A-Frame-Templated High-Coordinate Platinum(IV) cis-Bis(boryl) Complexes

The addition of Et2O·BF3 or Me2S·BCl3 to the BNBN-cumulene-bridged Pt(II) A-frame complexes (μ-1,1-BNBN(TMS)2)(μ-dmpm)2Pt2X2 (TMS = SiMe3, dmpm = CH2(PMe2)2, X = Br 1Br, I 1I) resulted in the oxidative addition of one B–F or B–Cl bond, respectively, to the internal BN bond of the bridging, iminoborane-like B–N≡B–N moiety, and coordination of one Pt(II) center to the resulting adjacent BF2 (complex 2Br-F) or BCl2 (complexes 2Br-Cl and 2I-Cl) moiety, respectively. X-ray crystallographic and multinuclear NMR-spectroscopic data show that the Pt→BF2 interaction in 2Br-F is very weak and merely electrostatic, while the Pt→BCl2 interaction in 2Br-Cl and 2I-Cl is a stronger donor–acceptor bond. In contrast, the reaction of Me2S·BBr3 with 1Br yielded a ca. 3:2 mixture of the analogous B–Br addition product to the iminoborane, 2Br-Br, and the product of a subsequent oxidative addition of one B–Br bond of the chelating BBr2 moiety to the adjacent platinum center, the mixed-valence boranediyl-bridged, Pt(II)–Pt(IV)-bromoboryl complex 3-Br5. The analogous reactions of Me2S·BI3 with 1Br and Me2S·BBr3 with 1I yielded complex product mixtures of Pt(II)–Pt(II)-borane (2Br-I and 2I-Br, respectively) and Pt(II)–Pt(IV)-boryl complexes (3-BrnI5–n, n = 1–3) analogous to 2X-Y and 3-Br5, respectively, the proportion of the latter increasing with the proportion of iodide in the precursor mixture. Both multinuclear NMR-spectroscopic and X-ray crystallographic data show evidence of complex and extensive inter- and intramolecular bromide–iodide exchanges between the soft, iodide-affine platinum centers and the harder, more bromide-affine boron centers. A clue to the mechanism of these halide exchanges is provided by the reactions of BBr2Ar (Ar = 2,4,6-Me3C6H2 (Mes), 2,3,5,6-Me4C6H (Dur)) with 1Br, which yielded the cationic Pt(II)–Pt(II)-borenium analogues of 2Br-Br, the complexes 4Br-Ar, generated by the sterics-induced displacement of the bromide substituent from the chelating Pt→BBrAr moiety, and displaying a rare metal→borenium donor–acceptor bond.