Modulation of calcium-sensitive potassium (BK) channels by oxygen is important in several mammalian tissues, and in the carotid body it is crucial to respiratory control. However, the identity of the oxygen sensor remains unknown. We demonstrate that hemoxygenase-2 (HO-2) is part of the BK channel complex and enhances channel activity in normoxia. Knockdown of HO-2 expression reduced channel activity, and carbon monoxide, a product of HO-2 activity, rescued this loss of function. Inhibition of BK channels by hypoxia was dependent on HO-2 expression and was augmented by HO-2 stimulation. Furthermore, carotid body cells demonstrated HO-2–dependent hypoxic BK channel inhibition, which indicates that HO-2 is an oxygen sensor that controls channel activity during oxygen deprivation.

Reaction of the arachno-[B₉H₁₄]^– or nido-[B₉H₁₂]^– anions with [IrCl(PPh₃)₃] at ca. 298 K gives, in addition to previously reported species, pale violet [7,7,9-(PPh₃)₃-isonido-7-IrB₉H₁₀]2 in yields of ca. 2%. Single crystal X-ray diffraction analysis and multinuclear NMR spectroscopy reveal a ten-vertex {IrB₉} cluster of an ‘isonido’ type that is based upon the closo eleven-vertex structure of [B₁₁H₁₁]^2– from which a four-connected vertex is removed to generate a four-membered open face. There are two fluxional bridging hydrogen atoms associated with the open face. Thermolysis at ca. 355 K of solutions in 1,1,2,2-tetrachloroethane of yellow [5,7-(PPh₃)₂-5-H-5-(o-Ph₂P[graphic omitted]]4 result in loss of hydrogen and the formation of deep purple [8-Cl-7,9-(PPh₃)₂-7-(o-Ph₂P[graphic omitted]0]5(0–5%) and royal blue [7,9-(PPh₃)₂-7-(o-Ph₂P[graphic omitted]0]6(0–5%). Single-crystal X-ray diffraction shows that compounds 5 and 6 also have isonido cluster structures, although 5 does not have bridging hydrogen atoms on the open face, suggesting that any extra electrons required for it to be isoelectronic with 2 and 6 may originate from the metal vertex. The structure of the crystal of compound 6 examined was found to contain ca. 25 mole % of the isostructural 3-chloro derivative 6a. The ten-vertex clusters 2, 5 and 6 are part of a structural continuum ranging from closo through isonido to isocloso. It is proposed that these isonido compounds represent intermediates in a variety of reactions involving nido to isocloso cluster oxidations and nido to nido rearrangements.

For the structural extent of the polyhedral molecular chemistry of boron to rival that of the organic chemistry of the carbon hydrides, (a) the individual boron-containing polyhedra need to be fused together to make larger contiguous covalent assemblies, and (b) an intermolecular chemistry needs to be developed. Contemporary examples are given that illustrate novel chemistry in both these areas and that may point the way to some future developments.