Ab Initio/IGLO/NMR Investigation of nido-C(4)B(7)H(11) Configurations: Structural Assignment of the Three Known Isomers and Reconfirmation of Empirical Carbon Placement Patterns

The ab initio/IGLO/NMR method has been successfully applied to establish the structures of the three known isomers of nido-C(4)B(7)H(11). The method confirms the previously proposed structure, nido-7,8,9,10-C(4)B(7)H(11), 1a, as one of the three known isomers. Of four candidates considered for the second isomer, one of the previously proposed structures, nido-1,7,8,10-C(4)B(7)H(11), 2b, is selected. Of four candidates considered for the third isomer, structure nido-2,7,9,10-C(4)B(7)H(11), 3b, which had not been previously proposed, is established. The relative order of stability is 1a > 2b > 3b. A comparison of the relative energies of the nine cage structures considered in this study shows that, in complete agreement with previous empirically determined patterns, the most stable structures are those in which the carbons occupy low coordinate sites. This preference is more important than avoiding carbon-carbon connections.     

Synthesis, characterization, and computational studies of 6-(RR'N)-nido-5,7-C2B8H11: a polyborane cluster with a cage-boron having an exopolyhedral dative boron-nitrogen double bond

The reactions of the arachno-4,6-C 2B 7H 13 carborane with the secondary and primary amines, Me 2NHBH 3 and ( t )BuNH 2BH 3, in ionic liquid media result in both boron-insertion into the cage at a position across the two cage-carbons and additional hydrogen-elimination via the reaction of a hydridic B-H with a protonic amine N-H hydrogen to produce the 6-(RR'N)- nido-5,7-C 2B 8H 11 carboranes. Computational characterizations of these compounds and the previously reported 6-ClC 6H 4-9-(RR'N)- nido-6-NB 9H 10 azaboranes indicate that the amine-nitrogens form unique exopolyhedral dative BN double bonds with a cage-boron.     

Phosphacarborane chemistry: the 7,8,9,11-, 7,9,8,10- and 7,8,9,10-isomers of nido-P2C2B7H9--diphosphadicarbaborane analogues of 7,8,9,10-C4B7H11

The reaction between the carborane arachno-4,6-C2B7H13 (1) and PCl3 in dichloromethane in the presence of a "proton sponge" (PS = 1,8-dimethylaminonaphthalene) resulted in the isolation of the eleven-vertex nido-diphosphadicarbaboranes 7,8,9,11-P2C2B7H, (2) and 3-Cl-7,8,11-P2C2B7H, (3-Cl-2) in yields of 54 and 7%, respectively. Replacement of the PS by NEt3 in the same reaction gave diphosphadicarbaboranes 2 and 3-CI-2 together with the isomeric species nido-7,9,8,10-P2C2B7H, (3) in yields of 28, 15 and 3%, respectively. The reaction between the isomeric carborane arachno-4,5-C2B7H13 (4) and PCl3 in dichloromethane in the presence of PS gave the asymmetrical isomer, nido-7,8,9,10-P2C2B7H, (5). along with the chloro derivatives 4-Cl-7,8,9,10-P2C2B7H8 (4-Cl-5) and 11-Cl-7,8,9,10-PC2B7,H8 (11-Cl-5) (yields of 21, 1 and 13%, respectively). The structures of the chlorinated derivatives 3-Cl2 and 11 -Cl-5 were determined by X-ray diffraction analysis. In addition, the structures of all compounds isolated were geometry-optimised and confirmed by comparison of experimental 11B chemical shifts with those calculated by the GIAO-SCF/II//RMP2(fc)/6-31G* method. The calculations also include the structure and 11B NMR shifts of the isomer nido-7,10,8,9-P2C2B7H9 (6) which has not yet been isolated.     

