionalized Carbazole Ligands

the prospects of both improving polymer processability and

mediating potential interchain interactions in polymer ﬁlms.⁸

There is also considerable interest in the solid-state structures

of the polymeric materials due to evidence of interchain

interactions which inﬂuence their optoelectronic properties. In

this context, an analysis of intermolecular interactions in the

crystal structures of the ligand precursors and of suitable model

complexes may lead to a better understanding of the

interactions in the polymers.⁹Mono- and diynes are viewed

as building blocks for the high molecular-weight poly-ynes, and

valuable information on their properties (e.g., electronic

structure and optical absorption) can be obtained by studying

these model systems. Mono- and diyne compounds are also

often more easily crystallized than the corresponding poly-ynes,

allowing for a detailed structural analysis and, thus, an

assessment of structure−property relationships.^9b

Several carbazole-based chromophores have been prepared,

and their photophysical properties investigated in order to

explore their potential applications in organic electrolumines-

cent devices.¹⁰Several recent reports have also studied the

incorporation of carbazole-based auxiliaries into conjugated

MOFs.^8b,11In this work, we report the synthesis and

characterization of a set of acetylide-functionalized carbazole

ligands and their mono-, di-, and polynuclear Pt(II) σ-acetylide

complexes. The crystal structures of two acetylide ligand

precursors, together with those of mononuclear and dinuclear

Pt(II) σ-acetylide complexes, are also reported. The optoelec-

tronic properties are characterized, compared to those of

related organometallic complexes and polymers, and inves-

tigated through computational modeling.

RESULTS AND DISCUSSION

■

Synthesis. Alkylation of 2,7- and 3,6-dibromo carbazoles

using NaH in anhydrous DMF was performed in high yields by

following the reported literature method.¹²However, the

subsequent removal of DMF was found to be a tedious and

time-consuming process, which also resulted in signiﬁcant loss

of the ﬁnal products. Yang et al. alkylated phenothiazine (pKa

∼ 23) using potassium tert-butoxide (KO^tBu) as the base and

THF as the solvent.¹³Since carbazole has a pKa of ∼20, we

successfully applied Yang’s method to alkylate carbazoles. No

reaction occurred upon stirring at room temperature for 6−8 h,

but reﬂuxing the reaction mixture overnight gave the desired

products in quantitative yield (Scheme 1). After completion of

the reaction, removal of the solvent in vacuo, followed by

puriﬁcation using column chromatography, gave the products

in very high yield, making the modiﬁed alkylation method more

convenient than the existing literature method.^12a

The mono- and bis-ethynyl carbazole derivatives were

synthesized by a sequence of coupling and proto-desilylation

reactions (Scheme 2). A total of six trimethylsilyl-protected

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Scheme 3. Synthesis of the Mono-Nuclear Pt(II) Mono- and Diynes M1−M3, Incorporating Carbazole-2-yl Moiety

Scheme 4. Synthesis of the Mono-Nuclear Pt(II) Mono- and Diynes M4−M6, Incorporating Carbazole-3-yl Moiety

alkynyl ligand precursors were prepared through a scheme

adapted from published procedures involving a Pd^II/Cu^I-

catalyzed cross-coupling reaction of the monobromo- or

Conversion of the protected ligand precursors into their

terminal alkynes was accomplished by smooth removal of the

trimethylsilyl group(s) with dilute aqueous KOH in MeOH/

THF, yielding 2-ethynylcarbazole (L1), 3-ethynylcarbazole

(L2), 2,7-bis(ethynyl)carbazole (L3a), 2,7-bis(ethynyl)-N-(2-

ethylhexyl)carbazole (L3b), 3,6-bis(ethynyl)carbazole (L4a),

and 3,6-bis(ethynyl)-N-(2-ethylhexyl)carbazole (L4b). The

products were puriﬁed by silica-gel column chromatography

and isolated as oﬀ-white to light-yellow solids or viscous liquids

in 88−99% yields. The trimethylsilyl-protected mono- and bis-

alkynes were stable to air and light and were characterized by

infrared (IR) spectroscopy, nuclear-magnetic resonance (NMR;

¹H and ¹³C), electron-impact/electrospray ionization mass

spectrometry (EI-MS/ESI-MS), and elemental analyses. The

mono- and bis-terminal alkynes were found to be somewhat

less stable and hence were freshly prepared before reaction with

the Pt(II) bis-phosphine dihalide complexes.

i

dibromocarbazoles with trimethylsilylethyne in an Pr₂NH/

THF solvent:^4c,8b,11,142-trimethylsilylethynylcarbazole (1-

TMS), 3-trimethylsilylethynylcarbazole (2-TMS), 2,7-bis-

(trimethylsilylethynyl)carbazole (3a-TMS), 2,7-bis-

(trimethylsilylethynyl)-N-(2-ethylhexyl)carbazole (3b-TMS),

3,6-bis(trimethylsilylethynyl)carbazole (4a-TMS), and 3,6-bis-

(trimethylsilylethynyl)-N-(2-ethylhexyl)carbazole (4b-TMS).

Wong et al. reported that alkylation at the carbazole N atom

should precede the cross-coupling reaction with trimethylsily-

lethyne,^11apresumably due to the instability of the N−H group

i

in Pr₂NH. However, we were able successfully to introduce

trimethylsilylethynyl groups onto the nonalkylated 2,7- and 3,6-

carbazoles by conducting the cross-coupling reactions in a

i

mixture of Pr₂NH and THF.

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Scheme 5. Synthesis of the Di-nuclear Pt(II) Diynes M7 and M8 and the Corresponding Poly-ynes P1 and P2

Scheme 6. Synthesis of the Pt(II) Diynes M9 and M10 and the Corresponding Poly-ynes P3 and P4

The reaction of the terminal monoalkynes L1 and L2 with

one equivalent of trans-[(Ph)(Et₃P)₂PtCl] in Pr₂NH/CH₂Cl₂

dinculear diynes M7−M9, and poly-ynes P1−P3 were reported

previously.^11e,f

i

Spectroscopic Characterization. Preliminary character-

ization of the acetylide-functionalized carbazole ligands and

their Pt(II) mono-, di- and poly-ynes was carried out using IR

spectroscopy and ¹H, ¹³C, and ³¹P NMR. The IR spectra of the

Pt(II) mono-, di-, and poly-yne compounds provided clear

evidence for the presence of the CC bond from its

characteristic absorption at around ∼2095 cm⁻¹.^{4d,e,8b,11,16}

The single, sharp ν_CCabsorptions indicate a trans-

conﬁguration of the alkynyl bridging ligands around the

bis(trialkynylphosphine) Pt(II) moieties. The Pt(II) mono-,

di-, and poly-ynes display lower ν_CCvalues than those of the

terminal alkynes (L1−L4) which is attributed to metal-to-

alkynyl ligand back bonding. The ν_CCvalues for the terminal

alkynes (L1−L4; 2104−2108 cm⁻¹) were found to be lower

than those of the trimethylsilyl-protected alkynes (1-TMS−4-

TMS; 2148−2152 cm⁻¹); the fact that terminal alkynes (HC

C−R) have lower ν_CCfrequencies than their protected

counterparts RCC−R (typically around 50 cm⁻¹) is well-

known and thus expected.¹⁷

in the presence of CuI at room temperature readily aﬀorded the

mononuclear Pt(II) monoynes M1 and M4, respectively.

Treating L1 and L2 with 0.5 equiv of trans-[(Et₃P)₂PtCl₂]

and trans-[(PⁿBu₃)₂PtCl₂] yielded the corresponding mono-

nuclear Pt(II) diynes M2 and M5, and M3 and M6,

respectively (Schemes 3 and 4). On the other hand, the

reaction of trans-[(Ph)(Et₃P)₂PtCl] with the terminal dialkynes

L3 and L4 (1:2 equiv) under similar conditions gave the

dinuclear Pt(II) diynes M7−M10 (Schemes 5 and 6). The CuI-

catalyzed dehydrohalogenation polycondensation reaction

between trans-[(PⁿBu₃)₂PtCl₂] and the terminal dialkynes L3

and L4 (1:1 equiv) under similar reaction conditions readily

aﬀorded the poly-ynes P1−P4 (Schemes 5 and 6). It is worth

noting that ethynylation with Pt(II) chloride complexes

proceeded smoothly in basic medium despite the presence of

acidic NH protons. We attribute this to the stabilization of

carbazole NH protons through H-bonding with solvent as well

as other carbazole moiety. The involvement of carbazole NH

protons in H-bond possibly prompted the terminal acetylenic

protons for the preferential dehydro-halogenation reaction.¹⁵

Puriﬁcation of the Pt(II) mono- and diynes (M1−M10) was

performed using silica column chromatography, while the

Pt(II) poly-ynes (P1−P4) were puriﬁed by chromatography on

alumina. The synthesis of the Pt(II) mononuclear diyne M6,

The ¹H and ¹³C NMR spectra of all the compounds

contained peaks corresponding to the expected alkyl, aryl, and

alkynyl fragments. The peak area ratios in the ¹H NMR spectra

were found to agree with the feed mole ratio of the precursors

for the mono-, di-, and poly-ynes. The IR and ³¹P{¹H}-NMR

spectral features are similar to those observed in other

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previously reported Pt(II) mono-, di-, and poly-ynes¹⁸and

indicate an all-trans conﬁguration and hence a rigid-rod like

structure. The mass-spectrometry results conﬁrm the molecular

masses expected for the acetylide ligands and the mono- and

dinuclear platinum-acetylide complexes.

Gel-permeation chromatography (GPC), calibrated against a

polystyrene standard, gave weight-average molecular weights

for the poly-ynes P1−P4 in the range of 17 900−40 500 g/mol,

corresponding to degrees of polymerizations (DPs) between 22

and 43. The polydispersity index (PDI) was found to vary

between 1.6 and 1.9. These molecular weights should, however,

be viewed with caution, due to the diﬃculties associated with

characterizing rigid-rod polymers using GPC. This technique

does not give absolute values of the molecular weights, but

rather provides a measure of hydrodynamic volume, and rod-

like polymers in solution show very diﬀerent hydrodynamic

properties to more ﬂexible polymers. Therefore, calibration of

the GPC with a polystyrene standard is likely to artiﬁcially

increase the measured molecular weights to some extent.

However, the lack of discernible resonances corresponding to

end groups in the NMR spectra does indicate a high degree of

polymerization in these poly-ynes.

