Ab Initio Studies On Some Borane Ions And Their Derivatives

In this paper, some borane and their derivatives regarding as candidates ofBNCT (Boron Neutron Capture Therapy) are investigated systematically.The sandwich complexes [(η5-C5H5)M(MeSiB10H10)]-1 (M=Co, Rh andIr), as the models for [(η5-C5Me5)M(MeSiB10H10)]-1, are calculated byusing Density Functional Theory (DFT) with relativistic pseudopotential tostudy the optimized geometry, electronic structure and vibrational spectrumat B3LYP level. It is a commonly accepted technique in ab initiocalculations to use H to stand for methyl for save the computationalresources. The complexes were proposed to take the Cs symmetry in all thecalculations. For the relativistic pseudopotential, the RECP proposed by Hayand Wadt is used for the calculations. RECP with f type basis set is adoptedfor Co and Rh and RECP with g type basis set for Ir. Simultaneously, thebasis set 6-311G* is adopted for Si, B, C, and 6-311G for H. Since therelativistic effect is important for the transition metals, the relativisticeffective core potentials were employed for Co, Rh and Ir. Co, Rh and Irpossess negative charge of -0.10, -0.44 and -0.55, and the correspondingsilicon atoms have positive charge of 0.95, 0.99 and 1.07 respectively. Itseems that the negative charge tends to locate on transition metals in thecomplexes containing direct transition metal-silicon bonds. This may informthat strong interaction exists between metal atom M and silicon atom.The silaborate sandwich anions [(η5-C5H5)M(MeSiB10H10)]-1 ( M=Co, Rhand Ir) are an isoelectronic system, in which there are 86 valence electronsoccupying 43 molecule orbitals. For simplicity, we take the complex[(η5-C5H5)Ir(MeSiB10H10)]-1 as an example for illustration. There are 9molecular orbitals (the 16th, 30th, 31th 27th, 29th, 32th, 33th, 34th and 39thorbitals) that have the bonding character between the metal and ligands.These 9 orbials are filled by 18 electrons, thus the bonding of the centermetal satisfies the 18-electron configuration. It is interesting that silaborate,as a new kind of ligand to the sandwich transition metal complexes, stillenables the center metal to follow 18-electron configuration. The energydeference of HOMO-LUMO is 0.177 a.u., indicating the high stability of thecomplex.The silaborate cage has a very strong coordinate ability to transitionmetals. As shown from our calculations, the transition metal atom possessesnegative charge, the Si atom has positive charge and the metal-silicon bondlength is short. Most transition metal-silicate complexes have been provedphotochemically active. Whether the complexes discussed above arephotochemically active remains to be a problem for the further experiment.The discussion on the properties of the frontier orbitals and electronicstructure is not only meaningful for the investigation of the thermal stabilityof the complexes, but also important for probing into the electron transitions.The [(η5-C5H5)Ir(MeSiB10H10)]-1 sandwich complex has totally 102fundamental vibrational frequencies with the Cs symmetry.To obtain high veracity, the various isomers of macropolyhedral boranesions [B20H18]n (n=0, -2, -4) are calculated using density functional theory(DFT) to study the optimized geometries, electronic structures, vibrationalfrequencies, and natural populations at RB3LYP/6-31+G* andRB3LYP/6-31G* levels. Three isomers (1)-(3) of dianions have C2h, Cs andC1 Symmetries, respectively. Tetraanions (4)-(6) have D4h, Cs and CsSymmetries, respectively. Molecule (7) has C1 symmetry. The objects weinvestigated are all singlet states of molecules. These isomers are confirmedto be local minima on the potential energy surface. They include:[e2-B20H18]2-(1), [ae-B20H18]2-(2), [a2-B20H18]2-(3), [a2-B20H18]4-(4),[e2-B20H18]4-(5), [ae-B20H18]4-(6), [a2-B20H18]0 (7), where "a" and "e" insquare brackets denote apical and equatorial sites in single polyhedral cageB10H9, respectively. B10H9 is a half of macropolyhedral boranes [B20H18]. Inthe following text, the "oa" denotes outer apex of two polyhedral cagesB10H9 and the "ia" denotes inside apex locating opposite position of "oa" ineach polyhedral cage B10H9. For mentioned seven isomers, all calculatedvibrational frequencies are real, so they should be stable. The species (1),(2), (4), (5) and (6) have been characterized by experiments. The species (3)and (7) represent two new boranes that are considered in this paper for thefirst time. We can find that the calculated bond lengths are in agreementwith available experimental values. The shortest B-B bond lengths areRoa-e associated with the distances between outer apex and equatorialatoms, and the second shortest are Re-ia associated with the distancesbetween equatorial atoms and inside apex. Yet the longest B-B bond lengthsare Re-e associated with distances between both equatorial atoms andequatorial atoms. Moreover, the B-B bond Rjoint linking two polyhedralcages B10H9 become longer with the changing of valence number of anionsin the