Gas-phase electron diffraction studies on two 11-vertex Dicarbaboranes, closo-2,3-C2B9H11 and nido-2,9-C2B9H13

The molecular structures of two carbaboranes, closo-2,3-C(2)B(9)H(11) and nido-2,9-C(2)B(9)H(13), were determined experimentally for the first time using gas-phase electron diffraction (GED). For closo-2,3-C(2)B(9)H(11), a model with C(2)(v)() symmetry was refined to give C-B bond distances ranging 158.3-167.0 pm and B-B distances ranging 177.4-200.0 pm. The structure of nido-2,9-C(2)B(9)H(13) was refined using a model with C(s)() symmetry to give C-B bond lengths ranging 160.3-171.9 pm and B-B lengths ranging 173.0-196.1 pm. Ab initio computations (up to MP2/6-311+G) were also carried out on these and the related nido-7,8-C(2)B(9)H(13), which was not sufficiently stable to allow determination of its molecular structure by GED.     

Cooperative metal-boron interactions in the reaction of nido-1,2-(Cp*RuH)2B3H7, Cp* = eta5-C5Me5, with HC(triple bond)CPh

Products of the reaction of nido-1,2-(CpRuH)(2)B(3)H(7), 1, and phenylacetylene demonstrate the ways in which cluster metal and main group fragments can combine with an alkyne. Observed at 22 degrees C are (a) reduction to mu-alkylidene Ru-B bridges (isomers nido-1,2-(CpRu)(2)(1,5-mu-C{Ph}Me)B(3)H(7), 2, and nido-1,2-(CpRu)(2)(1,5-mu-C{CH(2)Ph}H)B(3)H(7), 3), (b) reduction to exo-cluster alkyl substituents on boron (nido-1,2-(CpRuH)(2)-3-CH(2)CH(2)Ph-B(3)H(6), 4), (c) cluster insertion with extrusion of a BH(2) fragment into an exo-cluster bridge (nido-1,2-(CpRu)(2)(mu-H)(mu-BH(2))-4-or-5-Ph-4,5-C(2)B(2)H(5), 5), (d) combined insertion with BH(2) extrusion and reduction (nido-1,2-(CpRu)(2)(mu-H)(mu-BH(2))-3-CH(2)CH(2)Ph-5-Ph-4,5-C(2)B(2)H(4), 6), (e) insertion and loss of borane with and without reduction (nido-1,2-(CpRu)(2)-5-Ph-4,5-C(2)B(2)H(7), 7, and isomers nido-1,2-(CpRu)(2)-3-CH(2)CH(2)Ph-4-(and-5-)Ph-C(2)B(2)H(6), 8 and 9), and (f) insertion and borane loss plus reduction (nido-1,2-(CpRu)(2)-3-(trans-CH=CHPh)-5-Ph-4,5-C(2)B(2)H(6), 10). Along with 7, 8, and 10, the reaction at 90 degrees C generates products of insertion and nido- to closo-cluster closure (closo-4-Ph-1,2-(CpRuH)(2)-4,6-C(2)B(2)H(3), 11, closo-1,2-(CpRuH)(2)-3-CH(2)CH(2)Ph-5-Ph-7-CH(2)CH(2)Ph-4,5-C(2)B(3)H(2), 12, closo-1,2-(CpRuH)(2)-5-Ph-4,5-C(2)B(3)H(4), 13, and isomers closo-1,2-(CpRuH)(2)-3-and-7-CH(2)CH(2)Ph-5-Ph-4,5-C(2)B(3)H(3), 14 and 15). The clusters with an exo-cluster bridging BH(2) groups are shown to be intermediates by demonstrating that the major products 5 and 6 rearrange to 13 and convert to 14, respectively. 14 then isomerizes to 15, thus connecting low- and high-temperature products. Finally, all available information shows that the high reactivity of 1 with alkynes can be associated with the "extra" two Ru-H hydrides on the framework of 1 which are required to meet the nido-cluster electron count.     

Two isomeric phosphacarboranes 2,1- and 6,1-PCB(8)H(9), the first representatives of the 10-vertex closo phosphacarborane Series

The reaction between arachno-4-CB(8)H(14) and PCl(3) in the presence of PS (PS = proton sponge = 1,8-dimethylamino naphthalene) (dichloromethane, rt, 24 h) produced the neutral phosphacarborane closo-2,1-PCB(8)H(9) (35% yield), while a similar reaction of nido-1-CB(8)H(12) gave the isomeric compound closo-6,1-PCB(8)H(9) (27% yield). The structures of both compounds were derived on the basis of the combined ab initio/GIAO/NMR ((1)H, (11)B, (13)C) approach. The optimized structures at a correlated level of theory (MP2) with 6-31G* basis set were used as a basis for calculations of the (11)B and (13)C chemical shifts at GIAO-SCF/II and GIAO-MP2/II, the latter showing excellent agreement with experimental data.     