Optical-Absorption Spectroscopy. The TMS-protected

carbazole ligands, the ten Pt(II) complexes, and the four

polymers were found to be readily soluble in common organic

solvents, and we therefore measured the room-temperature

absorption spectra of all 20 compounds in dilute CH₂Cl₂

solution. Table 1 summarizes absorption spectral data of the

Table 1. Optical Absorption Data for Protected Ligands and

a

Pt(II) Complexes Incorporating Carbazole Spacers

Figure 1. Solution absorption spectra (CH₂Cl₂) of the 2- and 3-

trimethylsilylacetylide carbazole ligands 1-TMS (a) and 2-TMS (b),

together with those of the corresponding mononuclear Pt(II)

complexes M1−M3 (c, e, g) and M4−M6 (d, f, h).

compound

λ_max,1

ε₁

λ_2max

ε₂

λ_3max

ε₃

1-TMS

2-TMS

3a-TMS

3b-TMS

4a-TMS

4b-TMS

M1

258

246

253

265

257

261

242

243

241

242

250

266

291

244

262

265

264

257

1.60

1.25

1.71

1.50

1.78

1.69

1.36

1.85

1.65

1.73

1.80

1.89

1.39

1.68

1.83

1.47

1.75

2.62

1.95

315

280

325

269

290

297

350

354

357

290

292

291

354

373

315

318

323

400

293

350

1.57

1.63

1.25

1.47

1.73

1.74

1.50

1.67

1.78

1.14

1.21

1.42

0.51

1.62

1.48

0.94

0.54

1.48

1.60

1.78

contrast, M1 displays an intense, narrow absorption band at

350 nm, with higher-energy shoulders at 330 and 310 nm. The

spectra of M2 and M3 are similar in form, with both displaying

broad, asymmetric peaks with maxima around 360 nm; this

shape might be likened to the spectrum of M1, but with the

lower-energy transitions broadened and slightly red-shifted.

The spectrum of M2 displays a noticeable shift in the

absorption edge with respect to M1 and M3, although the

low-energy absorptions in all three complexes are signiﬁcantly

enhanced with respect to 1-TMS. This points toward extended

delocalization in the complexes, across the Pt(II) center and

between the aromatic ligands; the red shift seen in the

absorption edge of M2 and M3 compared to M1 may be

explained by the higher degree of electronic delocalization

possible over the two carbazole ligands in these complexes,

versus the single carbazole ligand and phenyl ring in M1.

In contrast, the spectrum of 2-TMS has a sharp maximum

and shoulder feature at ∼280 and 290 nm, respectively, with

some ﬁne structure at longer wavelengths including two

comparatively much weaker peaks around 335 and 350 nm.

The considerable blue shift compared to the 2-carbazole

acetylide analogue is mirrored in the spectra of the Pt(II)

complexes M4−M6. As with M1−M3, complexation leads to

an enhancement of the lower-energy absorption features, with

the spectrum of M4 displaying sharper features than those of

M5 and M6. The ﬁrst absorption band in the spectrum of M4

occurs around 370 nm but is considerably weaker than the clear

peak ∼325 nm. As for M2 and M3, the spectra of M5 and M6

335

1.60

M2

M3

M4

325

342

340

0.83

0.87

1.15

M5

M6

M7

M8

M9

323

328

352

0.52

0.83

0.46

M10

P1

P2

P3

335

0.47

P4

a

Absorption spectra were taken in CH₂Cl₂at room temperature (ε ×

10⁻⁵/M⁻¹cm⁻¹, λ_max/nm).

compounds. Room-temperature absorption spectra of the 2-

and 3-trimethylsilylacetylide carbazole ligands (1-TMS and 2-

TMS) and corresponding mononuclear Pt(II) complexes

(M1−M3 and M4−M6, respectively) are shown in Figure 1.

The spectrum of 1-TMS displays a pair of strong absorption

bands at 258 and 315 nm, which overlap to form an asymmetric

peak, and two weaker shoulder features at ∼340 and 350 nm. In

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Figure 2. Solution absorption spectra (CH₂Cl₂) of the 2,7- and 3,6-functionalized TMS-protected carbazole ligands 3a/3b-TMS (a, b) and 4a/4b-

TMS (c, d) and the corresponding dinucelar Pt(II) complexes M7−M10 (e−h) and polymers P1−P4 (i−l).

Figure 3. Schematic illustration of the electronic delocalization across the carbazole-2,7-diyl and carbazole-3,6-diyl spacers. Adapted with permission

consist of broad low-energy absorption peaks but with the

maxima blue-shifted to 340−350 nm. However, the absorption

edge of both 3-carbazole complexes is noticeably broader than

that of the 2-carbazole analogues. We also recorded and

compared room-temperature absorption spectra of the 2,7- and

3,6-functionalized TMS-protected carbazole ligands (3a/3b-

TMS and 4a/4b-TMS, respectively) and the corresponding

dinuclear Pt(II) complexes (M7−M10) and polymers (P1−

P4) (Figure 2).

Comparing Figure 2a,b and c,d, the changes to the spectra on

alkylating the N atom are subtle but noticeable. The ﬁrst

absorption maximum of 3a-TMS occurs at ∼325 nm, with

shoulder features at 335−340 and approximately 350 nm, while

the maximum of 3b-TMS was measured at 335 nm, and the

shoulder features at ∼360 and 380 nm. The spectrum of the

alkylated 2,7-acetylide carbazole also has sharper features than

the H form, particularly in the low-energy part of the spectrum,

although this diﬀerence is not apparent for the 3,6-function-

alized carbazoles. 4a-TMS has an absorption maximum at ∼290

nm, a shoulder around 300 nm, and a weaker absorption at

approximately 340 nm, whereas in the alkylated 4b-TMS the

maximum and shoulders occur at 295−300 and 310/320 nm,

with a pair of smaller peaks at 350 and 365 nm. For both the

2,7- and 3,6-carbazole systems, alkylation thus leads to a slight

enhancement of the long-wavelength part of the absorption

spectrum. The spectroscopy suggests that alkylation induces a

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small red shift in the absorption, although this is secondary to

the signiﬁcant blue shift observed between the 2,7- and 3,6-

substitution patterns; the latter is analogous to the blue shift

between the 2- and 3-functionalized carbazoles evident in

Figure 1 and can be understood qualitatively as a loss in

electronic delocalization in the 3,6-substituted ligand (Figure

3), which possesses a so-called “break junction” in the

molecular structure. As per the ﬁgure, complete delocalization

across the 3,6-diyl spacer is not possible without generating

pentavalent carbon centers (Figure 3, middle), which is not

possible. Another possibility of delocalization is involving the

lone pair on the nitrogen atom, leading to the generation of

charged centers. Overall, delocalization was favored in the case

of 2,7-isomers, while it is less in 3,6-diyl-isomers.

Crystal Structures. In order to complement the spectro-

scopic characterization of the polymer precursors, we

attempted to grow single crystals suitable for X-ray diﬀraction

of several of the key precursors and intermediates in the

synthesis. We were able to obtain suitable single crystals of the

2,7- and 3,6-carbazole trimethylsilyl-acetylide ligands (3a-TMS,

4a-TMS) and of the mono- and dinuclear Pt(II) σ-acetylide

compounds M3 and M7, and thus, crystallographic studies

were carried out on these systems. The structures of 3a-TMS,

4a-TMS, and the model compounds M3 and M7 are shown in

Figures 4−8, and key crystallographic parameters are

summarized in Table 2.

The photophysical properties of M7−M9 and P1−P3 in the

solid state have been previously reported,^11fand we observed

no signiﬁcant diﬀerences in the band shapes and absorption

maxima in thin ﬁlms and solution, except for some very small

shifts in band positions, which can be attributed to the

diﬀerences in environment. As for the mononuclear complexes,

incorporation of the ligands into dinuclear Pt(II) complexes

lead to a red shift in the absorption edge. In contrast, however,

the clear trend of an enhancement of the low-energy absorption

is less clear-cut for M7−M10. There is a sharp increase in

intensity of the absorption edge of M8, whereas there seems to

be a reduction in the intensity of the long-wavelength part of

the spectrum of M7, although this is relative to the bright band

at ∼250 nm and not on an absolute scale. With reference to the

spectra of the protected carbazole ligands, the spectra of M9

and M10 appear to show a broadening of the sharp features in

the spectra of 4a- and 4b-TMS, respectively, in addition to the

apparent red shift in the feature positions. Compared to the

dinuclear complexes M7−M10, the polymers P1−P4 show a

further red shift in the absorption, consistent with the extended

electronic delocalization, together with a general broadening of

the spectral features. The polymers also show a relative increase

in long-wavelength absorption, which is particularly noticeable

in the cases of P2 and P4.

Figure 4. Structure of 4a-TMS showing the atom-numbering scheme;

thermal ellipsoids are set at 40% probability.

Molecules of 4a-TMS (Figure 4), which lack N functional-

ization but have the 3,6-alkynyl substitution pattern, aggregate

through a hydrogen bond between the N−H group and the

midpoint of the C(15)C(16) bond of a neighboring

molecule [N−H···midpoint distance 2.392 Å; N−H−midpoint

angle 162.90°]. However, this has no signiﬁcant impact on the

length of the alkyne bond [1.208(5) Å] in comparison to that

of the nonhydrogen bonded group [C(5)C(6); 1.202(4) Å].

In contrast, the structure of 3a-TMS (Figure 5), which has the

alkynyl groups at the 2,7 positions, has too much steric

crowding around the N−H to allow for this interaction. All the

intramolecular bond lengths and angles for both 3a-TMS and

4a-TMS lie within the expected ranges.

The molecular structure of the mononuclear Pt(II) diyne M3

is illustrated in Figure 6. The molecule occupies a crystallo-

graphic center of symmetry, which is coincident with the

central platinum atom. This metal exhibits the trans-square

planar geometry suggested by the spectroscopic character-

ization, which is consistent with that found in the related

platinum poly-yne molecules.^4e,8bThe platinum−alkynyl σ-

interaction has a Pt(1)−C(3) distance of 2.002(2) Å, which is

within the expected range. The ethynylenic unit is almost linear

[∠Pt(1)−C(3)−C(4) 175.7(2)°, ∠C(3)−C(4)−C(5)

172.3(3)°], and the C(3)−C(4) bond is similar, at 1.210(3)

Å, to that observed in the precursor 3a-TMS. The carbazolyl

unit is essentially planar (maximum deviation from the C(5)−

N(17) unit of −0.038 Å for C(5); RMSD 0.019 Å), and the

bond parameters are not signiﬁcantly diﬀerent to those

To summarize, from the spectroscopic characterization, we

extract four general observations:

(1) The 3- and 3,6-substituted carbazoles and their

complexes display a blue-shifted absorption compared

to the 2- and 2,7-substituted analogues, which may be

attributed to a reduction in electronic delocalization in

the former ligands due to the presence of a break

junction.

(2) Incorporation of the carbazole spacers into Pt(II)

complexes generally leads to an enhancement of the

absorption in the longer-wavelength part of the proﬁle.

(3) Functionalization of the carbazole N atom can in

principle allow the absorption proﬁle to be tuned.

(4) The polymers show a further red shift and a broadening

of the absorption proﬁles with respect to the model

dinuclear complexes.

