Photochemically Induced Cyclometalations at Simple Platinum(II) Precursors

Photochemical cycloplatinations of 2-arylpyridines and related C∧N ligands, as well as terdentate heteroaromatic N∧N∧C, N∧C∧N, and N∧C∧C compounds, are demonstrated using (Bu4N)2[Pt2Cl6] or [PtCl2(NCPh)2] as precursors at room temperature. Mono- or bis-cyclometalated Pt(II) complexes with C∧N ligands are obtained depending on excitation wavelength and precursor. Monitoring experiments show that photoexcitation enables both the N-coordination and the subsequent C–H metalation. Photochemical synthetic protocols have been developed, which are advantageous with respect to the established thermal procedures and have allowed the synthesis of the first Pt(II) complexes with N∧C∧C ligands.

The experimental setup for irradiations with green light is similar to previously reported designs. 11 It consisted of a 2 L crystallizing dish with a commercial RGB LED strip working in green color (Inspire LEDFLEXI KIT, model 2128KS-5-RGBW, 2.5 m, 4.8 W/m, lmax = 516 nm) fixed to the inner wall, in which a 50 mL Schlenk tube containing the reaction mixture and a magnetic stir bar was introduced. The entire system was covered with aluminium foil and placed on a magnetic stirrer. The radiant flux impacting on the Schlenk tube was ca. 800 mW based on an optical power measurement. The emission spectrum of the LEDs is shown in Figure S1.
Irradiations with violet light (lmax = 405 nm) were performed in flat-bottom, 20 mL Carius tubes (AE = 20 mm) made of borosilicate glass and fitted with a PTFE vacuum stopcock, which were placed on top of individual LED emitters (LED Engin LuxiGen™ LZ1-10UB0R-00U8) fixed to an aluminium heat sink and cooled by a fan. The radiant flux at the bottom of the tube was ca. 1380 mW based on technical specifications. The emission spectrum of the LEDs is shown in Figure S1. Figure S1. Emission spectra of the violet (left) and green (right) LED emitters employed for irradiations.

Spectroscopic and analytical methods
NMR spectra were recorded on Bruker Avance 300, 400, or 600 MHz spectrometers at 298 K. Chemical shifts are referred to residual signals of non-deuterated solvent and are given in ppm downfield from tetramethylsilane. Electronic absorption spectra were recorded on a Perkin-Elmer Lambda 750S spectrophotometer. Elemental analyses were carried out with a LECO CHNS-932 microanalyzer. Infrared spectra were recorded on a Jasco FT/IR-4600 spectrometer equipped with an attenuated total reflectance (ATR) module.

Synthesis of (Bu4N)2[Pt2Cl6]
The published method for the synthesis of this compound involves heating of an aqueous solution of K2PtCl4 and Bu4NCl (1:3 molar ratio) at 80 °C. 12 We present an alternative procedure.

Thermal cycloplatinations of dmtppyH2 and dPhOppyH2
The precursors (Bu4N)2[Pt2Cl6] and [PtCl2(NCPh)2] did not react with dmtppyH2 or dPhOppyH2 in acetone at room temperature in the dark. When a EtOH/CH2Cl2 mixture was employed as solvent, metalation of dPhOppyH2 was observed at room temperature in the dark using (Bu4N)2[Pt2Cl6] as precursor, but the yield was 24% after 3 days. By refluxing a mixture of (Bu4N)2[Pt2Cl6] and dPhOppyH2 in MeCN in the presence of Na2CO3, a mixture was obtained, which contained only a small proportion of complex 7. The reaction of [PtCl2(NCPh)2] with dPhOppyH2 in toluene at reflux temperature in the presence of Na2CO3 gave a mixture in which complex 10 was a minor product.

S9
The reactions between K2[PtCl4] and dmtppyH2 or dPhOppyH2 in AcOH at reflux temperature led to complex mixtures under an inert atmosphere. In the case of the dmtppyH2, refluxing in AcOH under aerobic conditions produced pure material of a very low solubility, which we postulate as the dimeric Pt(IV) complex [Pt2Cl2(µ-Cl)2(dmtppy)2] (12; 51% yield) on the basis of its elemental analysis and its reaction with g-picoline to give the mononuclear complex [PtCl2(dmtppy)(γ-picoline)] (13) (Scheme S1), whose identity was confirmed by an X-ray diffraction analysis ( Figure S5).

X-Ray structure determinations
Single crystals suitable for X-ray diffraction were grown by slow liquid-liquid diffusion from CH2Cl2/n-pentane (8) or CH2Cl2/Et2O (11 and 13). Diffraction data were collected on a Bruker D8 QUEST diffractometer with monochromated Mo-Ka radiation performing j and w scans. The structures were solved by direct methods and refined anisotropically on F 2 using the program SHELXL-2018 (G. M. Sheldrick, University of Göttingen). 24,25 Numerical details are presented in Table S1. Methyl hydrogens were included as part of rigid idealized methyl groups allowed to rotate but not tip; other hydrogens were included using a riding model. reflections; w -1 = s 2 (F 2 ) + (aP) 2 + bP, where P = (2Fc 2 + Fo 2 )/3 and a and b are constants set by the program. S11 Figure S3. Crystal structure of 8 (thermal ellipsoids at 50% probability). Hydrogen atoms are omitted.  Figure S4. Crystal structure of 11 (thermal ellipsoids at 50% probability). Hydrogen atoms are omitted.  Figure S5. Crystal structure of 13 (thermal ellipsoids at 50% probability). Hydrogen atoms are omitted.            Figure S18. 1 H NMR spectra (600 MHz) of an acetone-d6 solution of (Bu4N)2[Pt2Cl6], ppyH and piperidinomethyl(polystyrene) after different times of irradiation with green LEDs at room temperature (aromatic region). Conditions: (Bu4N)2[Pt2Cl6] (10 mg, 0.01 mmol), ppyH (3 µL, 0.02 mmol), piperidinomethyl(polystyrene) (11 mg, ca. 0.04 mmol), acetone-d6 (0.5 mL). Note that the base is not totally effective in capturing the released HCl and part of the free ppyH is protonated to give ppyH·HCl, resulting in a gradual deshielding of the resonance of the H ortho to the N atom.  Figure  S18). Two of them can be unequivocally assigned on the basis of the 1 H NMR data of trans-N,N-[PtCl(ppy)(ppyH)]. 26,27 The resonance at 9.27 ppm is somewhat broadened at the base because of unresolved Pt satellites and corresponds to the proton ortho to the N atom of the coordinated ppyH. The resonance at 8.38 ppm that integrates as 2 protons with respect to the previous one can be assigned to the protons in positions 2 and 6 of the pendant phenyl ring, proving that the ppyH ligand has not undergone cyclometalation. Therefore, this intermediate can only be the anionic complex [PtCl3(ppyH)] -.   Additional comments. In this experiment, complex (OC-6-32)-[PtCl2(tpy)2] 28 (Scheme S2) presumably arises from the reaction between [PtH(Cl)(tpy)2] and HCl released from the electrophilic metalation of the first tpyH ligand. However, the major product after 20 min of irradiation is its C2-symmetrical isomer (OC-6-33)-[PtCl2(tpy)2], which must be produced upon photoisomerization. 29 Scheme S2. (