Diphosphacarbollide analogues of the C5H5- anion: isolation of the nido-di- and triphosphacarboranes 7,8,9-P2CB8H10, [7,8,9-P2CB8H9]-, [7,8,10-P2CB8H9]-, and 7,8,9,10-P3CB7H8

Treatment of a solution of excess PCl(3) and PS (PS = "proton sponge" = 1,8-dimethylamino naphthalene) with arachno-4-CB(8)H(14) (1) in CH(2)Cl(2), followed by hydrolysis of the reaction mixture, resulted in the isolation of the eleven-vertex diphosphacarbaborane nido-7,8,9-P(2)CB(8)H(10) (2) (yield 34%) as the main product. Other products isolated from this reaction were the phosphacarboranes nido-7,8,9,10-P(3)CB(7)H(8) (3) (yield 5%) and closo-2,1-PCB(8)H(9) (4) (yield 15%). Compound 2 can be deprotonated by PS in CH(2)Cl(2) or NaH in diethyl ether to give the [nido-7,8,9-P(2)CB(8)H(9)](-) (2(-)()) anion, which gives back the original compound, 2, upon re-protonation. Thermal rearrangement of anion 2(-) (Na(+) salt) at 350 degrees C for 2 h produced the isomeric [nido-7,8,10-P(2)CB(8)H(9)](-) (5(-)()) anion, which was isolated as a PPh(4)(+) salt (yield 86%). Multinuclear ((1)H, (11)B, (31)P, and (13)C), two-dimensional [(11)B-(11)B] COSY, (1)H{(11)B(selective)}, (1)H{(31)P(selective)}, and gradient-enhanced ([(1)H-(13)C] HSQC) magnetic resonance measurements led to complete assignments of all resonances which are in excellent agreement with the structures proposed. Coupling constants, (1)J((31)P,(13)C), (2)J((31)P,C,(1)H), and (1)J((31)P,(31)P), were calculated using the DFT method B3LYP/6-311+G(d,p). The molecular geometries of all compounds were optimized ab initio at a correlated level of theory (RMP2(fc)) using the 6-31G basis set, and their correctness was assessed by comparison of the experimental (11)B and (13)C chemical shifts with those calculated by the GIAO-SCF/II//RMP2(fc)/6-31G method. The computations also include the structures and chemical shieldings of the still unknown isomers [nido-7,10,8-P(2)CB(8)H(9)](-) (6(-)) and [nido-7,9,8-P(2)CB(8)H(9)](-) (7(-)).     

Ruthenacarboranes from the reaction of nido-1,2-(Cp*RuH)2B3H7 with HC [triple bond] CCO2Me, Cp* = eta5-C5Me5. Hydrometalation, alkyne incorporation, and functional group modification via cooperative metal-boron interactions within a metallaborane cluster framework