Extended conjugation across the metal centers in these

complexes is consistent with previous results on related

systems,^{10a,11a,14,19}and the red shifts in the spectra of the

complexes and polymers compared to those of the protected

ligands, suggestive of electronic excitations between highly

delocalized states, is consistent with observations made from

other Pt(II) poly-ynes incorporating carbocyclic and hetero-

cyclic spacers.^3a,14,16a

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C bond [1.212(12) Å] which, as in the structure of 4a-TMS, is

not signiﬁcantly diﬀerent from the length of the other alkyne

bond [C(33)C(34); 1.208(13) Å]. The two Pt(II) centers

adopt the expected square planar geometry, as observed in a

wide variety of diplatinum diyne complexes with a range of

central spacer groups.^{4d,e,8b,11a−d,16}The dihedral angle between

the two platinum-containing planes is 35.5°. The terminal

phenyl rings on each platinum are planar and essentially

perpendicular to the Pt-coordination planes; the angle between

the planes containing the atoms Pt(1)P(1)P(2)C(13)C(19)

and the C(13)−C(18) ring is 84.5° and that between the

planes containing the atoms Pt(2)P(3)P(4)C(34)C(35) and

C(35)−C(40) is 90.0°. The acetylenic units show only slight

deviations from linearity, with ∠Pt(1)−C(19)−C(20)

175.3(8)°, ∠Pt(2)−C(34)−C(33) 171.6(12)°, ∠C(19)−

C(20)−C(21) 177.1(10)°, and ∠C(31)−C(33)−C(34)

171.0(13)°. The central carbazole group is essentially planar,

with angles of 2.2° and 1.3° between the C(21)−C(26) and

N(1)C(24)C(25)C(27)C(28) planes and between the N(1)-

C(24)C(25)C(27)C(28) and C(24)C(27)C(29)C(30)C(31)-

C(32) planes, respectively. The interplanar angle between

Pt(1)P(1)P(2)C(13)C(19) and C(21)−C(26) is 45.7° and

that between the Pt(2)P(3)P(4)C(34)C(35) and C(24)C(27)-

C(29)C(30)C(31)C(32) planes is 82.9°.

The Pt−P distances average 2.286 Å, and the Pt−C(C)

bond lengths average 2.01 Å, which is slightly shorter than the

average Pt−C(Ph) bond length of 2.03 Å. These values are

consistent with the geometries of related diplatinum

diynes.^{4d,e,8b,11a−d,16}The two acetylenic CC bond lengths

average 1.21 Å, which is, again, within the expected range. The

bond parameters within the carbazole spacer group do not

show any signiﬁcant deviations from those reported in 3-

(trimethylsilylethynyl)carbazole²⁰and are consistent with those

in the mononuclear platinum complex trans-bis(carbazol-3-

ylethynyl)bis-tri-n-butylphosphine-platinum(II).^11e

Figure 5. Structure of 3a-TMS showing the atom-numbering scheme;

thermal ellipsoids are set at 40% probability.

Figure 6. Structure of M3 showing the atom-numbering scheme;

thermal ellipsoids are set at 40% probability. Selected geometric data

(distances in Å, angles in °): Pt(1)−C(3) 2.002(2), Pt(1)−P(2)

2.3200(6), C(3)−C(4) 1.210(3), ∠Pt(1)−C(3)−C(4) 175.7(2), and

∠C(3)−Pt(1)−P(2) 91.61(7).

observed in 3a-TMS, the diyne M7 (Figure 7), and in other,

related, reported carbazole structures.^11a,eThe carbazolyl ring

plane makes an angle of 81.88° with the square planar Pt unit

(Pt(1), P(2), C(3), P(2i), C(3i)); the “i” atoms are related to

those in the asymmetric unit by the symmetry operation (−x, −

y, −z).

Across all four structures, the C₁₀−C₁₁, C₁₁−C₁₂, and C₁₂−

C₁₃bond distances in the heterocycle (numbering as in Chart

1), and the bond lengths in the C−CC groups, are the same

within experimental error. Thus, the proposed enhanced

delocalization within the carbazole-2,7-diyl derivatives com-

pared to the 3,6-substituted analogues (Figure 3) appears to

have no discernible eﬀect on the bond lengths. The data are

also consistent with previously reported structures of Pt, Au,

and Hg complexes of carbazole-3,6-diyls.^8b

M7 crystallizes as a methanol solvate [PhPt(PEt₃)₂CCC

C(PEt₃)₂PtPh]·MeOH, with one molecule each of the complex

and solvent in the asymmetric unit (Figure 7). The methanolic

oxygen atom acts as a hydrogen-bond acceptor²⁰to the

carbazole N−H group [H(1)···(1) 2.02 Å, N(1)···O(1)

2.869(10) Å, ∠N(1)−H(1)···O(1) 161.9°]. The methanolic

hydrogen atom acts as a hydrogen-bond donor to the CC

C(19)−C(20) on an adjacent molecule (Figure 8), though

again this has no signiﬁcant inﬂuence on the length of the C

Figure 7. Structure of M7 showing the atom-numbering scheme and the interaction with the methanol solvent molecule. Only one orientation of the

disordered component of the structure (labeled Ca) is shown for clarity; C(3a) and C(11) are obscured by C(6a) and C(12), respectively. Thermal

ellipsoids are set to 40% probability. Selected geometric data (distances in Å, angles in °): Pt(1)−P(1) 2.294(3), Pt(1)−P(2) 2.277(3), Pt(2)−P(3)

2.282(3), Pt(2)−P(4) 2.291(3), Pt(1)−C(13) 2.064(9), Pt(1)−C(19) 2.007(9), Pt(2)−C(35) 2.072(3), Pt(1)−C(34) 2.013(9), C(19)−C(20)

1.212(12), C(33)−C(34) 1.208(13), ∠Pt(1)−C(19)−C(20) 175.3(8), ∠Pt(2)−C(34)−C(33) 171.6(12), ∠C(19)−C(20)−C(21) 177.1(10), and

∠C(31)−C(33)−C(34) 171.0(13).

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Figure 8. Association of the M7 diyne molecules through solvent-mediated hydrogen bonding. Selected geometric data (distances in Å, angles in °):

N−H(1)···O(1) 2.02, ∠N−H(1)···O(1) 161.7, O−H···midpoint C(19′)−C(20′) 2.31, and ∠O−H···midpoint C(19′)−C(20′) 157.0.

Figure 9. Simulated gas-phase absorption spectra of 1-TMS (a), 2-TMS (b), M3 (c), and M6 (d) from time-dependent density-functional theory

(TD-DFT) using adiabatic B3LYP.²¹Each plot shows the simulated absorption proﬁle obtained from the spin-allowed (singlet) states (blue), with

the spin-forbidden (triplet) states marked by red lines/gold stars. Isosurface plots of the highest-occupied and lowest-unoccupied molecular orbitals

(HOMOs/LUMOs) for each system are shown to the right of the simulated spectra. These plots were prepared using the VESTA software.²²

Computational Modeling. To complement the optical

spectroscopy and in particular to understand the origin of the

spectral changes observed between the carbazole ligands with

diﬀerent substitution patterns and the Pt(II) complexes, we

performed gas-phase molecular quantum-chemical calculations

on 1-TMS, 2-TMS, M3, and M6.^11eThe four models were

optimized at the hybrid density-functional theory (DFT) level

of theory, and the minima were veriﬁed by computing

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Table 3. HOMO−LUMO Gaps (E_H‑L) and Energies, Wavelengths, Oscillator Strengths (f), and Assignments of the “Brightest”

Low-Energy Transitions (λ > 300 nm) in Gas Phase 1-TMS, 2-TMS, M3, and M6, Obtained from Time-Dependent Density-

a

,

Functional Theory (TD-DFT) Using Adiabatic B3LYP²¹

model

E_H‑L/eV

E/eV

λ/nm

f

assignment

1-TMS

2-TMS

4.417

4.579

3.860

3.945

302.3

313.6

0.757

0.024

HOMO → LUMO (85.3%)

HOMO → LUMO (87.6%)

HOMO − 1 → LUMO + 1 (10.5%)

HOMO → LUMO (92.9%)

M3

M6

4.085

3.720

3.877

333.3

319.9

1.471

0.137

HOMO → LUMO + 2 (80.5%)

HOMO → LUMO + 3 (17.5%)

a

The assignments are based on the percentage of the sum of the squared coeﬃcients (given in brackets), and only the major components of the

overall transition are listed.

vibrational frequencies. We then performed time-dependent

DFT (TD-DFT) calculations on these optimized models to

investigate the optical properties and to characterize the

electronic excitations. The computational methodology is

described in detail in the Experimental Section.

Figure 9 shows the simulated absorption spectra of the four

complexes, together with isosurface plots of the frontier

highest-occupied and lowest-unoccupied molecular orbitals

(HOMOs/LUMOs). The simulated spectra compare reason-

ably well with the corresponding solution spectra in Figure 1

and in particular mirror the key trends observed from these

measurements, viz., the red shift and stronger absorption of the

2-substituted carbazoles and their Pt(II) complexes compared

to the 3-substituted analogues and also of both Pt(II)

complexes compared to the TMS-protected ligands.

Aside from some small shifts in the positions of absorption

bands, a notable discrepancy between the simulated and

experimentally recorded spectra is the presence of additional

ﬁne structure (e.g., shoulders) in the latter. Two possible

reasons for this are that (1) a constant peak broadening was

assumed when generating the simulated spectra (see Exper-

imental Section), whereas in general each state may have a

diﬀerent line width, and (2) low-energy singlet excitations with

small oscillator strengths, of which there are several in the M3

and M6 Pt(II) complexes, could be enhanced relative to the

brighter transitions at ﬁnite temperature by geometric

distortions induced through thermal vibration. Furthermore,

as evident in Figure 9, all four complexes possess low-lying

triplet excitations; these are formally spin forbidden but in

principle could also contribute to small features in the spectra.

To interpret the spectroscopic activity of the four complexes,

we chose to analyze the “brightest” low-energy transition in

each, i.e., the excitations with the highest oscillator strengths, f,

among the transitions with excitation wavelengths above 300

nm. Table 3 lists the excitation energies/wavelengths, oscillator

strengths, and an assignment of each state in terms of

contributions from individual orbital transitions; the calculated

HOMO−LUMO gap is also given for reference.

In 1-TMS, there are two predicted transitions with λ > 300

nm. The brightest of these, at 302 nm, can be assigned as a

HOMO−LUMO excitation, although there is a considerably

weaker transition between the HOMO − 1 and LUMO orbitals

at 321 nm (f = 0.032). Mirroring the blue shift observed

experimentally, 2-TMS has a single, comparatively weak

calculated transition above 300 nm (λ = 314 nm, f = 0.024),

which can be assigned to the HOMO → LUMO absorption

with a small contribution from the HOMO − 1 → LUMO + 1

excitation. From inspection of the frontier orbitals, it is

apparent that both the HOMO and LUMO in 1-TMS are

more delocalized than the corresponding orbitals in 2-TMS,

which could account for the smaller energy gap and longer

excitation wavelengths of 1-TMS.

