The Synthesis and Crystallographic Characterization of Emissive Pt(II) and Au(I) Compounds Exploiting the 2-Ethynylpyrimidine Ligand

: The luminescent properties of Au(I) and Pt(II) compounds are commonly tuned by exploiting the alkynyl ligand with varying electron density. Herein, we describe the synthesis of three new emissive transition metal compounds, t bpyPt(C 2 pym) 2 , Ph 3 PAuC 2 pym, and Cy 3 PAuC 2 pym (where HC 2 pym = 2-ethynylpyrimidine), veri ﬁ ed by 1 H-NMR, EA, and a single-crystal X-ray diffraction analysis. The t bpyPt(C 2 pym) 2 complex crystallized as an Et 2 O solvate in the orthorhombic space group Pbca with Z = 24 with three unique Pt(II) species within the unit cell. The Cy 3 PAuC 2 pym species crystallizes in a monoclinic space group with one unique complex in the asymmetric unit. Changing the identity of the phosphine from Cy 3 P to Ph 3 P in ﬂ uences interactions within the unit cell. Ph 3 PAuC 2 pym, which also crystalizes in a monoclinic space group, has an aurophilic bonding interaction Au–Au distance of 3.0722(2) Å, which is not present in crystalline Cy 3 PAuC 2 pym. Regarding optical properties, the use of an electron-de ﬁ cient heterocycle provides an alternate approach to blue-shifting the emission of Pt(II) transition metals’ compounds, where the aryl moiety is made more electron-de ﬁ cient by exploiting nitrogen within this moiety instead of the typical strategy of decorating the aryl ring with electron withdrawing substituents (e.g., ﬂ uorines). This is indicated by the blue-shift in emission that occurs in t bpyPt(C 2 pym) 2 ( λ max , emission = 512 nm) compared to the previously reported t bpyPt(C 2 2-py) 2 (where HC 2 2-py = 2-ethynylpyridine) complex ( λ max , emission = 520 nm).


Introduction
A systematic way to tune the emission and absorption of transition metal compounds is to modify the electronics of a pendant alkynyl ligand [1][2][3].This method has been previously exploited with various Pt(II) [4,5] and Au(I) [6] compounds to generate luminescent materials, which have applications in numerous devices including OLEDs [7].Despite progress in developing OLEDs using organometallic phosphors, the development of materials with deep blue emission still receives significant attention [8][9][10][11], and thus general strategies to blue-shift emission are important.Regarding Pt(II) species, the emission of these compounds can be blue-shifted by decreasing the electron density of the alkynyl ligand.For instance, the -C2C6F6 ligand (where HC2C6F6 = 1-ethynyl-2,3,4,5,6-pentafluorobenzene) is commonly used as an electron-deficient ligand to tune the emission of Pt(II) species [4,5,12] as it raises the energy of the metal-to-ligand charge-transfer (MLCT) and the ligand-to-ligand charge-transfer (LL'CT) excited states (ESs) [1,4,5,[12][13][14][15][16].For example, in the series of compounds t bpyPt(C2R)2 (Figure 1), the t bpyPt(C2C6F5)2 complex has the most blue-shifted emission (λmax = 501 nm in CH2Cl2) [4,17].The electrondeficient -C2C6F6 ligand has also been exploited to tune Au(I) phosphors [6].An alternative approach to the fluorine-substituted -C2C6F6 ligand is the introduction of electron-deficient heteroatoms to decrease the electron density of the aryl ring appended to the alkyne.For instance, pyridine is more electron-deficient than benzene due to the presence of the nitrogen atom, and pyrazine rings containing two nitrogen atoms are even more electron-deficient.This is reflected in the basicity of the pyridine [18] and pyrazine [19,20] ring systems.In emissive transition metal compounds, the presence of the pyridyl ring in t bpyPt(C2-2py)2 (where HC2-2py = 2-ethynylpyridine) blue-shifts emission relative to the t bpyPt(C2Ph)2 derivative (Figure 1), indicating the lowering of the MLCT and LL'CT ES.Similar to the aforementioned Pt(II) alkynyl compounds, the emission of Au(I) species of the form Cy3PAuC2R can be blue-shifted when the alkynyl group is made more electron-deficient [21].Herein, we describe the synthesis and characterization of three new alkynyl compounds, which exploit 2-ethynylpyrimidine (HC2pym) to provide insight into how this electron-deficient heterocycle can impact the properties of these transition metal compounds.