Reaction of nido-1,2-(Cp*RuH)2B3H7, 1, and methyl acetylene monocarboxylate under kinetic control generates nido-1,2-(Cp*Ru)2(mu-C[[CO2Me]Me])B3H7 (a pair of geometric isomers, 3 and 5) and nido-1,2-(Cp*Ru)2(1,3-mu-C[[CH2CO2Me]H])B3H7, 4, which display the first examples of exo-cluster mu-alkylidene Ru-B bridges generated by hydrometalation of an alkyne on the cluster framework. Both 3 and 5, but not 4, rearrange into arachno-2,8-mu(C)-5-eta1(O)-Me[CO2Me]C-1,2-(Cp*Ru)2B3H7, 2, in which an unprecedented intramolecular coordination of the carbonyl oxygen atom of the alkyne substituent to a boron framework site opens the ruthenaborane skeleton. Compound 2, in turn, is an intermediate in the formation of the ruthenacarborane nido-1,2-(Cp*Ru)2-3-OH-4-OMe-5-Me-4,5-C2B2H5, 12, in which the carbonyl-oxygen double bond has been cleaved as its oxygen atom inserts into a B-H bond and the carbonyl carbon inserts into the metallaborane framework. In a parallel reaction pathway, nido-1,2-(Cp*Ru)2-5-CO2Me-4,5-C2B2H7, 6, nido-1,2-(Cp*Ru)2-4-B(OH)2-5-CO2Me-4,5-C2B2H6, 16, and nido-1,2-(Cp*Ru)2(mu-H)(mu-BH2)-3-(CH2)2CO2Me-CO2Me-4,5-C2B2H4 (a pair of geometric isomers, 7 and 14, which contain an unusual Ru-B borane bridge) are formed. On heating, 7 rearranges to yield nido-1,2-(Cp*Ru)2-3-(CH2)2CO2Me-4-BH2-5-CO2Me-4,5-C2B2H5, 13, whereas 14 converts to nido-1,2-(Cp*Ru)2-3-(CH2)2CO2Me-4-CO2Me-4,5-C2B2H6, 8. Under thermodynamic control, nido-1,2-(Cp*Ru)2-4,5-B[(CH2)2CO2Me]CO(MeO)[C(CH2)CO2Me]-4,5-C2B2H6, 11, is the major product accompanied by lesser amounts of 6 and 1,2-(Cp*Ru)2-4-OMe-5-Me-4,5-C2B2H6, 10. Compound 11 features a five-membered heterocycle containing a boron atom. The structure of 7, which is an intermediate in the formation of 11, provides the basis for an explanation of this complex condensation of three alkynes. A previously unrecognized role for an exo-cluster bridging borene generated from the metallaborane skeleton by addition of the alkyne is also a feature of this chemistry. Reinsertion or loss of this boron fragment accounts for much of the chemistry observed. NMR experiments reveal labile intermediates, and one has been sufficiently characterized to provide mechanistic insight on the early stages of the alkyne-metallaborane addition reaction. All isolated compounds have been spectroscopically characterized, and most have been structurally characterized in the solid state.     

Reductive degradation of nido-1-CB8H12 into smaller-cage carborane systems via new monocarbaboranes [arachno-5-CB8H13](-) and closo-2-CB6H8

Treatment of the nido-1-CB8H12 (1) carborane with NaBH4 in THF at ambient temperature led to the isolation of the stable [arachno-5-CB8H13]- (2(-)), which was isolated as Na+[5-CB8H13]-.1.5 THF and PPh4 +[5-CB8H13]- in almost quantitative yield. Compound 2(-) underwent a boron-degradation reaction with concentrated hydrochloric acid to afford the arachno-4-CB7H13 (3) carborane in 70 % yield, whereas reaction between 2(-) and excess phenyl acetylene in refluxing THF gave the [closo-2-CB6H7]- (4-) in 66 % yield. Protonation of the Cs+4(-) salt with concentrated H2SO4 or CF3COOH in CH2Cl2 afforded a new, highly volatile 2-CB6H8 (4) carborane in 95 % yield, the deprotonation of which with Et3N in CH2Cl2 leads quantitatively to Et3NH+[2-CB6H7](-) (Et3NH+4(-)). Both compounds 4- and 4 can be deboronated through treatment with concentrated hydrochloric acid in CH2Cl2 to yield the carbahexaborane nido-2-CB5H9 (5) in 60 % yield. New compounds 2-, 3, and 4 were structurally characterised by the ab initio/GIAO/MP2/NMR method. The method gave superior results to those carried out using GIAO-HF when relating the calculated 11B NMR chemical shifts to experimental data.     