As can be seen clearly from the data in Table 3, incorporation

of the carbazole ligands into Pt(II) complexes leads to a red

shift in the absorption energies and a substantial enhancement

of the oscillator strengths, which once again mirrors the

observations made from the spectroscopy. The very strong low-

energy absorption at 333 nm in M3 is due to the HOMO−

LUMO transition, and from inspection of the orbitals, the red

shift can be explained by delocalization of the HOMO orbitals

in the two carbazole ligands across the metal center. A similar

pattern is observed in M6, for which the composite HOMO →

LUMO and HOMO −1 → LUMO − 1 transition in 2-TMS

undergoes a red shift and an increase in oscillator strength (λ =

343 nm, f = 0.054). However, in this complex, there is also a

considerably brighter transition at 320 nm, corresponding to a

redistribution of electron density from the HOMO to the

LUMO + 2 and LUMO + 3 orbitals, and it is this excitation

which makes the largest contribution to the long-wavelength

tail feature in the simulated spectrum. The oscillator strength of

this transition, however, is still an order of magnitude smaller

than the HOMO−LUMO transition in M3. In summary, the

computational modeling presented here conﬁrms the key

inferences drawn from the optical spectroscopy and conﬁrms

(1) that the frontier orbitals in the 2-substituted carbazole

ligands are more delocalized than those in the 3-substituted

analogues and (2) that the red shift and enhancement of the

absorption in the Pt(II) complexes can be explained by

delocalization of the ligand HOMO orbitals across the Pt

centers.

CONCLUSION

■

We have synthesized and characterized a series of acetylide-

functionalized carbazole ligands and Pt(II) mono-, di-, and

poly-ynes incorporating 2-, 3-, 3,6-, and 2,7-carbazole spacer

groups. We have reported crystal structures of several of these

precursors and model compounds, providing insight into

possible intermolecular interactions in the polymeric systems,

viz., hydrogen bonding between the carbazole N−H and alkyne

groups on neighboring molecules, which may be mediated by

included solvent molecules. This study has also shed valuable

light on the relationship between molecular and electronic

structure in this family of systems, in particular showing that

there is a signiﬁcantly higher degree of electronic delocalization

within the complexes and polymers with the 2- and 2,7-

functionalized ligands compared to those with the 3- and 3,6-

substituted ones, which leads to a marked red shift in the

absorption spectra. Incorporation of the carbazole ligands into

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alkyl CH₂), 0.81−0.67 (m, 6H, alkyl CH₃). ¹³C NMR (176 MHz,

CDCl₃, ppm) δ 162.56, 139.61, 129.09, 123.61, 123.33, 112.20, 110.87

(aromatic), 47.55 (N−CH₂), 39.41 (alkyl CH), 32.15, 30.88, 28.64,

24.25 (alkyl CH₂), 13.73, 10.66 (alkyl CH₃). ESI-MS: m/z 437.15

(M⁺). Anal. Calc. for C₂₀H₂₃Br₂N: C, 54.94; H, 5.30. Observed: C,

55.02; H, 5.31%.

Pt(II) complexes produces a further red shift and an

enhancement of the low-energy absorption proﬁle, which can

be explained by electronic delocalization between ligands across

the Pt(II) centers. These hypotheses have been conﬁrmed both

spectroscopically and using computational modeling. Organo-

metallic rigid-rod polymers based on carbazole-2,7-diyl units

are expected to display enhanced electron transport within the

chains, whereas polymers based on the carbazole-3,6-diyl

analogue are expected to show poorer transport properties.

The systematic study carried out in this work should assist in

the future rational design of conjugated Pt(II) poly-ynes with

improved optoelectronic properties for a range of technological

applications.

2-(Trimethylsilyl)-9H-carbazole, 1-TMS. To a solution of 2-

i

bromocarbazole 1 (1.00 g, 4.06 mmol) in Pr₂NH/THF (70 mL, 1:4

v/v) under an Ar atmosphere were added catalytic amounts of CuI (10

mg), Pd(OAc)₂(10 mg), and PPh₃(52 mg). The solution was stirred

for 30 min at room temperature, and then trimethylsilylethyne (0.87

mL, 6.09 mmol) was added under vigorous stirring. The reaction

mixture was then reﬂuxed overnight. The completion of the reaction

was conﬁrmed by silica TLC and IR spectroscopy. After being cooled

to room temperature, the mixture was ﬁltered and concentrated in

vacuo. The impure residue was redissolved in CH₂Cl₂and puriﬁed by

silica column chromatography using a hexane/CH₂Cl₂(1:1 v/v)

eluent to yield a pale yellow solid (0.85 g, 79%, mp 218 °C). IR (ATR,

EXPERIMENTAL SECTION

■

General Procedures. All reactions were performed under a dry Ar

atmosphere using the standard Schlenk technique. Solvents were

predried and distilled before use according to standard procedures.²³

All chemicals, except where stated otherwise, were obtained from

Sigma-Aldrich and used as received. The compounds 3a,²⁴trans-

[(Ph)(PEt₃)₂PtCl],²⁵and trans-[(PⁿBu₃)₂PtCl₂]²⁶were prepared

according to literature procedures. NMR spectra were recorded in

CDCl₃using Bruker WM-250 and AM-400 spectrometers and a

Bruker Avance III HD 700 MHz spectrometer equipped with 5 mm

1

diamond): ν/cm⁻¹3402 (N−H), 2151 (−CC−). H NMR (700

MHz, CDCl₃, ppm): δ, 8.04 (d, J = 8.1 Hz, 2H, H-4, 5), 7.99 (d, J =

8.0 Hz, 1H, H-8), 7.55 (s, 1H, H-1), 7.43 (m, 2H, H-6, 7), 7.35 (d, J =

8.0 Hz, 1H, H-3), 5.76 (s, 1H, NH), 0.28 (s, 9H, SiMe₃). ¹³C NMR

(176 MHz, CDCl₃, ppm): δ 140.24, 139.03, 132.27, 126.55, 123.76,

123.10, 120.71, 120.25, 120.14, 119.94, 114.32, 110.85 (aromatic),

106.39, 93.70 (−CC−), 0.22 (SiMe₃). ESI-MS: m/z 261.9 (M⁺).

Anal. Calc. for C₁₇H₁₇NSi: C, 77.52; H, 6.51. Observed: C, 77.55; H,

6.53%.

1

TCI H/C/N cryoprobe. The H and ¹³C{¹H} NMR spectra were

referenced to solvent resonances, and the ³¹P{¹H}NMR spectra were

referenced to external trimethylphosphite or 85% H₃PO₄. IR spectra

were recorded either in CH₂Cl₂solutions in a NaCl cell using a

PerkinElmer 1710 FT-IR spectrometer or directly on the sample as

attenuated total reﬂectance (ATR) on Diamond using a Cary 630 FT-

IR spectrometer. UV/vis spectra were recorded with Shimadzu UV-

2450 spectrometer. Mass spectra were acquired using a Kratos MS 890

spectrometer using electron-impact (EI) and electrospray-ionization

(ESI) techniques. Microanalyses were carried out at the University

Chemical Laboratory, University of Cambridge. Preparative thin-layer

chromatography was carried out on commercial Merck plates with a

0.25 mm layer of silica. Column chromatography was performed using

either Kieselgel 60 silica gel (230−400 mesh) or Brockman grade II−

III alumina.

Caution! All chemicals used in the current work are sensitive to skin,

eyes, and the respiratory system. Therefore, all reactions were performed in

a well-ventilated fume hood. Inhalation of silica/alumina and low boiling

solvents like dichloromethane and hexane may cause injuries to internal

organs. Safety glasses, gloves, masks, and lab coats were worn during the

experiments.

3-(Trimethylsilyl)-9H-carbazole, 2-TMS. A similar procedure to

that used for the synthesis of 1-TMS was adopted using 3-

bromocarbazole 2. The compound was obtained as a white solid

(0.79 g, 73%, mp 189 °C). IR (ATR, diamond): ν/cm⁻¹3392 (N−H),

1

2148 (−CC−). H NMR (700 MHz, CDCl₃, ppm): δ 8.22 (s, 1H,

H-4), 8.11 (br s, 1H, NH), 8.04 (d, J = 7.8 Hz, 1H, H-5), 7.53 (d, J =

8.3 Hz, 1H, H-8), 7.48−7.39 (m, 3H, H-2, 6, 7), 7.33 (d, J = 8.3 Hz,

1H, H-1), 0.29 (s, 9H, SiMe₃). ¹³C NMR (176 MHz, CDCl₃, ppm): δ

139.89, 139.35, 129.97, 126.47, 124.67, 123.36, 123.05, 120.62, 120.11,

114.00, 110.88, 110.60 (aromatic), 106.65, 91.91 (−CC−), 0.31

(SiMe₃). ESI-MS: m/z 261.9 (M⁺). Anal. Calc. for C₁₇H₁₇NSi C,

77.52; H, 6.51. Observed: C, 77.60; H, 6.50%.

2,7-Bis(trimethylsilylethynyl)carbazole, 3a-TMS. A similar proce-

dure to that used for the synthesis of 1-TMS was adopted using 3,6-

dibromocarbazole 3 (1.20 g, 3.69 mmol), CuI (7 mg), Pd(OAc)₂(8

mg), PPh₃(48 mg), and trimethylsilylethyne (1.30 mL, 9.22 mmol).

The product was isolated as a pale brown solid (1.2 g, 90%, mp 184.7−

185.2 °C). IR (CH₂Cl₂): ν/cm⁻¹3401 (N−H), 2150 (−CC−). ¹H

NMR (700 MHz, CDCl₃, ppm): δ 8.06 (br s, 1H, NH), 7.95 (d, J =

8.1 Hz, 2H, H-4, 5), 7.58 (s, 2H, H-1, 8), 7.54 (d, J = 8.3 Hz, 2H, H-3,

6), 0.28 (s, 18H, SiMe₃). ¹³C NMR (176 MHz, CDCl₃, ppm): δ

140.81, 139.51, 124.10, 123.92, 123.10, 122.95, 121.67, 120.57, 120.31,

120.09, 114.29, 113.80 (aromatic), 105.99, 95.04 (−CC−), 0.03

(SiMe₃). ESI-MS: m/z 359.6 (M⁺). Anal. Calc. for C₂₂H₂₅Si₂N: C,

73.50; H, 7.01. Observed: C, 73.74; H, 7.02%.

Ligand Synthesis. 2,7-Dibromo-N-(2-ethylhexyl)carbazole, 3b.

