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.

The photophysics of the two isomers of octadecaborane(22), anti- and syn-B18H22, have been studied by UV–vis spectroscopic techniques and theoretical computational methods. In air-saturated hexane, anti-B18H22 shows fluorescence with a high quantum yield, ΦF = 0.97, and singlet oxygen O2(1Δg) production (ΦΔ ∼ 0.008). Conversely, isomer syn-B18H22 shows no measurable fluorescence, instead displaying much faster, picosecond nonradiative decay of excited singlet states. Computed potential energy hypersurfaces (PEHs) for both isomers rationalize these data, pointing to a deep S1 minimum for anti-B18H22 and a conical intersection (CI) between its S0 and S1 states that lies 0.51 eV higher in energy. Such an energy barrier to nonradiative relaxation is not present in the PEH of syn-B18H22, and the system therefore has sufficient initial energy on excitation to reach the (S0/S1) CI and to then decay to the ground state without fluorescence. The computational analysis of the geometries at stationary points along the PEH of both isomers shows that the determining factor for the dissimilar photophysics of anti- and syn-B18H22 may be due to the significant differences in the geometrical rearrangements at their respective conical intersections. Thus, the syn isomer shows one very large, B–B elongation of 1.2 Å from 1.8 Å in the ground state to 3.0 Å at the CI, whereas the anti isomer shows smaller elongations (below 1 Å) in several B–B connectivities at its (S0/S1)CI. The absorbed energy in S1 for the anti-B18H22 is therefore redistributed vibrationally into several regions of the molecule rather than almost completely into a single vibrational mode as in the case for the syn isomer. The consequent prolonged S1 lifetime for the anti isomer allows for relaxation via fluorescence.

Reaction of the arachno-[ B,H,,] -or nido-[ BsHIZ] -anions with [ lrCl( PPh,),] at ca. 298 K gives, in addition to previously reported species, pale violet [7,7,9-( PPh,),-isonido-7-IrB,H,,] 2 in yields of ca. 2%. Single crystal X-ray diffraction analysis and multinuclear NM R spectroscopy reveal a ten-vertex {IrB,} cluster of an 'isonido' type that is based upon the closo eleven-vertex structure of [BllHl,]Z-from which a fourconnected vertex is removed to generate a four-membered open face. There are two fluxional bridging hydrogen atoms associated with the open face. Thermolysisat ca. 355 K of solutions in 1,1,2,2-tetrachIoroethane of yellow [5,7-(PPh,),-5-H-5-(o-Ph,PC6H,)-nido-5-lr~,H,,-2] 4 result in loss of hydrogen and the formation of deep purple [8-Cl-7,9-(PPh,),-7-(o-Ph,PC6H,)-ison~do-7-lrBsH6-1 01 5 (0-5%) and royal blue [7,9-(PPh,),-7-(o-Ph,P~6H,)-~son~do-7-IrB,H,-l 0 1 6 (045%). Single-crystal X-ray

The facile synthesis of the metallaheteroborane [8,8-(PPh 3) 2- nido-8,7-RhSB 9H 10] ( 1) makes possible the systematic study of its reactivity. Addition of pyridine to 1 gives in high yield the 11-vertex nido-hydridorhodathiaborane [8,8,8-(PPh 3) 2H-9-(NC 5H 5)- nido-8,7-RhSB 9H 9] ( 2). 2 reacts with C 2H 4 or CO to form [1,1-(PPh 3)(L)-3-(NC 5H 5)- closo-RhSB 9H 8] [L = C 2H 4 ( 3), CO ( 4)]. In CH 2Cl 2 at reflux temperature 2 undergoes a nido to closo transformation to afford [1,1-(PPh 3) 2-3-(NC 5H 5)- closo-1,2-RhSB 9H 8] ( 5). Reaction of 2 with alkenes leads to hydrogenation and isomerization of the olefins. NMR spectroscopy indicates the presence of a labile phosphine ligand in 2, and DFT calculations have been used to determine which of the two phosphine groups is labile. Rationalization of the hydrogenation mechanism and the part played by the 2 --> 3 nido to closo cluster change during the reaction cycle is suggested. In the proposed mechanism the classical hydrogen transfer from hydride metal complexes to olefins occurs twice: first upon coordination of the alkene to the rhodium centre in 2, and second concomitant with formation of a closo-hydridorhodathiaborane intermediate by migration of a BHB-bridging hydrogen atom to the metal. Reaction of H 2 with 3 or 5 regenerates 2, closing a reaction cycle that under catalytic conditions is capable of hydrogenating alkenes. Single-site versus cluster-bifunctional mechanisms are discussed as possible routes for H 2 activation.

The tuning of the photophysical properties of the highly fluorescent boron hydride cluster anti-B18H22 (1), by straightforward chemical substitution to produce 4,4'-(HS)2-anti-B18H20 (2), facilitates intersystem crossing from excited singlet states to a triplet manifold. This subsequently enhances O2((1)Δg) singlet oxygen production from a quantum yield of ΦΔ ∼ 0.008 in 1 to 0.59 in 2. This paper describes the synthesis and full structural characterization of the new compound 4,4'-(HS)2-anti-B18H20 (2) and uses UV-vis spectroscopy coupled with density functional theory (DFT) and ab initio computational studies to delineate and explain its photophysical properties.

Formation of metallaheteroboranes by insertion of heteroatoms into an existing metallaborane cluster, rather than adding the metal moiety to the heteroborane which is normally the case, has been achieved in the cases of carbon, nitrogen, and sulfur to produce nine-, ten-, and eleven-vertex metallaheteroborane clusters. Thus passage of acetylene through refluxing p-xylene solutions of either arachno-[(PMe3)2(CO)HIrB8H12] (1a) or nido-[(PMe3)2(CO)IrB8H11] (2a) afforded the 11-vertex cluster nido-[9,9,9-(PMe3)2(CO)-9,7,8-IrC2B8H11] (5a) in up to 44% yield. With the cage substituted species arachno-[(PMe3)2(CO)HIrB8H11Cl] (1b), the analogous product is nido-[5-Cl-9,9,9-(PMe3)2(CO)-9,7,8-IrC2B8H10] (5b) and the position of the Cl substituent in the product suggests that the acetylene moiety attacks across the open face of the intermediate nido-iridanonaborane species and that cage rearrangement does not occur during the course of the reaction. Similarly, reaction of 1a with H2S under identical conditions results in the formation nido-[2,2,2-(PMe3)2H-2,6-IrSB8H10] (9a), closo-[2,2,2-(PMe3)2H-2,1-IrSB8H8] (10), and nido-[(PMe3)2HIrS2B8H8] (11a) in overall yields of 13%, 7%, and 20%, respectively. Additionally, refluxing a solution of 1a and anhydrous hydrazine in xylene afforded a 10% yield of the novel iridaazanonaborane nido-[2,2,2-(PMe3)3-2,9-IrNB7H9] (12). The compound is the first metallaazanonaborane cluster to be described, and its formation indicates that methods similar to those used to generate azaboranes may be used on metallaboranes to generate metallazaboranes. In addition to characterization by a combination of 1H, 31P, and 11B NMR and IR spectroscopy and high-resolution mass spectrometry, single-crystal X-ray diffraction studies were carried out on compounds 5a, 9a, 10, and 12.

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