Synthesis, structure, and reactivity of 13-vertex carboranes and 14-vertex metallacarboranes

Syntheses, properties, and synthetic applications of 13-vertex closo- and nido-carboranes are reported. Reactions of the nido-carborane salt [(CH2)3C2B10H10]Na2 with dihaloborane reagents afforded 13-vertex closo-carboranes 1,2-(CH2)3-3-R-1,2-C2B11H10 (R = H (2), Ph (3), Z-EtCH=C(Et) (4), E-(t)BuCH=CH (5)). Treatment of the arachno-carborane salt [(CH2)3C2B10H10]Li4 with HBBr2.SMe2 gave both the 13-vertex carborane 2 and a 14-vertex closo-carborane (CH2)3C2B12H12 (8). On the other hand, the reaction of [C6H4(CH2)2C2B10H10]Li4 with HBBr2.SMe2 generated only a 13-vertex closo-carborane 1,2-C6H4(CH2)2-1,2-C2B11H11 (9). Electrophilic substitution reactions of 2 with excess MeI, Br2, or I2 in the presence of a catalytic amount of AlCl3 produced the hexa-substituted 13-vertex carboranes 8,9,10,11,12,13-X6-1,2-(CH2)3-1,2-C2B11H5 (X = Me (10), Br (11), I (12)). The halogenated products 11 and 12 displayed unexpected instability toward moisture. The 13-vertex closo-carboranes were readily reduced by groups 1 and 2 metals. Accordingly, several 13-vertex nido-carborane dianionic salts [nido-1,2-(CH2)3-1,2-C2B11H11][Li2(DME)2(THF)2] (13), [[nido-1,2-(CH2)3-1,2-C2B11H11][Na2(THF)4]]n (13a), [[nido-1,2-(CH2)3-3-Ph-1,2-C2B11H10][Na2(THF)4]]n (14), [[nido-1,2-C6H4(CH2)2-1,2-C2B11H11][Na2(THF)4]]n (15), and [nido-1,2-(CH2)3-1,2-C2B11H11][M(THF)5] (M = Mg (16), Ca (17)) were prepared in good yields. These carbon-atom-adjacent nido-carboranes were not further reduced to the corresponding arachno species by lithium metal. On the other hand, like other nido-carborane dianions, they were useful synthons for the production of super-carboranes and supra-icosahedral metallacarboranes. Interactions of 13a with HBBr2.SMe2, (dppe)NiCl2, and (dppen)NiCl2 gave the 14-vertex carborane 8 and nickelacarboranes [eta5-(CH2)3C2B11H11]Ni(dppe) (18) and [eta5-(CH2)3C2B11H11]Ni(dppen) (19), respectively. All complexes were fully characterized by various spectroscopic techniques and elemental analyses. Some were further confirmed by single-crystal X-ray diffraction studies.     

Diphosphacarborane analogues of ferrocene: the synthesis of two isomeric twelve-vertex closo-[(eta5-C5H5)FeP2CB8H9] complexes

The reaction of the Tl+ salt of the [nido-7,8,9-P2CB8H9]- anion (1-) with [CpFe(CO)2I](Cp =eta(5)-C5H5) in refluxing mesitylene for 12 h gives mixed-sandwich [1-Cp-closo-1,2,3,4-FeP2CB8H9] (2) (yield 63%). Reaction of the PPh4+ salt of the isomeric [nido-7,8,10-P2CB8H9]- anion 3- with [CpFe(CO)2I] in refluxing mesitylene gives [1-Cp-closo-1,2,3,5-FeP2CB8H9]4 (yield 56%), isomeric with 2. Compound 4 also results (yield 92%) from the sublimation of 2 under argon at ca. 350 degrees C. The constitution of all compounds is established by mass spectrometry, IR spectroscopy and multinuclear NMR spectroscopy (1H, 11B, 31P, and 13C; two-dimensional [11B-11B]-COSY, and 1H- 11B(selective)), further confirmed in the case of 4 by a single-crystal X-ray diffraction analysis.     

Synthesis and structural characterization of symmetrical closo-4,7-I2-1,2-C2B10H10 and [(CH3)3NH][nido-2,4-I2-7,8-C2B9H10