A mixture of 2,7-carbazole 3a (1.00 g, 3.08 mmol) and potassium tert-

butoxide (KO^tBu) (0.415 g, 3.70 mmol) in dry THF (80 mL) was

placed in a two-necked ﬂask and stirred at room temperature for 20

min under an Ar atmosphere. 2-Ethylhexyl bromide (0.640 mL, 3.70

mmol) was added to the reaction mixture, which was then reﬂuxed

overnight. After removing the solvent, the crude product was puriﬁed

using a silica-gel column eluted with hexane, yielding a white solid

(1.32 g, 97.8%). ¹H NMR (700 MHz, CDCl₃, ppm) δ 7.89 (d, J = 8.2

Hz, 2H, H-4, 5), 7.34 (dd, J = 8.2, 1.2 Hz, 2H, H-3, 6), 7.26 (s, 2H, H-

1,8), 4.07 (d, J = 8.0 Hz, 2H, N−CH₂), 2.02 (hept, J = 6.7 Hz, 1H,

alkyl CH), 1.45−1.22 (m, 8H, alkyl CH₂), 0.89 (ddt, J = 31.4, 24.3,

12.3 Hz, 6H, alkyl CH₃). ¹³C NMR (176 MHz, CDCl₃, ppm) δ

141.86, 122.53, 121.43, 121.25, 119.66, 112.31 (aromatic), 47.70 (N−

CH₂), 41.06 (alkyl CH), 31.90, 29.72, 28.56, 24.34 (alkyl CH₂), 14.08,

10.91 (alkyl CH₃). ESI-MS: m/z 437.21 (M⁺). Anal. Calc. for

C₂₀H₂₃Br₂N: C, 54.94; H, 5.30. Observed: C, 55.01; H, 5.32%.

3,6-Dibromo-N-(2-ethylhexyl)carbazole, 4b. The same procedure

as used to synthesize 3b using 3,6-dibromocarbazole 4a was followed.

3,6-Bis(trimethylsilylethynyl)carbazole, 4a-TMS. A similar proce-

dure to that used for the synthesis of 1-TMS was adopted using 3,6-

dibromocarbazole 4a and aﬀorded a pale yellow solid (60.1%). IR

1

(CH₂Cl₂): ν/cm⁻¹3394 (N−H), 2149 (−CC−). H NMR (700

MHz, CDCl₃,): δ 8.32 (br s, 1H, NH), 8.18 (s, 2H, H-4, 5), 7.54 (dd, J

= 8.3, 1.1 Hz, 2H, H-1, 8), 7.33 (d, J = 8.3 Hz, 2H, H-2, 7), 0.29 (s,

18H, SiMe₃). ¹³C NMR (176 MHz, CDCl₃) δ 139.61, 130.40, 124.84,

122.96, 114.65, 110.78 (aromatic), 106.32, 92.31 (−CC−), 0.33,

0.32, 0.32, 0.30, 0.16, 0.14 (SiMe₃). ESI-MS: m/z 359.5 (M⁺). Anal.

Calc. for C₂₂H₂₅NSi₂: C, 73.51; H, 7.01. Observed: C, 73.66; H, 7.08%.

2,7-Bis(trimethylsilylethynyl)-N-(2-ethylhexyl)carbazole, 3b-TMS.

A similar procedure to that followed in the synthesis of 1-TMS was

adopted using 3b and aﬀorded a pale yellow solid (71%). IR

1

(CH₂Cl₂): ν/cm⁻¹2151 (−CC−). H NMR (700 MHz, CDCl₃,

1

ppm): δ 7.96 (d, J = 8.0 Hz, 2H, H-4, 5), 7.47 (s, 2H, H-1, 8), 7.33

(dd, J = 8.0, 1.0 Hz, 2H, H-3, 6), 4.12 (ddd, J = 36.3, 14.8, 7.6 Hz, 2H,

N−CH₂), 2.12−2.01 (m, 1H, alkyl CH), 1.44−1.22 (m, 8H, alkyl

The product was obtained as a colorless viscous liquid (96.2%). H

NMR (700 MHz, CDCl₃, ppm) δ 7.90 (s, 2H, H-4, 5), 7.39 (dd, J =

8.7, 1.8 Hz, 2H, H-1, 8), 7.06 (d, J = 8.7 Hz, 2H, H-2, 7), 3.92−3.82

(m, 2H, N−CH₂), 1.87−1.77 (m, 1H, alkyl CH), 1.26−1.06 (m, 8H,

CH₂), 0.92−0.82 (m, 6H, alkyl CH₃), 0.33−0.24 (m, 18H, SiMe₃). ¹³

C

L

DOI: 10.1021/acs.inorgchem.6b00523

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

NMR (176 MHz, CDCl₃): δ 140.97, 123.18, 122.54, 120.22, 120.19,

112.64 (aromatic), 106.35, 93.75 (−CC−), 47.51 (N−CH₂), 39.09

(alkyl CH), 30.70, 28.49, 24.37, 23.00 (alkyl CH₂), 14.00, 10.92 (alkyl

CH₃), 0.03, 0.01 (SiMe₃). ESI-MS: m/z 472.0 (M⁺). Anal. Calc. for

C₃₀H₄₁Si₂N: C, 76.39; H, 8.76. Observed: C, 76.45; H, 8.75%.

3,6-Bis(trimethylsilylethynyl)-N-(2-ethylhexyl)carbazole, 4b-TMS.

A similar procedure to that used in the synthesis of 1-TMS was

followed using 4b and aﬀorded a pale yellow viscous liquid (80%). IR

CH₃). ESI-MS: m/z 328.2 (M⁺). Anal. Calc. for C₂₄H₂₅N: C, 88.03; H,

7.70. Observed: C, 88.14; H, 7.65%.

3,6-Bis(ethynyl)carbazole, L4a. A similar procedure as was used to

synthesize L1 was followed using 4a-TMS and aﬀorded the product as

a colorless microcrystalline solid (98.3%). IR (CH₂Cl₂): ν/cm⁻¹2105

1

(−CC−), 3302 (CC−H). H NMR (700 MHz, CDCl₃, ppm) δ

8.20 (s, 1H, NH), 8.13 (s, 2H, H-4, 5), 7.57 (d, J = 8.3 Hz, 2H, H-1,

8), 7.36 (d, J = 8.3 Hz, 2H, H-2, 7), 3.07 (s, 2H, CC−H). ¹³C NMR

(176 MHz, CDCl₃, ppm) δ 139.73, 138.55, 130.41, 129.27, 124.89,

124.00, 123.25, 122.40, 113.57, 112.96, 112.22, 110.88 (aromatic),

84.40, 84.04 (−CC−). ESI-MS: m/z 216.3 (M⁺). Anal. Calc. for

C₁₆H₉N: C, 89.28; H, 4.19. Observed: C, 89.48; H, 4.21%.

1

(CH₂Cl₂): ν/cm⁻¹2152 (−CC−). H NMR (700 MHz, CDCl₃,

ppm): δ 8.19 (s, 2H, H-4, 5), 7.56 (dd, J = 8.4, 1.5 Hz, 2H, H-1, 8),

7.28 (dd, J = 8.3, 3.8 Hz, 2H, H-2, 7), 4.12 (d, J = 7.6 Hz, 2H, N−

CH₂), 2.00 (hep, J = 6.8 Hz, 1H, alkyl CH), 1.39−1.14 (m, 8H, alkyl

CH₂), 0.89 (t, J = 7.4 Hz, 3H, alkyl CH₃), 0.82 (t, J = 7.2 Hz, 3H, alkyl

CH₃), 0.31−0.14 (m, 18H, SiMe₃). ¹³C NMR (176 MHz, CDCl₃,

ppm): δ 141.05, 129.85, 124.79, 122.70, 113.82, 109.24 (aromatic),

106.50, 92.29 (−CC−), 47.66 (N−CH₂), 39.32 (alkyl CH), 30.60,

28.87, 24.17, 23.06 (alkyl CH₂), 14.13, 10.91 (alkyl CH₃), 0.28, 0.28,

0.25, 0.19, 0.16, 0.12 (SiMe₃). ESI-MS: m/z 472.0 (M⁺). Anal. Calc.

for C₃₀H₄₁Si₂N: C, 76.39; H, 8.76. Observed: C, 76.45; H, 8.75%.

2-(Ethynyl)-9H-carbazole, L1. 1-TMS (0.600 g, 2.28 mmol) was

proto-desilylated in THF/methanol (20 mL, 4:1, v/v) using aqueous

KOH (0.19 g, 3.45 mmol). The reaction mixture was stirred at room

temperature for 1 h, during which time TLC and IR revealed that all

the protected compounds had been converted to the terminal alkyne

ligand. The solvent was then removed, and the residue was dissolved

in CH₂Cl₂and puriﬁed by column chromatography on silica using

hexane/CH₂Cl₂(1:1, v/v) as eluent, to give the product as a pale

brown solid (0.391 g, 89.8%). IR (ATR, diamond): ν/cm⁻¹2104

(−CC−), 3276 (CC−H). ¹H NMR (250 MHz, CDCl₃, ppm): δ

9.11 (s, 1H, NH), 7.56 (s, 1H, H-1), 7.54 (d, J = 7.9 Hz, 1H, H-8),

7.51 (d, J = 8.1 Hz, 1H, H-4), 7.40 (d, J = 7.9 Hz, 1H, H-5), 7.16 (d, J

= 7.4 Hz, 1H, H-3), 7.08−7.00 (m, 2H, H-6, 7), 3.06 (s, 1H, CC−

H). ¹³C NMR (100 MHz, CDCl₃, ppm): δ 135.52, 133.43, 131.14,

126.05, 125.63, 124.10, 123.22, 121.98, 120.15, 118.52, 114.31, 112.53

(aromatic), 105.9, 82.4 (−CC−). ESI-MS m/z 191.99 (M⁺). Anal.

Calc. for C₁₄H₉N: C, 87.93; H, 4.74. Observed: C, 88.01; H, 4.73%.

3-(Ethynyl)-9H-carbazole, L2. A similar procedure to the one

followed for the synthesis of L1 was adopted using 2-TMS and

aﬀorded a pale yellow solid (87.6%). IR (ATR, diamond): ν/cm⁻¹

3,6-Bis(ethynyl)-N-(2-ethylhexyl)carbazole, L4b. A similar proce-

dure to that used in the synthesis of L1 was followed using 4b-TMS

and aﬀorded the product as a yellow viscous liquid (90.0%). IR

(CH₂Cl₂): ν/cm⁻¹2104 (−CC−), 3299 (CC−H). ¹H NMR

(250 MHz, CDCl₃, ppm): δ 7.99 (d, J = 7.6 Hz, 2H, H-4, 5), 7.51 (s,

2H, H-1, 8), 7.36 (dd, J = 13.8, 7.3 Hz, 2H, H-3, 6), 4.10−4.12 (m,

2H, N−CH₂), 3.15 (s, 2H, CC−H), 2.05−2.08 (m, 1H, alkyl CH),

1.25−1.29 (m, 8H, alkyl CH₂), 0.87−0.88 (m, 6H, alkyl CH₃). ¹³C

NMR (100.6 MHz, CDCl₃, ppm): δ 142.53, 137.63, 132.49, 130.10,

123.22, 122.83, 120.95, 120.02, 119.46, 117.46, 113.01, 112.31

(aromatic), 106.56, 89.91 (−CC−), 48.10 (N−CH₂), 40.32

(CH), 30.95, 28.62, 24.41, 22.94 (alkyl CH₂), 14.97, 11.24 (alkyl

CH₃). ESI-MS: m/z 327.97 (M⁺). Anal. Calc. for C₂₄H₂₅N: C, 88.03;

H, 7.70. Observed: C, 88.14; H, 7.65%.

