Compound 3i

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Synthetic procedure: See article for the definitive version of this procedure and for full experimental details.

Synthesis of 3i: 3 (10.0 mg, 0.0029 mmol) and K₃PO₄ (8.5 mg, 0.040 mmol) were added along with a flea micro stir bar to a 4-mL glass vial, which was then sealed with a PTFE/silicone cap under ambient conditions. The vial was then purged and backfilled with N₂ three times before being transferred into the glovebox. In the glovebox, the vial was opened and 300 µL anhydrous DMF was added, followed by mPEGthiol₃₅₆ (12.27 µL, 0.038 mmol). The vial was sealed again and set to stir at 400 rpm for 5 hours. The vial was transferred out of the glovebox, and its contents were transferred into an NMR tube for in situ ¹⁹F NMR spectroscopy to ensure nearly quantitative conversion and in situ ¹¹B NMR spectroscopy to ensure structural integrity of the cluster. The crude mixture was then transferred into a 20-mL glass vial and lyophilized for solvent removal. A 1.25 cm x 35 cm glass column was packed with Sephadex LH20 medium in MeOH (23 cm packed height), and the crude product mixture containing 3i was loaded onto the column with MeOH. 15 1-2 mL fractions were collected, dried via rotary evaporation, and subjected to characterization via ¹H, ¹¹B, and ¹⁹F NMR spectroscopy. The pure product fractions as indicated by NMR spectroscopy were combined and dried via rotary evaporation to obtain an isolated yield of 17.1 mg (78 %). ¹H NMR (400 MHz, CD₃OD): δ 7.64 – 7.46 (br m, 24H, OCH₂-Ar), 7.26 – 7.18 (br m, 24H, OCH₂-Ar), 5.65 – 5.61 (br m, 24H, OCH₂), 3.70 (t, 24H, SCH₂CH₂), 3.62 – 3.44 (m, 288H, SCH₂CH₂O(CH₂CH₂O)₆), 3.30 – 3.28 (m, 36H, (CH₂CH₂O)₆CH₃), 3.14 (t, 24H, SCH₂). ¹¹B NMR (128 MHz, CD₃OD): δ -15.3. ¹⁹F NMR (376 MHz, CD₃OD): δ -136.4 – -136.5 (m, 24F, -meta), -145.3 (m, 24F, -ortho). HRMS (Q-Exactive Plus): m/z calculated for C₃₃₆H₄₄₄B₁₂F₄₈O₉₆S₁₂ (M^2-), 3721.3430 Da; found, 3721.3395 Da.