order of 0, -2 to -4. They are 1.59 ?, 1.68-1.70 ? and 1.74-1.78 ?respectively and all lie in the range of normal B-B bond lengths.Of the boranes (1)-(7), the three dianion species (1)-(3) have the lowesttotal energies in the range of -508.1774 a.u. — -508.1634 a.u. with stabilitysequence (1)>(2)>(3). The neutral molecule (7) is the second low-lying(-507.9487 a.u.). The three tetraanion species (4)-(6) are the third low-lying(-507.8138 a.u. — -507.7907 a.u.) with the stability sequence being (4)>(6)> (5). It seems, for tetraanion, that coulomb repulsion energies ofsuperabundance 4 electrons are bigger than their electron-nucleus attractionenergies, while for dianion, electron-nucleus attraction energies ofsuperabundance 2 electrons are bigger than their coulomb repulsion energies,reversely. The two systems (3) and (7) which probably exist as stableminima have not been studied up to now both theoretically andexperimentally. They desire further experimental confirmation.At present, the mno electron-counting rule has been applied tomacropolyhedral boranes. In the mno rule, the electronic requirements formacropolyhededral boranes were evaluated as F (e) =n+m+o+p-q-r, where nis the number of vertexes in the polyhedral skeleton, m is the number ofindividual polyhedral fragments, o is the number of single-vertex-sharingjunctions, p is the number of missing vertexes in the idealized closo skeleton,q is the number of capping vertexes, and r is the number of stuffed atoms.Counting of skeletal bonding electrons shows that the structures (1)-(6)match this rule, whereas (7) do not. For the ions (1)-(3), there are only 18exo-B-H bonds in 60 valence electrons of 20 B atoms, leaving 21 electronpairs used for skeletal bonding. According to the mno rule, the number ofskeletal bonding electron pairs required should be n+m=20+2=22, requiringtwo more electrons for aromatic stability. So their stable ions should bedianion. For the ions (4), (5) and (6), there are only 20 exo-B-H bonds in 60valence electrons, leaving 20 electron pairs used for skeletal bonding.According to the mno rule, the number of skeletal bonding electron pairsrequired should be n+m=20+2=22, requiring four more electrons foraromatic stability. Therefore their stable ions should be tetraanion. For thestructure (7), the number of skeletal bonding electron pairs required shouldalso be 22, requiring two more electrons for aromatic stability, so it shouldbe dianion. However, calculations indicate that it is neutral and can stablyexist. Surely, structure (7) provides new examples that violate the electronicrequirements based on the mno rule.In the contour maps and energies of MOs of [a2-B20H18]4-(4), LUMO(4-62) and LUMO-1 (4-63) are two pπ bonding MOs, they do not bond toadjacent equatorial boron atoms. Therefore, if two electrons are added toLUMO, the B-B bond Rjoint will be strengthened and the B-B bond Re-iaXIbecome weaker. Thus tetraanion may be reduced to hexaanion. In thecontour maps and energies of MOs of [e2-B20H18]2-(1), the "double-linking"between two polyhedra in dianion and neutral molecule make bond lengthsRjoint shorter. Moreover, HOMO (1-60) and LUMO (1-61) are all thebonding MOs. Consequently, LUMO of [e2-B20H18]2-(1) readily accept twoelectrons, reducing dianion to tetraanion. In the contour maps and energiesof MOs of [a2-B20H18]0 (7), LUMO energy (-0.2443 a.u.) is very low, easilygaining electrons and being reduced to dianion.Using Gaussian 98, 20 different derivatives of macropolyhedral boranesB20 were built and geometry-optimized beginning with substituting theorigin H atoms of B20H18 with OH, SH, O, S, NH3, ON and NO viacomputation at B3LYP/6-31G* level with density functional theory method.They include: B20H16(NH3)2 (1), [B20H17NH3]1-(2), [B20H17OH]2-(3),[B20H17NH3]1-(4), [B20H17NH3]1-(5), [B20H17NH3]3-(6), [B20H17OH]4-(7),[B20H16S]4-(8), [B20H16O]4-(9), [B20H17OH]4-(10), [B20H17NH3]3-(11),[B20H17OH]4-(12), [B20H17OH]2-(13), [B20H17O]3-(14), [B20H17S]3-(15),[B20H17SH]2-(16), [B20H17ON]2-(17), [B20H17NO]2-(18), [B20H18OH]3-(19),[B20H18OH]3-(20). Among them, the structures [B20H17NH3]1-(4),[B20H16S]4-(8), [B20H17S]3-(15), [B20H17SH]2-(16), [B20H17ON]2-(17),[B20H17NO]2-(18) were designed by us. The continued calculatedvibrational frequencies are all real, so all of these derivatives could be stablestructures. The substituent groups of these derivatives are examined withtheir special influences to the macropolyhedral boranes. These influencesare different depending on the joint motheds between the two "B10H9". Tocompounds (1) -(5), the two "B10H9" linging tight, there is little infection tomacropolyhedral boranes after the substitution. The effects tomacropolyhedral boranes by OH, O substituent groups are more clearly thanSH, S groups with the others substitution methods. Electron structures arealso discussed, and the HOMO, LUMO their gaps and active sites areperceived comparing the foremost compounds. Greatly difference occurwhen the substitution of terminal OH, NH3, S, O, SH, NO, ON.