Synthesis of Pt(II) Mono-, Di-, and Poly-ynes. trans-[R−C

C−(PEt₃)₂Pt−Ph] (R = Carbazole-2-yl), M1. To a stirred mixture of L1

(0.100 g, 0.522 mmol) and trans-[Ph(PEt₃)₂PtCl] (0.284 g, 0.522

i

mmol) in Pr₂NH (20 mL) and CH₂Cl₂(20 mL) was added CuI (1

mg). The solution was stirred at room temperature under Ar over a

period of 18 h, after which all volatile components were removed

under vacuum. The crude product was taken up in CH₂Cl₂and passed

through a silica column with hexane/CH₂Cl₂(1:1, v/v) as eluent. The

product was obtained as a pale brown solid (0.300 g, 82.6%, mp 170.2

°C). IR (ATR, diamond): ν/cm⁻¹3230.9 (N−H), 2079 (−CC−).

¹H NMR (700 MHz, CDCl₃, ppm): δ, 7.99 (d, J = 7.7 Hz, 1H, H-5),

7.92 (s, 1H, H-1), 7.88 (d, J = 8.0 Hz, 1H, H-4), 7.40−7.31 (m, 3H, H-

8 and H_orthoof Ph), 7.20 (dd, J = 13.8, 7.3 Hz, 1H, H-3), 6.97 (t, J =

7.1 Hz, 1H, H_paraof Ph), 6.90 (dt, J = 24.4, 7.1 Hz, 2H, H_metaof Ph),

6.81 (t, J = 7.1 Hz, 2H, H-6, 7), 5.29 (br s, 1H, NH), 1.92−1.33 (m,

12H, PCH₂), 1.14−0.94 (m, 18H, alkyl CH₃). ¹³C NMR (176 MHz,

CDCl₃, ppm): δ 139.90, 139.87, 139.37, 136.94, 128.02, 127.45,

127.20, 125.24, 123.84, 123.48, 122.04, 121.36, 120.74, 120.08, 119.78,

119.46, 113.17, 113.09 (aromatic), 111.26, 110.49 (−CC−), 15.43−

13.56 (PCH₂), 8.27−7.86 (alkyl CH₃). ³¹P{¹H}NMR (122 MHz,

1

2108 (−CC−), 3300 (CC−H). H NMR (250 MHz, CDCl₃,

ppm): δ 9.93 (s, 1H, NH), 7.70 (d, J = 7.7 Hz, 1H, H-1), 7. 55 (d, J =

7.8 Hz, 1H, H-8), 7.50 (d, J = 8.0 Hz, 1H, H-4), 7.40 (d, J = 8.2 Hz,

1H, H-5), 7.24 (d, J = 7.7 Hz, 1H, H-2), 7.08 (t, J = 7.2 Hz, 1H, H-7),

7.00 (t, J = 7.4 Hz, 1H, H-6), 3.06 (s, 1H, CC−H). ¹³C NMR

(100.6 MHz, CDCl₃, ppm): δ 138.43, 134.25, 132.15, 131.17, 127.46,

125.11, 123.26, 123.01, 122.36, 120.06, 116.10, 113.46 (aromatic),

107.3, 105.9 (−CC−). ESI-MS: m/z 192.2 (M⁺). Anal. Calc. for

C₁₄H₉N: C, 87.93; H, 4.74. Observed: C, 87.98; H, 4.72%.

1

DMSO-d₆, ppm): δ 11.46, J_Pt−P= 2631.5 Hz. ESI-MS: m/z 696.9

(M⁺). Anal. Calc. for C₃₄H₄₃NP₂Pt: C, 63.72; H, 7.20. Observed: C,

63.80; H, 7.22%.

2,7-Bis(ethynyl)carbazole, L3a. A similar procedure to that used to

synthesize L1 was adopted using 3a-TMS and aﬀorded a pale brown

solid (93.0%). IR (CH₂Cl₂): ν/cm⁻¹2105 (−CC−), 3302 (CC−

H). ¹H NMR (700 MHz, CDCl₃, ppm) δ 8.12 (br s, 1H), 7.88 (d, J =

8.3 Hz, 2H, H-4, 5), 7.58 (s, 2H, H-1, 8), 7.36 (d, J = 8.2 Hz, 2H, H-3,

6), 3.14 (s, 2H, CC−H). ¹³C NMR (176 MHz, CDCl₃, ppm) δ

140.15, 138.81, 125.39, 123.90, 122.93, 121.78, 120.23, 119.50, 114.57,

113.68, 110.62, 109.26 (aromatic), 86.75, 84.29 (−CC−). ESI-MS:

m/z 216.1 (M⁺). Anal. Calc. for C₁₆H₉N: C, 89.28; H, 4.19. Observed:

C, 89.70; H, 4.26%.

2,7-Bis(ethynyl)-N-(2-ethylhexyl)carbazole, L3b. A similar proce-

dure to that followed to synthesize L1 was adopted using 3b-TMS and

aﬀorded the product as a pale yellow viscous liquid (98.7%). IR

(CH₂Cl₂): ν/cm⁻¹2105 (−CC−), 3298 (CC−H). ¹H NMR

(700 MHz, CDCl₃, ppm) δ 8.00 (d, J = 8.0 Hz, 2H, H-4, 5), 7.53 (s,

2H, H-1, 8), 7.37 (d, J = 8.0 Hz, 2H, H-3, 6), 4.13 (qd, J = 14.8, 7.7

Hz, 2H, N−CH₂), 3.16 (s, 2H, CC−H), 2.06 (m, 1H, alkyl-CH),

1.44−1.22 (m, 8H, alkyl CH₂), 0.94−0.83 (m, 6H, alkyl CH₃). ¹³C

NMR (176 MHz, CDCl₃, ppm) δ 141.03, 123.24, 122.81, 120.49,

119.33, 113.06 (aromatic), 84.90 (−CC−), 47.64 (N−CH₂), 39.26

(alkyl CH), 30.87, 28.64, 24.37, 23.06 (alkyl CH₂), 14.02, 10.92 (alkyl

trans-[R−CC−(PEt₃)₂Pt−CC−R] (R = Carbazole-2-yl), M2. A

similar procedure as was used in the synthesis of M1 was followed

using L1 (0.100 g, 0.522 mmol) and trans-[Pt(PEt₃)₂Cl₂] (0.132 g,

0.261 mmol) and CuI (1 mg). The monomer was obtained as a pale

brown solid (0.326 g, 77.3%, mp 280.4 °C). IR (ATR, diamond): ν/

1

cm⁻¹3395 (N−H), 2092 (−CC−). H NMR (700 MHz, CDCl₃,

ppm): 8.00 (d, J = 7.5 Hz, 2H, H-5, 5′), 7.95 (s, 2H, H-1, 1′), 7.90 (d,

J = 7.7 Hz, 2H, H-4, 4′), 7.43 (dd, J = 14.6, 8.1 Hz, 2H, H-8, 8′), 7.20

(t, J = 7.4 Hz, 4H, H-6, 6′,7, 7′), 7.14 (d, J = 8.0 Hz, 2H, H-3, 3′), 5.28

(s, 2H, NH, NH′), 1.31−1.25 (m, 12H, PCH₂), 1.16−1.08 (m, 18H,

CH₃). ¹³C NMR (176 MHz, CDCl₃, ppm): δ 139.94, 139.75, 136.13,

134.25, 129.84, 127.66, 125.56, 125.42, 123.71, 123.38, 123.25, 121.30,

120.19, 120.16, 119.90, 119.86, 119.54, 112.67 (aromatic), 110.56,

110.54 (−CC−), 18.35−14.72 (PCH₂), 8.60−8.13 (alkyl CH₃).

1

³¹P{¹H}NMR (122 MHz, CDCl₃, ppm): δ 11.98, J_Pt−P= 2769.4 Hz.

ESI-MS: m/z 810.3 (M⁺). Anal. Calc. for C₄₀H₄₆N₂P₂Pt: C, 59.18; H,

5.71. Observed: C, 59.23; H, 5.73%.

trans-[R−CC-(Bu₃P)₂Pt−CC−R] (R = Carbazole-2-yl), M3. A

similar procedure as was used in the synthesis of M1 was followed

using L1 (0.100 g, 0.522 mmol), trans-[(PBu₃)₂PtCl₂] (0.175 g, 0.261

mmol), and CuI (1 mg). The product was obtained as a biege powder

M

DOI: 10.1021/acs.inorgchem.6b00523

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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(0.224 g, 87.5%, mp 242.6 °C). IR (ATR, diamond): ν/cm⁻¹3383

previously reported.^11f¹H NMR (250 MHz, CDCl₃, ppm): δ 7.81 (d, J

= 7.7 Hz, 2H, H-4, 5), 7.35 (d, J = 7.9 Hz, 4H, H_orthoof Ph), 7.25 (s,

2H, H-1, 8), 7.14 (dd, J = 13.2, 7.0 Hz, 2H, H-3, 6), 6.97 (t, J = 7.6 Hz,

4H, H_metaof Ph), 6.88 (t, J = 8.0 Hz, 2H, H_paraof Ph), 4.05−4.06 (m,

2H, NCH₂), 1.81−1.78 (m, 24H, PCH₂), 1.62−1.57 (m, 1H, alkyl

CH), 1.43−1.38 (m, 8H, alkyl CH₂), 1.20−1.14 (m, 36H, alkyl CH₃),

0.88 (t, J = 7.8 Hz, 6H, alkyl CH₃). FAB-MS: m/z 1342.3 (M⁺).

trans-[(Ph)(Et₃P)₂Pt−CC−R−CC−Pt(PEt₃)₂(Ph)] (R = Carba-

zol-3,6-diyl), M9. The compound was prepared as previously

reported.^11f¹H NMR (250 MHz, CDCl₃, ppm): δ 7.53 (d, J = 7.9

Hz, 2H, H-4, 5), 7.41 (d, J = 8.0 H, 2H, H-2, 7), 7.32 (dd, J = 14.0, 7.4

Hz, 4H, H_orthoof Ph), 7.26 (d, J = 8.0 Hz, 2H, H-1, 8), 6.96 (t, J = 7.9

Hz, 4H, H_metaof Ph), 6.80 (t, J = 7.9 Hz, 2H, H_paraof Ph), 9.55 (s, 1H,

NH), 1.82−1.75 (m, 24H, PCH₂), 0.86 (t, J = 7.2 Hz, 36H, CH₃).

FAB-mass spectrum: m/z 1230.1 (M⁺).