The boron-atom insertion reaction of nido-9,11-I(2)-7,8-C(2)B(9)H(9)(2-), with the HBCl(2):SMe(2) complex yields closo-4,7-I(2)-1,2-C(2)B(10)H(10), 1, in excellent yield. Although the two boron atoms (B3 and B6) nearest to the carbon atoms in 1 are equally available for attack by nucleophiles, the boron-degradation reaction of 1 with alkoxide ion occurs only at the B6 vertex, yielding regioselectively [(CH(3))(3)NH][nido-2,4-I(2)-7,8-C(2)B(9)H(10)], 2. The molecular structures of 1 and 2 have been determined by X-ray diffraction studies. Crystallographic data are as follows. For 1, monoclinic, space group P2(1)/n, a = 6.9199(19) Angstroms, b = 23.9560(7) Angstroms, c = 7.2870(2) Angstroms, beta = 94.081(4) degrees, V = 1204.9(6) Angstroms(3), Z = 4, rho(calcd) = 2.18 g cm(-3), R = 0.020, R(w) = 0.0610; for 2, orthorhombic, space group Pca2(1), a = 14.1141(7) Angstroms, b = 7.0276(4) Angstroms, c = 16.4602(9) Angstroms, V = 1632.7(15) Angstroms(3), Z = 4, rho(calcd) = 1.81 gcm(-3), R = 0.022, R(w) = 0.0623.     

The structure of arachno-[B6H11]-, at -25 degrees C in (CD3)2O,is resolved via the ab initio/IGLO-GIAO/NMR procedure

The arachno-[B6H11]- solution structure at -25 degrees C was clarified as fluxional compound 2 by applying the ab initio/IGLO/NMR method. The anion 2 can be derived from arachno-B6H12, 1, by the removal of the B2/B3 bridging hydrogen (2). No minimum on the potential energy surface could be found for an asymmetric complex, a, between [B5H8]- and BH3, which had been proposed originally. A Cs-symmetric [mu-(BH3)B5H8]- complex, A, only 3.2 kcal mol-1 higher in energy than 2, is the intermediate in the fluxional rearrangement observed on the NMR time scale. The transition structure [D] connecting 2 (Erel = 0.0) and A (Erel = 3.2) has a relative energy of 9.7 kcal mol-1. The elimination of both a and A as "most stable structure" candidates of arachno-[B6H11]- reinforces the early geometrical bonding systematics for boranes and carboranes.     

Dimethylsulfide-dicarbaborane chemistry. Isolation and characterisation of isomers [9-(SMe2)-nido-7,8-C2B9H10-X-Me] (where X = 1, 2, 3 and 4) and some related compounds. An unusual skeletal rearrangement

Oxidative coupling by FeCl(3) of the [nido-7,8-C(2)B(9)H(11)-9-Me](-) anion 1a with SMe(2) yields a mixture of four isomers of 9-(SMe(2))-nido-7,8-C(2)B(9)H(10)-X-Me, where X = 1, 2, 3 and 4 (compounds 2a, 2b, 2c and 2d respectively). On high dilution of the reaction mixture, the 9-(SMe(2))-nido-7,8-C(2)B(9)H(10)-10-Me 2e isomer is also isolated in a low yield. The isomers are separated by HPLC, and are identified and characterised by NMR spectroscopy and by single-crystal X-ray diffraction analyses of 2c and 2d. The formation of the products implies an unexpected cluster rearrangement, which is discussed in terms of dsd and vertex-flip reaction pathways. Two additional isomers, 9-(SMe(2))-nido-7,8-C(2)B(9)H(10)-5-Me 2f and 9-(SMe(2))-nido-7,8-C(2)B(9)H(10)-6-Me 2g occur when [nido-7,8-C(2)B(9)H(11)-5-Me](-) 1b is used as the starting substrate, in a reaction in which no cluster rerarrangement is observed. The corresponding bromide, [nido-7,8-C(2)B(9)H(11)-5-Br](-) 1c, behaves similarly, forming only 9-(SMe(2))-nido-7,8-C(2)B(9)H(10)-5-Br 2h and 9-(SMe(2))-nido-7,8-C(2)B(9)H(10)-6-Br 2i.     

Structure of 1-(arylselanyl)naphthalenes. 2. G dependence in 8-G-1-(p-YC(6)H(4)Se)C(10)H(6).

