A visible-light photoactivatable di-nuclear PtIV triazolato azido complex

A novel PtIV triazolato azido complex [3]-[N1,N3] has been synthesised via a strain-promoted double-click reaction (SPDC) between a PtIV azido complex (1) and the Sondheimer diyne (2).

of the trapped azidyl radical (a α N = 0.314 mT) were considered. For the DMPO-OH adduct, only the first two hyperfine couplings were included. An isotropic g-tensor (g =2 .00587 mT) was used in both cases.

Preparation of cell-free extract
The paediatric high grade glioma cell line KNS42 was grown in DMEM media (ThermoFisher, 61965-026) containing 10% foetal bovine serum (Sigma, F7524). KNS42 cells were grown in a CO 2 incubator (5% CO 2 ) at 37°C. To prepare the cell-free lysate, cell flasks were washed with 5mL HBSS (ThermoFisher, 14170120) and cells digested in 2mL of TryplExpress (ThermoFisher, 12604021). Following complete digestion, TryplEpress was inhibited with an equal volume of DMEM media and the cell suspension was centrifuged at 1200 rpm x 5 min at 4 o C to form a pellet. The cell pellet was washed in dH 2 O and centrifuged to pellet 3 times. The final cell pellet was resuspended in 1 ml of dH 2 O (1.0 x 10 7 cells/ml). To ensure complete lysis, the cell suspension was minimally sonicated. Cell debris was removed by centrifugation at 13,0000 rpm for 15 min at 4 o C. The cell-free lysate was kept on ice prior to running EPR experiments which were conducted as soon as possible after lysate preparation.

Synthetic procedures
Caution! No problems were encountered during this work, however heavy metal azides are known to be shock sensitive detonators, therefore it is essential that platinum azido complexes are handled with care. In addition, due to their photosensitivity, the platinum azido complexes (particularly solutions) were handled and stored with minimal light exposure.
The Pt IV complex trans,trans,trans-[Pt(N 3 ) 2 (OH) 2 (py) 2 ] (1) was synthesised as previously reported and purified by HPLC before use. 3 The Sonheimer diyne (5,6,11,12-Tetradehydrodibenzo[a,e]cyclooctene, 2) was synthesised and purified by column chromatography as outlined below: 4,5 ortho-(Phenylsulfonylmethyl)benzonitrile: 3.92 g of alpha-bromotolunitrile (20 mmol) was added to benzene sulfinic acid sodium salt dihydrate (4.80 g, 24.0 mmol, ChemCruz) in DMF (30 ml) under an inert N 2 atmosphere. The reaction mixture was stirred at 80 o C for two hours and then cooled to room temperature. The reaction mixture was worked up with H 2 O/EtOAc. The organic layer was isolated, dried with MgSO 4 , filtered and the solvent removed in vacuo to yield 5.65 g of crude product. The crude product was then recrystallised with minimal hot EtOAc and hexane until the clouding point was reached, yielding 2.58 g of solid white product (10 mmol, 50%). 1 1 M, in THF/hexane, 7.9 ml, 7.9 mmol) was added dropwise at -78 o C. The reaction mixture was stirred at -78 o C for two hours and then quenched with aqueous NH 4 Cl (100 ml, 1 M). The reaction mixture was worked up with H 2 O (1 x 100 ml) and EtOAc (2 x 200 ml). The organic layers were combined and the solvent removed in vacuo to yield 870 mg of crude product. The product was purified by column chromatography (DCM:hexane 2:3) to yield 45 mg of pure product as a yellow solid (0.23 mmol, 11%). 1

X ray crystallographic tables
Single crystals of (3)-[N1,N3] were grown from MeCN; the Pt complex co-crystallised with three molecules of MeCN in the unit cell.
Single crystal X-ray structure determination: Single-crystal X-ray diffraction data were collected using an Oxford Diffraction Supernova dual-source diffractometer equipped with a 135 mm Atlas CCD area detector. Crystals were selected under Paratone-N oil, mounted on micromount loops and quenchcooled using an Oxford Cryosystems open flow N2 cooling device. Data were collected at 150 K using mirror monochromated Cu Kα radiation (λ = 1.5418 Å; Oxford Diffraction Supernova) and processed using the CrysAlisPro package, including unit cell parameter refinement and inter-frame scaling (which was carried out using SCALE3 ABSPACK within CrysAlisPro). 6 Equivalent reflections were merged and diffraction patterns processed with the CrysAlisPro suite. Structures were subsequently solved using direct methods and refined on F2 using the SHELXL package. 7,8,9 Table S1 Crystal data and structure refinement for (3)

Photochemistry of 3-[N1,N3]
Mass spectrometry           For elimination of two radicals from one Pt IV centre, we suggest that this involves two stepwise one-electron donations from azido and hydroxyl radicalor two hydroxyl radicals, depending on the radicals generated -to the Pt centre. 10 The formation of a Pt III centre is a plausible intermediate, such species having previously been detected by ESI-MS of photoactivated Pt IV prodrugs by ourselves 11 and by ESI-MS collision-induced dissociation of Pt IV prodrugs by others. 12 Detailed experimental and theoretical investigations of the electrochemical reduction of Pt IV to Pt II have also determined that electron transfer and Pt−L bond cleavage occurs in a stepwise fashion, with a metastable six-coordinate Pt(III) intermediate formed upon addition of a single electron; the loss of both axial ligands is then associated with the second electron transfer. 13 Although Pt III species are likely to be transient intermediates following the ejection of either an azido or a hydroxyl radical, closed-shell Pt II or Pt IV centres are significantly more stable than Pt III ; the latter typically only being stable in the presence of suitable bulky ligands over which the Pt III unpaired electron can be delocalised. 14 We suggest that for species where two ligands have been photoejected, that this is more likely to occur from the same Pt IV centre, forming a Pt IV -Pt II photoproduct, as opposed to a Pt III -Pt III species.
Of the photoproducts observed by ESI-MS, those which could be assigned as consistent with observed masses are shown below, with suggested formulae and putative oxidation states: This formally charged complex is formed through loss of azide anion (N 3 -). It is likely that in aqueous solution that this species is hydrated in the vacant coordination site thus: [Pt IV (N 3 )(Py) 2 (OH) 2 (C 16 H 8 N 6 )Pt IV (Py) 2 (OH) 2 (OH 2 )] + , with an appropriate counterion.   Note: The photoproducts detected by ESI-MS are by necessity either formally charged, or sodiated/protonated. These species therefore represent only a subset of the possible photoproducts -those which are uncharged, or negatively charged not being unobserved. Furthermore, 195 Pt NMR spectroscopy is a relatively insensitive slow technique, and unlikely to detect minor -or transientphotoproducts. Isolation and complete structural characterisation of the stable photoproducts (and determining the photochemical pathways, subsequent photochemistry of intermediates etc.) is outside the scope of this communication, these more detailed experiments are currently underway.