1

(N−H), 2084 (−CC−). H NMR (700 MHz, CDCl₃, ppm): δ,

8.00 (d, J = 7.7 Hz, 2H, H-4, 4′), 7.94 (s, 2H, H-1, 1′), 7.89 (d, J = 8.0

Hz, 2H, H-5, 5′), 7.39 (d, J = 7.9 Hz, 2H, H-8, 8′), 7.35 (dd, J = 15.1,

7.2 Hz, 2H, H-3, 3′), 7.19 (t, J = 8.1 Hz 4H, 6, 6′,7, 7′), 5.29 (br s, 2H,

NH, NH′), 1.72−1.42 (m, 36H, alkyl CH₂), 0.94 (m, 18H, alkyl CH₃).

¹³C NMR (176 MHz, CDCl₃, ppm): δ 139.80, 139.76, 126.88, 125.33,

123.77, 123.42, 120.88, 120.09, 119.76, 119.81, 112.80, 110.50

(aromatic), 110.08, 108.19, 108.11, 108.03 (−CC−), 26.56−24.01

(alkyl CH₂), 14.01−13.86 (alkyl CH₃). ³¹P{¹H}-NMR (122 MHz,

1

CDCl₃, ppm): δ 4.04, J_Pt−P= 2368.0 Hz. ESI-MS: m/z 980.3 (M⁺).

Anal. Calc. for C₅₂H₇₀N₂P₂Pt: C, 63.72; H, 7.20. Observed: C, 63.90;

H, 7.21%.

trans-[R−CC−(PEt₃)₂Pt−Ph] (R = Carbazole-3-yl), M4. A similar

procedure to that used to synthesize M1 was followed using L2 and

aﬀorded a pale brown solid (76.6%, mp 154.1 °C). IR (ATR,

trans-[(Ph)(Et₃P)₂Pt−CC−R−CC−Pt(PEt₃)₂(Ph)] (R = N-(2-

Ethylhexylcarbazole-3,6-diyl)), M10. To a stirred mixture of L4b

(0.150 g, 0.461 mmol) and trans-[Pt(PEt₃)₂PhCl] (0.543 g, 1.00

1

diamond): ν/cm⁻¹3406 (N−H), 2085 (−CC−). H NMR (700

MHz, CDCl₃, ppm): δ 8.04 (d, J = 7.7 Hz, 1H, H-5), 8.00 (s, 1H, H-

4), 7.97 (s, 1H, NH), 7.41−7.37 (m, 2H, H_orthoof Ph), 7.20 (dd, J =

9.1, 4.5 Hz, 2H, H-1, 8), 6.97 (t, J = 7.1 Hz, 3H, H_para, H_metaof Ph),

6.93 (d, J = 6.0 Hz, 1H, H-2), 6.81 (t, J = 7.0 Hz, 2H, H-7, 6), 1.40 (m,

12H, PCH₂), 0.90 (m, 18H, alkyl CH₃). ¹³C NMR (176 MHz, CDCl₃,

ppm): δ 1156.88, 139.93, 139.44, 137.38, 129.61, 128.00, 127.65,

127.38, 125.95, 125.72, 123.47, 123.39, 122.44, 121.25, 120.93, 120.46,

119.55, 119.33 (aromatic), 110.71, 110.65, 110.20, 109.31 (−CC−),

15.43−13.45 (PCH₂), 8.28−7.85 (alkyl CH₃). ³¹P{¹H}NMR (122

i

mmol) in Pr₂NH (20 mL) and CH₂Cl₂(20 mL) was added CuI (1

mg). The solution was stirred at room temperature under Ar over a

period of 18 h, after which all volatile components were removed

under reduced pressure. The crude product was taken up in CH₂Cl₂

and puriﬁed using silica column chromatography with hexane/CH₂Cl₂

(1:1, v/v) as the eluent. The product was obtained as a brown solid

1

(0.480 g, 77.5%). IR (CH₂Cl₂): ν/cm⁻¹2090 (−CC−). H NMR

(700 MHz, CDCl₃, ppm) δ 8.08 (s, 1H, H-4), 7.88 (s, 1H, H-5), 7.48

(dd, J = 8.7, 1.9 Hz, 1H, H-1), 7.40 (ddd, J = 21.9, 8.5, 1.7 Hz, 1H, H-

8), 7.29 (d, J = 7.0 Hz, 2H, H-2, 7), 7.17−7.13 (m, 4H, H_orthoof Ph),

6.90 (t, J = 7.4 Hz, 4H, H_metaof Ph), 6.74 (dd, J = 14.1, 6.9 Hz, 2H,

H_paraof Ph), 3.60−3.56 (m, 2H, NCH₂), 1.99−1.88 (m, 1H, alkyl

CH), 1.77−1.65 (m, 24H, PCH₂), 1.34−1.10 (m, 8H, alkyl CH₂),

1.10−0.91 (m, 36H, alkyl CH₃), 0.85−0.69 (m, 6H, alkyl CH₃). ¹³C

NMR (176 MHz, CDCl₃, ppm) δ 138.77, 138.28, 137.95, 128.91,

127.98, 126.89, 126.25, 123.42, 122.41, 122.21, 121.96, 121.35, 120.63,

120.13, 110.92 (aromatic), 110.20, 109.67, 109.29, 107.73 (−CC−),

52.42, 46.66, 46.52, 38.35, 38.32, 29.97, 29.94, 28.70, 27.81, 27.74,

23.34, 22.03, 21.99, 21.70 (alkyl CH₂), 14.23, 14.14, 14.04, 13.13,

13.03, 12.99, 9.88, 9.86, 7.09, 6.82 (alkyl CH₂).³¹P{¹H}-NMR (101.3

1

MHz, CDCl₃, ppm): δ 10.73, J_Pt−P= 2653.5 Hz. ESI-MS: m/z 698.0

(M⁺). Anal. Calc. for C₃₄H₄₃NP₂Pt: C, 63.72; H, 7.20. Observed: C,

63.81; H, 7.22%.

trans-[R−CC−(PEt₃)₂Pt−CC−R] (R = Carbazole-3-yl), M5. A

similar procedure to the synthesis of M2 using L2 aﬀorded a pale

yelllow solid (69.2%, mp 209 °C). IR (ATR, diamond): ν/cm⁻¹3387

(N−H), 2096 (−CC−). ¹H NMR (700 MHz, CDCl₃, ppm): δ 8.03

(s, 2H, H-4, 4′), 7.81 (d, J = 7.6 Hz, 2H, H-5, 5′), 7.56−7.51 (m, 4H,

H-1, 1′,8, 8′), 7.37 (d, J = 7.9 Hz, 2H, H-2, 2′), 7.16−7.11 (m, 4H, H-

6, 6′,7, 7′), 5.22 (s, 2H, NH, NH′), 1.25−1.11 (m, 12H, PCH₂), 0.83−

0.73 (m, 18H, alkyl CH₃). ¹³C NMR (176 MHz, CDCl₃, ppm): δ

139.96, 137.72, 137.57, 136.23, 134.52, 134.38, 134.33, 134.29, 134.25,

134.04, 130.30, 130.05, 129.84, 129.54, 129.39, 127.89, 127.82, 127.66,

125.97, 125.83, 123.43, 123.28, 122.54, 120.47, 119.57, 119.45

(aromatic), 110.70, 110.68, 110.35, 110.26 (−CC−), 14.75−14.65

(PCH₂), 8.64−8.26 (alkyl CH₃). ³¹P{¹H}NMR (122 MHz, CDCl₃,

1

MHz, CDCl₃): δ − 131.8, J_Pt−P= 2637 Hz. FAB-MS: m/z 1342.1

(M⁺). Anal. Calc. for C₆₀H₉₃P₄Pt₂N: C, 53.69; H, 6.98. Observed: C,

53.72; H, 6.91%.

trans-[(Bu₃P)₂Pt−CC−R−CC−]_n(R = Carbazole-2,7-diyl), P1.

The compound was prepared according to the previously reported

procedure.^11f¹H NMR (250 MHz, CDCl₃, ppm): δ 7.81 (dd, J = 7.2,

2,2 Hz, 2H, H-4, 5), 7.27 (s, 2H, H-1, 8), 7.15 (d, J = 7.0 Hz, 2H, H-3,

6), 5.65 (s, 1H, NH), 2.18−2.10 (m, 12H, PCH₂), 1.58−1.30 (m,

1

ppm): δ 11.89, J_Pt−P= 2104.1 Hz. ESI-MS: m/z 810.0 (M⁺). Anal.

Calc. for C₄₀H₄₆N₂P₂Pt: C, 59.18; H, 5.71. Observed: C, 59.21; H,

5.70%.

trans-[R−CC-(Bu₃P)₂Pt−CC−R] (R = Carbazole-3-yl), M6. A

similar synthesis to that used to prepare M3 was followed using L2 and

aﬀorded a pale brown solid (81.6%, mp 187.3 °C). IR (ATR,

24H, alkyl CH ), 1.08−0.87 (m, 18H, alkyl CH ). GPC (THF): M =

̅

3

n

25 500 g mol⁻¹²(n = 31), M = 40 500 g mol⁻¹, PDI = 1.6.

̅

w

1

diamond): ν/cm⁻¹3397 (N−H), 2097 (−CC−). H NMR (700

trans-[−(Bu₃P)₂Pt−CC−R−CC−]_n(R = N-(2-Ethylhexylcar-

bazole-2,7-diyl)), P2. The compound was prepared according to the

previously reported procedure.^11f¹H NMR (250 MHz, CDCl₃, ppm):

δ 7.83 (d, J = 6.9 Hz, 2H, H-4, 5), 7.21 (s, 2H, H-1, 8), 6.99 (d, J = 7.1

Hz, 2H, H-3, 6), 4.04−4.01 (m, 2H, NCH₂), 2.66−2.57 (m, 1H, alkyl

CH), 2.22 (m, 12H, PCH₂), 1.58−1.46 (m, 32H, alkyl CH₂), 1.12−

MHz, CDCl₃, ppm): δ 8.02−7.96 (m, 4H, H-4, 4′, 5, 5′), 7.42−7.34

(m, 4H, H-1, 1′, 8, 8′), 7.30 (d, J = 7.1 Hz, 2H, H-2, 2′), 7.21 (t, J =

7.0 Hz, 4H, H-6, 6′,7, 7′), 2.24 (br s, 2H, NH, NH′), 1.69−1.65 (m,

12H, PCH₂), 1.80−1.39 (m, 24H, CH₂), 1.14−0.84 (m, 18H, alkyl

CH₃). ¹³C NMR (176 MHz, CDCl₃, ppm): δ 139.95, 137.48, 137.43,

129.48, 129.35, 125.77, 123.38, 122.60, 122.46, 120.85, 120.72, 120.34,

119.44, 110.70, 110.19, 109.35 (aromatic), 104.88, 104.86, 104.78,

104.70 (−CC−), 26.59−24.08 (alkyl CH₂), 14.85−13.02 (alkyl

CH₃). ³¹P{¹H}NMR (122 MHz, CDCl₃, ppm): δ 3.37, ¹J_Pt−P= 2380.2

Hz. ESI-MS: m/z 979.4 (M⁺). Anal. Calc. for C₅₂H₇₀N₂P₂Pt: C, 63.72;

H, 7.20, C. Observed: 63.90; H, 7.21%.