General Methods
UV-Vis: Absorption spectra were collected using a Cary-50 UV-Vis spectrophotometer (Agilent Technologies, Santa Clara, CA, USA).
Emission spectra: Emission spectra were collected using a Horiba (Kyoto, Japan) Scientific Fluorolog-3 spectrofluorometer.All emission spectra were recorded when compound concentration was at approximately 10 −5 M with the following excitation wavelengths: 380 nm for t bpyPt(C2pym) and 285 nm for Ph3PAuC2pym and Cy3PAuC2pym.
NMR spectra: 1 H NMR spectra were recorded on a JEOL (Peabody, MA, USA) ECX 400 MHz spectrometer. 1 H NMR resonances were referenced against tetramethylsilane using residual proton signals.
All reactions were performed under N2.The compounds t bpyPtCl2 [22], Ph3PAuCl [23], and Cy3PAuCl [24] were prepared according to previously reported procedures.CH2Cl2 and MeOH were dried with Al2O3 (passed through a column), (iPr)2NH was distilled over KOH, and all other reagents were used as received.

Single-Crystal X-ray Diffraction
Single-crystal X-ray diffraction data were collected at 100 K using a Bruker (Madison, WI, USA) D8 Venture diffractometer.The data were collected using phi and omega scans (0.50° frame width) with a Mo Kα (λ = 0.71073 Å) microfocus source and Photon 2 detector.Data were integrated (SAINT) and corrected for absorption using the multi-scan technique (SADABS), both within the Apex3 suite [25].The structures were solved by intrinsic phasing (SHELXT) and subsequently refined by full matrix least squares on F 2 (SHELXL) [26,27].All non-hydrogen atoms were refined anisotropically.Hydrogen atoms attached to carbon atoms were refined in calculated positions using the appropriate riding models.
The t bpyPt(C2pym)2 complex was found to crystallize as the diethyl ether solvate, with 0.25 molecules of Et2O per formula unit.Due to their partial occupancy in the structural voids, the disordered Et2O molecules were refined using several restraints to maintain chemically reasonable interatomic distances and anisotropic displacement parameters.One of the unique solvent molecules is present in half-occupancy due to symmetry constraints and the second unique solvent molecule is present in partial (0.25) occupancy in the voids.The latter occupancy was first tested by free variable refinement and found to be very close to 0.25, so it was fixed at this value for the final refinement.Elsewhere in this structure, disorder was observed in the orientation of two of the pyrimidine rings of the Pt1 complex and in one of the t-butyl groups of the Pt3 complex.Restraints were again employed in these instances to maintain similarity between the respective disordered contributions, where the site occupancies of the disordered parts were freely refined with a unity sum.The Ph3PAuC2pym complex crystallized as a dinuclear complex via aurophilic bonding and was refined in a fully ordered model without restraints.The Cy3PAuC2pym complex crystallized as a mononuclear complex and was likewise fully ordered and required no restraints.Crystallographic data are summarized in Table 1.CCDC 2359466-2359468 contain the complete supplementary crystallographic data for this paper and can be obtained from the Cambridge Crystallographic Data Centre.

Synthesis of Alkynyl Compounds
The compounds t bpyPt(C2pym)2, Ph3PAuC2pym, and Cy3PAuC2pym were synthesized from the chloride complexes t bpyPtCl2, Ph3PAuCl, and Cy3PAuCl, respectively (Scheme 1).For t bpyPt(C2pym)2, the strategy reported by Lu et al. was utilized to deprotonate HC2pym in the presence of CuI/iPr2NH to form the desired alkynyl compound.It was found that upon purifying this species using flash chromatography often [iPr2NH2]Cl would coelute with our desired product and remain with our product following Et2O precipitation.Therefore, a basic extraction (aqueous Na2CO3) was performed to remove this amine salt prior to column chromatography.Regarding the Ph3PAuC2pym and Cy3PAuC2pym alkynyl compounds, KOH was used to deprotonate HC2pym in an acetone/MeOH solution to form the desired Au(I) compounds.The excess KOH and the KCl by-products from these reactions were removed by dissolving the mixture in CH2Cl2 and filtering through a celite plug prior to precipitating (for Ph3PAuC2pym) or treating with hexanes (for Cy3PAuC2pym) to afford the desired solid.