The structures of 8-G-1-(p-YC(6)H(4)Se)C(10)H(6) (1 (G = Cl) and 2 (G = Br): Y = H (a), OMe (b), Me (c), Cl (d), Br (e), COOEt (f), and NO(2) (g)) were investigated by X-ray crystallographic analysis, NMR spectroscopy, and ab initio MO calculations. The structures of all members in 1 and 2 are concluded to be type B, which is in striking contrast to the type A structure for 4d-g (4 (g(n)), where G = H). The Se-C(i) bond of the p-YC(6)H(4)Se group in 8-G-1-(p-YC(6)H(4)Se)C(10)H(6) is almost perpendicular to the naphthyl plane in type A, and it is located on the plane in type B. The chlorine and bromine substitution at the 8-position in 1 and 2 dramatically changes the type A structure of 4 (g(n)) to type B. The nonbonded G- - -Se-C 3c-4e type interaction must contribute to stabilize the type B structure. The type B structure in 1 and 2 should also be more stabilized than the same structure in 4 by the 3c-4e type interaction: The structure of 4b is type B in the crystals and type B would be more stable for 4c and might be for 4a in solutions. Ab initio MO calculations are performed on 8-G-1-(p-YC(6)H(4)Se)C(10)H(6), 8-G-C(10)H(6)SeH-1, and models HG- - -SeH(2), where G = Cl, Br, and F, to clarify the reason for the dramatic change in the structures. The type B structure is optimized to be more stable than the type A for all species examined, which supports the observations. The energy differences between type B and type A are larger for the models than for the naphthalenes. While the superiority of the type B for the former is Br > Cl > F, that of the latter is Br approximately Cl >/= F. These results show that the main factor of the structural change from type A to type B is the nonbonded G- - -Se-C 3c-4e interaction. The electronic effect of halogens through the naphthalene pi-framework would also contribute to some extent, although the direct comparison of the evaluated values between the naphthalene systems and the models is not so easy. Factors to stabilize the two structures of 1, 2, 4, and 8-(MeSe)-1-(p-YC(6)H(4)Se)C(10)H(6) are reexamined from a viewpoint of the nonbonded G- - -Se-C 3c-4e interaction (G dependence), together with the electronic effect of Y (Y dependence).     

Molecular structures of arachno-decaborane derivatives 6,9-X2B8H10 (X = CH2, NH, Se) including a gas-phase electron-diffraction study of 6,9-C2B8H14

The molecular structures of the three heterodecaboranes arachno-6,9-C2B8H14, arachno-6,9-N2B8H12, and arachno-6,9-Se2B8H10 have been determined by ab initio MO theory. In addition, the structure of arachno-6,9-C2B8H14 was experimentally determined using gas-phase electron diffraction (GED). The accuracy of all four of these structures has been confirmed by the good agreement of the (11)B chemical shifts calculated at the GIAO-MP2 level with the experimental values. A comparison of the GIAO-HF and GIAO-MP2 methods shows that for these heteroborane clusters, electron correlation effects on the computed delta((11)B) values are quite substantial and that it is necessary to go beyond the HF level in the NMR computation.     

Formation of two isomeric closo-[(eta5-C5H5)FePC2B8H10] phosphadicarbaborane analogues of ferrocene via isolable eta1-bonded complexes of the [7-[(eta5-C5H5)Fe(CO)2]-(eta1-nido-PC2B8H10)] type

The reaction of nido-[7,8,9-PC(2)B(8)H(11)] (1) with [[CpFe(CO)(2)](2)] (Cp=eta(5)-C(5)H(5) (-)) in benzene (reflux, 3 days) gave an eta(1)-bonded complex [7-Fp-(eta(1)-nido-7,8,9,-PC(2)B(8)H(10))] (2; Fp=CpFe(CO)(2); yield 38 %). A similar reaction at elevated temperatures (xylene, reflux 24 h) gave the isomeric complex [7-Fp-(eta(1)-nido-7,9,10-PC(2)B(8)H(10))] (3; yield 28 %) together with the fully sandwiched complexes [1-Cp-closo-1,2,4,5-FePC(2)B(8)H(10)] 4 a (yield 30%) and [1-Cp-closo-1,2,4,8-FePC(2)B(8)H(10)] 4 b (yield 5%). Compounds 2 and 3 are isolable intermediates along the full eta(5)-complexation pathway of the phosphadicarbaborane cage; their heating (xylene, reflux, 24 h) leads finally to the isolation of compounds 4 a (yields 46 and 52%, respectively) and 4 b (yields 4 and 5%, respectively). Moreover, compound 3 is isolated as a side product from the heating of 2 (yield 10%). The structure of compound 4 a was determined by an X-ray structural analysis and the constitution of all compounds is consistent with the results of mass spectrometry and IR spectroscopy. Multinuclear ((1)H, (11)B, (31)P, and (13)C), two-dimensional [(11)B-(11)B]-COSY, and (1)H[(11)B(selective)] magnetic resonance measurements led to complete assignments of all resonances and are in excellent agreement with the structures proposed.     