0.90 (m, 24H, alkyl CH ). GPC (THF): M = 40 000 g mol⁻¹(n =

̅

3

n

43), M = 72 000 g mol⁻¹, PDI = 1.8.

̅

w

trans-[−(Bu₃P)₂Pt−CC−R−CC−]_n(R = Carbazol-3,6-diyl),

P3. The compound was prepared following the previously reported

procedure.^11f¹H NMR (250 MHz, CDCl₃, ppm): δ 7.50 (d, J = 7.7

Hz, 2H, H-4, 5), 7.41 (dd, J = 13.9, 7.4 Hz, 2H, H-1, 8), 7.25 (d, J =

8.2 Hz, 2H, H-2, 7), 5.57 (s, 1H, NH), 2.20−2.01 (m, 12H, PCH₂),

1.67−1.01 (m, 24H, alkyl CH₂), 0.93 (t, J = 7.9 Hz, 18H, alkyl CH₃).

GPC (THF): M = 17 900 g mol⁻¹(n = 22), M = 30 500 g mol⁻¹, PDI

trans-[(Ph)(Et₃P)₂Pt−CC−R−CC−Pt(PEt₃)₂(Ph)] (R = Carba-

zole-2,7-diyl), M7. The compound was prepared as previously

reported.^11f¹H NMR (250 MHz, CDCl₃, ppm): δ 7.80 (d, J = 7.9

Hz, 4H, H_orthoof Ph), 7.42 (d, J = 7.7 Hz, 2H, H-4, 5), 7.32 (s, 2H, H-

1, 8), 7.20 (dd, J = 13.9, 7.4 Hz, 2H, H-3, 6), 6.98 (t, J = 7.4 Hz, 4H,

H_metaof Ph), 6.82 (t, J = 7.3 Hz, 2H, H_paraof Ph), 5.20 (br s, 1H, NH),

1.80−1.61 (m, 24H, PCH₂), 1.07−0.98 (m, 36H, alkyl CH₃). FAB-

MS: mlz 1230.2 (M⁺).

̅

n

w

= 1.7.

trans-[−(Bu₃P)₂Pt−CC−R−CC−]_n(R = N-(2-ethylhexylcar-

bazole-3,6-diyl)), P4. The poly-yne was synthesized by mixing L4b

(0.100 g, 0.305 mmol), trans-[Pt(PBu₃)₂Cl₂] (0.205 g, 0.305 mmol),

and CuI (1 mg) in ⁱPr₂NH/CH₂Cl₂(50 mL, 1:1, v/v). After stirring at

room temperature overnight under Ar, the solvent was evaporated

under reduced pressure. The residue was dissolved in CH₂Cl₂and

trans-[(Ph)(Et₃P)₂Pt−CC−R−CC−Pt(PEt₃)₂(Ph)] (R = N-(2-

Ethylhexylcarbazole-2,7-diyl)), M8. The compound was prepared as

N

DOI: 10.1021/acs.inorgchem.6b00523

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

ﬁltered through a short alumina column, using hexane/CH₂Cl₂(1:1,

v/v) as eluent, to remove ionic impurities and catalyst residues. After

removal of the solvent, the crude product was puriﬁed twice by

precipitation in CH₂Cl₂from MeOH. Subsequent washing with

hexane and drying in vacuo gave a brown solid (0.226 g, 80.1%). IR

former were used to generate a simulated UV/vis absorption spectrum

according to the equation:

2

⎛

⎝

⎞

⎟

⎠

f

⎡ v − v ⎤

i

ε(v) =

1.3062974 × 10^{8 i}exp⎜−

∑

⎢

⎣

⎥

⎦

σ

i

1

(CH₂Cl₂): ν/cm⁻¹2097 (−CC−). H NMR (700 MHz, CDCl₃,

where the energies, v, are in wavenumbers (cm⁻¹), ε is the molar

extinction coeﬃcient in L mol⁻¹cm⁻¹, f_iis the (dimensionless)

oscillator strength, v_iis the band position, and σ is a uniform

bandwidth used to broaden the peaks, here set to 0.2 eV.

ppm) δ 7.94 (s, 2H, H-4, 5), 7.38 (d, J = 8.2 Hz, 2H, H-1, 8), 7.18 (d, J

= 8.3 Hz, 2H, H-2, 7), 4.14−4.02 (m, 2H, NCH₂), 3.29 (m, 1H, alkyl

CH), 2.21 (t, J = 21.4 Hz, 10H, PCH₂), 2.09−1.99 (m, 2H, PCH₂),

1.75−1.11 (m, 32H, alkyl CH₂), 0.99−0.75 (m, 24H, alkyl CH₃). ¹³C

NMR (176 MHz, CDCl₃, ppm) δ 138.07, 127.93, 123.10, 121.53,

121.22, 118.51 (aromatic), 108.27, 107.34 (−CC−), 46.27 (NCH₂),

38.35 (alkyl CH), 29.94, 28.70, 27.74, 25.44, 25.34, 25.12, 23.52,

23.49, 23.45, 23.34, 23.09, 22.99, 22.89, 22.05 (alkyl CH₂), 13.02,

12.92, 12.86, 9.90 (alkyl CH₃). ³¹P{¹H}-NMR (101.3 MHz, CDCl₃): δ

ASSOCIATED CONTENT

* Supporting Information

■

S

The Supporting Information is available free of charge on the

ACS Publications website at DOI: 10.1021/acs.inorg-

chem.6b00523.

1

− 137.55, J_Pt−P= 233 Hz. Anal. Calc. for [C₄₈H₇₇P₂PtN]_n: C, 62.31;

H, 8.39. Observed: C, 62.20; H, 8.36%. GPC (THF): M = 36 000 g

̅

n

mol⁻¹(n = 39), M = 70 000 g mol⁻¹, PDI = 1.9

CCDC reference numbers 1456923−1456926.(CIF)

̅

w

X-ray Crystallography. The crystals of 3a-TMSA, 4a-TMSA, M3,

and M7 were mounted in inert oil on glass ﬁbers. Data were measured

using Mo Kα radiation (λ = 0.71073 Å) with a Bruker Kappa CCD

diﬀractometer or an Agilent Gemini A-Ultra diﬀractometer (for M3)

both equipped with an Oxford Cryostream low-temperature attach-

ment. Structures were solved by direct methods (SHELXS-86)²⁷and

subjected to full-matrix least-squares reﬁnement on F²(SHELXL-

97).²⁸In both M3 and M7, there was extensive disorder in the alkyl

groups of the phosphine ligands, and in M7, two orientations of one of

the terminal phenyl groups were also observed. These features were

modeled over two or three sites using partial occupancies, summed to

unity, and additional constraints were placed on the bond parameters

to maintain reasonable bond lengths and angles. In M7, the two

partially occupied phenyl-ring positions were restrained using the

FLAT command. Except for some of the disordered carbon atoms in

the alkyl chains of the phosphine ligands, all the nonhydrogen atoms

were reﬁned anisotropically. Hydrogen atoms were included using

rigid methyl groups or a riding model and, again, partial occupancies

were included as appropriate. Reﬁnement continued until convergence

was reached, and in the ﬁnal cycles of reﬁnement, a weighting scheme

was introduced that aﬀorded a relatively ﬂat analysis of variance.

Molecular-Weight Measurements. Molar masses were deter-

mined by gel-permeation chromatography (GPC),²⁹using two PL Gel

30 cm, 5 μm mixed C columns at 30 C, running in THF at 1 cm³

min⁻¹with a Roth Mocel 200 high-precision pump. A DAWN DSP

(Wyatt Technology) multiangle laser-light scattering (MALLS)

apparatus with 18 detectors and an auxiliary Viscotek model 200

diﬀerential refractometer/viscometer detector was used to calculate

the molecular weights (the overall technique is referred to as GPC-

LS).

Computational Modeling. Molecular quantum-chemical calcu-

lations were carried out using the density-functional theory (DFT)

formalism, as implemented in the NWChem code.³⁰The B3LYP

hybrid functional³¹was used in conjunction with Pople split-valence

basis sets³²of 6-31g and 6-31g** quality for the H and non-H atoms,

respectively, and the LANL2DZ pseudopotential³³and corresponding

double-ζ basis set were used to describe Pt. The convergence

tolerances for the optimization of the electronic wave functions were

set to 10⁻⁶, 10⁻⁵, and 5 × 10⁻⁴a.u. on the total energy, density, and

gradients, respectively. The geometries of the four initial models were

optimized until the maximum and root-mean-square (RMS) gradients

on the ions were less than 4.5 × 10⁻⁴and 3 × 10⁻⁴a.u., respectively,

and the maximum and RMS Cartesian steps in the last iteration fell

below 1.8 × 10⁻³and 1.2 × 10⁻³a.u. The minima were then veriﬁed

by computing the vibrational frequencies using analytical gradients;

during these calculations, the tolerances on the energy, density, and

gradients during the electronic minimization were tightened to 10⁻⁸,

10⁻⁶, and 10⁻⁵a.u., respectively. Finally, time-dependent DFT (TD-

DFT) calculations were carried out on the optimized models using

adiabatic B3LYP. The 50 lowest-lying singlet (spin-allowed) and

triplet (spin-forbidden) electronic excitations were computed, and the

AUTHOR INFORMATION

Corresponding Authors

*E-mail: msk@squ.edu.om.

*E-mail: j.m.skelton@bath.ac.uk.

*E-mail: p.r.raithby@bath.ac.uk.

■

Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

M.S.K. gratefully acknowledges the Research Council (TRC),

Oman for funding (grant no. ORG/EI/SQU/13/015). This

work was conducted as part of a Strategic Project submitted to

Sultan Qaboos University (SQU), Oman for funding from His

Majesty’s Trust Fund (HMTF). R.A.A.-B. acknowledges SQU

for a PhD scholarship and M.S.K. for a research leave. J.M.S.

and P.R.R. are supported by an EPSRC programme grant

(grant no. EP/K004956/1). The computational modelling was

carried out using the UK Archer facility, accessed through the

UK Materials Chemistry Consortium, which is funded by the

EPSRC (grant no. EP/L000202). Some modelling was also

performed using the Balena HPC system at the University of

Bath, which is maintained by Bath University Computing

Services.

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DOI: 10.1021/acs.inorgchem.6b00523

Inorg. Chem. XXXX, XXX, XXX−XXX

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Experimental and Theoretical Investigation for the Level of Conjugation in Carbazole-Based Precursors and Their Mono-, Di-, and Polynuclear Pt(II) Complexes

DOI: 10.1021/acs.inorgchem.6b00523

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Article abstract of DOI:10.1021/acs.inorgchem.6b00523

Full text of DOI:10.1021/acs.inorgchem.6b00523

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