Crystallographic Characterization
The t bpyPt(C2pym)2 complex crystallized as an Et2O solvate in the orthorhombic space group Pbca with Z = 24.The asymmetric unit consists of three crystallographically unique complexes (Figure 2: Pt1, Pt2, Pt3), differing slightly in the relative rotational orientations of the pyrimidine rings and their attachment angle at the alkyne.The Pt atoms are all four-coordinate square planar with C-Pt-C angles of 91.7(5)° for Pt1, 91.0(6)° for Pt2, and 89.4(6)° for Pt3, with Pt-C ranging from 1.928(11) Å to 1.956(13) Å and Pt-N ranging from 2.038(10) Å to 2.074(10) Å.Though there are no metal complexes of LM(C2pym)2 coordination reported in the CSD for direct comparison, these are comparable values to those in similar LPt(C22-py)2 (L = benzo(h)quinoline [28], 4,4′-di-t-butyl-2,2′bipyridine [29], and 1-benzyl-2-(2-pyridyl)benzimidazole [30]) complexes.Also similar to the ethynylpyridine complexes, the ethynylpyrimidine complex t bpyPt(C2pym)2 here exhibits Pt-C≡C and C≡C-C bonds that can deviate from linearity to accommodate packing with neighboring molecules.The complexes form stacks along the b-axis, with the b-axis length of 20.2605(15) Å comprising six complexes and the unit cell in total containing four such stacks (Figure 3).Though the mean planes of the PtN2C2 cores of the complexes are not formally parallel in the stacks, this spacing suggests an average distance of 3.38 Å between complexes.The orientation of the complexes alternates within these stacks via rotation in the ac plane, allowing the t-butyl groups and the ethynylpyrimidine arms to be adequately accommodated.This is a similar motif to what was observed in the ethynylpyridine complex [29], t bpyPt(C22-py)2 (CSD refcode RUGWIC) with a b-axis stacking of six complexes over 20.4987( 17  The structure of the dinuclear Ph3PAuC2pym complex is shown in Figure 4.The Au-Au distance of 3.0722(2) Å is well within the range typically considered for an aurophilic bonding interaction [31], leading to our interpretation of Ph3PAuC2pym as a dinuclear complex.All the atoms of the complex are crystallographically unique.Aurophilic interactions are common among Ph3PAu complexes, though to our knowledge, there are only two examples in the CSD where aurophilic bonding occurs between Ph3P-Au-C≡C fragments: bis[(thiphen-2-yl)ethynyl]-bis(triphenylphosphine)-di-gold (CSD refcode VULBIQ, Au-Au = 3.144 Å, [32]) and bis(µ2-penta-1,4-diyn-3-one)-tetrakis(triphenylphosphine)-tetra-gold (CSD refcode XIKWOF, Au-Au = 2.983 Å, [33]).The Ph3PAuC2pym complex has similar features about its Au centers to those other multinuclear complexes, namely approximately linear P-Au-C angles of 179.33(9)° and 173.11(9)°,Au-P distances of 2.2790(8) Å and 2.2788(7) Å, and Au-C distances of 1.998(3) Å and 1.996(3) Å.The Ph3PAuC2pym complexes pack sufficiently well to not create large solvent-accessible voids (Figure 5a).In particular, one phenyl group of a Ph3PAuC2pym complex fits between the two pyrimidyl groups of a neighboring complex to form chains of the complexes propagating along the b-axis via C-H•••N interactions (Figure 5b).The Cy3PAuC2pym complex crystallized as a mononuclear gold complex (Figure 6) with one unique complex in the asymmetric unit.It exhibits similar features of linear P-Au-alkyne connectivity (P-Au-C angle of 176.36(4)°,Au-P distance of 2.2936(4) Å, and Au-C distance of 2.0044( 16) Å) to other Cy3PAu complexes [21].The use of 2-ethynylpyrimidine in the present study is again unique in the structural literature for Cy3PAu complexes.Here, it again enables the formation of C-H•••N interactions from one pyrimidyl group of a complex to one of the pyrimidyl nitrogen atoms of a neighboring complex, thereby creating chains along the b-axis (Figure 7a).The PCy3 groups of neighboring chains are then positioned for localized complementary packing, which again limits the solvent-accessible void space (Figure 7b).