Facile hydrometalation of alkynes by nido-1,2-(CpRuH)(2)B(3)H(7) yielding novel Ru-B edge-bridging alkylidenes. Stepwise conversion of HCtbd1;CC(O)OMe into nido-1,2-(CpRuH)(2)-3-HOB-4-MeC-5-MeOC-BH(3), Cp = eta(5)-C(5)Me(5)

The reaction of the alkyne HCCC(O)OMe with 7 sep 1,2-(Cp*RuH)2B3H7 leads to hydroboration plus hydroruthenation to produce nido-1,3-mu-Me{C(O)OMe}C-1,2-(Cp*Ru)2B3H7, a compound with an exocluster ruthenium-boron mu-alkylidene that exists in two isomeric forms. Both isomers undergo rearrangement with intramolecular chelation of the carbonyl oxygen at a boron site, thereby opening the cluster and generating arachno-2,3,-mu(C)-5-eta1(O)-Me{C(O)OMe}C-1,2-(Cp*Ru)2B3H7. Further heating leads to deoxygenation of the carbonyl fragment by a boron center concurrent with insertion of the carbon atom into the metallaborane cage to give nido-1,2-(Cp*RuH)2-3-HOB-4-MeC-5-MeOC-BH3.     

Microwave spectra and structures of 1,2-(ortho)- and 1,7-(meta)-carborane, C2B10H12

The microwave spectra of 1,2- and 1,7-dicarba-closo-dodecaborane(12), C(2)B(10)H(12) (ortho- and meta-carborane), have been recorded for the first time at room temperature in the 32-88 and 24-80 GHz spectral ranges, respectively. The spectra of the parent species (1,2-C(2)(11)B(10)H(12) and 1,7-C(2)(11)B(10)H(12)) have been assigned, together with those of four monosubstituted ((10)B) 1,2-C(2)(10)B(11)B(9)H(12) and 1,7-C(2)(10)B(11)B(9)H(12) isotopologues. The microwave spectra confirm that the structures of each of these two molecules are slightly distorted icosahedrons of C(2v) symmetry. A previous determination of the gaseous structures of these two carboranes by the gas electron-diffraction method was based on several assumptions about the B-B bond length differences. All B-B bond lengths have now been redetermined using the substitution (r(s)) method, which is independent of such restraints. Although several of the r(s) and electron-diffraction bond lengths are in good agreement, there are also differences of up to 0.026 ?. Quantum chemical calculations at the B3LYP/6-311++G(3df,3pd) level of theory have also been performed.     

Gas-phase stability of derivatives of the closo-hexaborate dianion B(6)H(6)(2-)

B(6)H(6)(2-) does not represent a stable gas-phase dianion, but emits spontaneously one of its excess electrons in the gas phase. In this work we address the question whether small stable gas-phase dianions can be constructed from the parent B(6)H(6)(2-) dianion by substitution of the hydrogens with appropriate ligands. Various hexa-, tetra-, and disubstituted derivatives B(6)L(6)(2-), B(6)H(2)L(4)(2-), and B(6)H(4)L(2)(2-) (L = F, Cl, CN, NC, or BO) are investigated with ab initio methods in detail. Four stable hexasubstituted B(6)L(6)(2-) (L = Cl, CN, NC, or BO) and three stable B(6)H(2)L(4)(2-) (L = CN, NC, or BO) gas-phase dianions could be identified and predicted to be observable in the gas phase. The trends in the electron-detachment energies depending on various ligands are discussed and understood in the underlying electrostatic pattern and the electronegativities of the involved elements.