Characterization of the Alkynyl Pyrimidine Compounds
1 H-NMR spectroscopy served as an initial indicator that t bpyPt(C2pym)2, Ph3PAuC2pym, and Cy3PAuC2pym were successfully synthesized.A key feature in all the spectra was the presence of a downfield doublet and triplet integrating in a 2:1 ratio indicative of the resonances on the pyrimidine ring.Elemental analyses confirmed the purity of all three species, and crystallographic characterization (vide infra) supported the proposed structures with full structural characterization.
UV-Vis, excitation, and emission spectra of t bpyPt(C2pym)2, Ph3PAuC2pym, and Cy3PAuC2pym were collected in CH2Cl2.Over the range of concentrations where data were collected, no indication of ground-state aggregation was observed as indicated by Beer's law plots (Supplementary Materials).For each sample, the excitation spectra reasonably match the absorption spectra (Supplementary Materials), which indicates that the emission is not resultant of an impurity.

Conclusions
Three new transition metal complexes bearing 2-ethynylpyrimidine ligands have been synthesized, structurally characterized by X-ray crystallography, and further characterized by excitation and emission spectroscopy to gauge the utility of the alkynyl ligands toward tuning the emissive properties of complexes.The complexes, t bpyPt(C2pym)2, Ph3PAuC2pym, and Cy3PAuC2pym, were synthesized by the deprotonation of HC2pym in the presence of t bpyPtCl2, Ph3PAuCl, and Cy3PAuCl, respectively, to form the mixed-ligand species.The t bpyPt(C2pym)2 complex is a square planar Pt(II) complex with three unique complexes in the asymmetric unit that form stacks along the b-axis.The Ph3PAuC2pym complex forms a dinuclear Au(I) complex of two linear Ph3PAuC2pym units joined through aurophilic bonding.The Cy3PAuC2pym is likewise a linear Au(I) complex, but without aurophilic interactions, thus crystallizing as a discrete complex.Of note, the emission of the t bpyPt(C2pym)2 compound is blue-shifted relative to the previously reported t bpyPt(C22-py)2 complex, indicating that the introduction of electron-deficient heteroatoms provides a means of tuning of the optical properties of emissive alkynyl compounds.This provides an alternative strategy to the use of fluorinated alkynes, mainly -C2C6F5, that are commonly utilized to blue-shift emission.Other pyrimidine derivatives (e.g., 2-ethynyl-5-fluoropyrimidine [39]) should be considered as ligands to blue-shift emission of transition metal compounds.

Figure 3 .
Figure 3. Stacks of complexes in t bpyPt(C2pym)2 • 0.25(Et2O): (a) Six complexes stack in rotated fashion, viewed as normal to b-axis.(b) Packing diagram viewed along b-axis.For clarity, hydrogen atoms are omitted and only major-occupied disordered contributions are shown.Carbon atoms are gray, nitrogen atoms are purple, platinum atoms are blue, and oxygen atoms (of the Et2O molecules shown in (b)) are red.

Figure 4 .
Figure 4. Crystal structure of Ph3PAuC2pym shown as displacement ellipsoids at 50% probability levels.

Figure 5 .
Figure 5. Packing of dinuclear complexes in Ph3PAuC2pym: (a) Packing diagram viewed along baxis.(b) Selected intermolecular interactions in Ph3PAuC2pym resulting in formation of chains propagating along b-axis via C-H•••N (dashed blue lines).Carbon atoms are gray, nitrogen atoms are purple, gold atoms are yellow, and phosphorus atoms are orange.

Figure 6 .
Figure 6.Crystal structure of Cy3PAuC2pym shown as displacement ellipsoids at 50% probability levels.

Figure 7 .
Figure 7. Packing of complexes in Cy3PAuC2pym: (a) Selected intermolecular interactions in Cy3PAuC2pym resulting in formation of chains propagating along b-axis via C-H•••N (dashed blue lines).(b) Packing diagram of Cy3PAuC2pym viewed along b-axis.Carbon atoms are gray, nitrogen atoms are purple, gold atoms are yellow, and phosphorus atoms are orange.