Trimetallic Chalcogenide Species: Synthesis, Structures, and Bonding.

In an attempt to isolate boron-containing tri-niobium polychalcogenide species, we have carried out prolonged thermolysis reactions of [Cp*NbCl4] (Cp* = ɳ5-C5Me5) with four equivalents of Li[BH2E3] (E = Se or S). In the case of the heavier chalcogen (Se), the reaction led to the isolation of the tri-niobium cubane-like cluster [(NbCp*)3(µ3-Se)3(BH)(µ-Se)3] (1) and the homocubane-like cluster [(NbCp*)3(µ3-Se)3(µ-Se)3(BH)(µ-Se)] (2). Interestingly, the tri-niobium framework of 1 stabilizes a selenaborate {Se3BH}- ligand. A selenium atom is further introduced between boron and one of the selenium atoms of 1 to yield cluster 2. On the other hand, the reaction with the sulfur-containing borate adduct [LiBH2S3] afforded the trimetallic clusters [(NbCp*)3(µ-S)4{µ-S2(BH)}] (3) and [(NbCp*)3(µ-S)4{µ-S2(S)}] (4). Both clusters 3 and 4 have an Nb3S6 core, which further stabilizes {BH} and mono-sulfur units, respectively, through bi-chalcogen coordination. All of these species were characterized by 11B{1H}, 1H, and 13C{1H} NMR spectroscopy, mass spectrometry, infrared (IR) spectroscopy, and single-crystal X-ray crystallography. Moreover, theoretical investigations revealed that the triangular Nb3 framework is aromatic in nature and plays a vital role in the stabilization of the borate, borane, and chalcogen units.

Hexagonal Planar [B6 H6 ] within a [B6 H12 ] Borate Complex: Structure and Bonding of [(Cp*Ti)2 (µ-ɳ6 : ɳ6 -B6 H6 )(µ-H)6 ].

Isolation of planar [B6 H6 ] is a long-awaited goal in boron chemistry. Several attempts in the past to stabilize [B6 H6 ] were unsuccessful due to the domination of polyhedral geometries. Herein, we report the synthesis of a triple-decker sandwich complex of titanium [(Cp*Ti)2 (µ-η6 : η6 -B6 H6 )(µ-H)6 ] (1), which features the first-ever experimentally achieved nearly planar six-membered [B6 H6 ] ring, albeit within a [B6 H12 ] borate. The small deviation from planarity is a direct consequence of the predicted structural pattern of the middle ring in 24 Valence Electron Count (VEC) triple-decker complexes. The large ring size of [B6 H6 ] in 1 brings the metal-metal distance into the bonding range. However, significant electron delocalization from the M-M bonding orbital to the bridging hydrogen and B-B skeleton in the middle decreases its bond strength.

Metal-Stabilized [B8 H8 ]2- Derivatives with Dodecahedral Structure in the Solid and Solution States: [(Cp2 MBH3 )2 B8 H6 ] (Cp=η5 -C5 H5 ; M=Zr (1-Zr) and Hf (1-Hf)).

Despite the synthesis and structural characterization of closo-hydroborate dianions, [Bn Hn ]2- (n=6-12) more than 50 years ago, some ambiguity remains about the structure of [B8 H8 ]2- . Although the solid-state structure of [B8 H8 ]2- was established by single-crystal X-ray studies in 1969, fast rearrangements in solution at accessible temperatures prevented its detailed characterization. We therefore stabilized a derivative of [B8 H8 ]2- by using Cp2 MBH3 and structurally characterized two new octaborane analogues, [(Cp2 MBH3 )2 B8 H6 ] (Cp=η5 -C5 H5 ; M=Zr (1-Zr) and Hf (1-Hf)), so that the dynamics of the B8 skeleton were arrested. The solid-state structures of both 1-Zr and 1-Hf comprise a dodecahedron core protected by {Cp2 MBH3 } moieties on both sides of the cluster. Spectroscopic characterization (11 B NMR) validates the intactness of the B8 dodecahedron core in solution as well. Theoretical calculations establish that the two exo-{Cp2 MBH3 } fragments provide structural and electronic structural stability to the B8 core and its intact dodecahedral dianionic nature. Furthermore, we propose isodesmic equations for the formation of higher analogues of the Bn core (n>8) guarded by different group 4 transition metals. Our analysis suggests that, as we move to higher polyhedra (n>10), the formation becomes unfavourable irrespective of metal.

Chemistry of Dimetallaoctaborane(12) with Chalcogen-Based Borate Ligands: Obedient versus Disobedient Clusters.

The reactions of dimetallaoctaboranes(12) [(Cp*M)2B6H10] [M = Co (1) or Rh (2); Cp* = η5-C5Me5] with different chalcogen sources, such as Li[BH2E3] and Li[BH3EPh] (E = S, Se, or Te), led to two unique reaction outcomes. For example, the formation of 10-vertex nido-[(Cp*M)2B6E2H6] (3, M = Co, E = S; 4, M = Co, E = Se; 5, M = Co, E = Te; 6, M = Rh, E = Se) from compounds 1 and 2 is a typical representation of a cluster growth reaction, while the formation of arachno-[(Cp*Co)2B6H9(EPh)] [E = S (9), Se (10), or Te (11)] is a rare method that yielded arachno clusters, keeping the core geometry identical. The formation of arachno-9-11 is a unique method that converts disobedient cluster 1 to obedient clusters 9-11. Further, the reactivity of nido-4 with various metal carbonyls presented sequential cluster growth reactions, which afforded 11-vertex nido-[(Cp*Co)2B6Se2H6{Fe(CO)3}] (7) and 13-vertex fused closo-[(Cp*Co)2B6Se2H6{Ru3(CO)8}] (8). The core geometry of nido-7 is uncommon and very similar to that of [C2B9H11]2- with a unique open pentahapto-coordinating five-membered face.

Cluster Fusion: Face-Fused Macropolyhedral Tetracobaltaboranes.

In an effort to isolate the 16-vertex supraicosahedral cobaltaborane [(Cp*Co)3B12H12Co{Cp*CoB4H9}] (Cp* = η5-C5Me5), we have pyrolyzed an in situ generated intermediate, obtained from the fast metathesis of [Cp*CoCl]2 and [LiBH4·THF], with an excess amount of [BH3·THF]. Although the objective of isolating the 16-vertex cobalt analogue was not achieved, the reaction yielded a closo-19-vertex face-fused cluster presenting icosahedral {Co3B9}, tetrahedral {B4}, and 10-vertex {CoB9} units. The reaction also yielded a 20-vertex face-fused cluster that contains icosahedral {Co4B8}, square-pyramidal {CoB4}, tetrahedral {Co2B2}, and nido-{CoB7} units.

Trithia-diborinane and Bis(bridging-boryl) Complexes of Ruthenium Derived from a [BH3(SCHS)]- Ion.

The field of diborinane is sparsely explored area, and not many compounds are structurally characterized. The room-temperature reaction of [{Cp*RuCl(µ-Cl)}2] (Cp* = η5-C5Me5) with Na[BH3(SCHS)] yielded ruthenium dithioformato [{Cp*Ru(µ,η3-SCHS)}2], 1, and 1-thioformyl-2,6-tetrahydro-1,3,5-trithia-2,6-diborinane complex, [(Cp*Ru){(η2-SCHS)CH2S2(BH2)2}], 2. To investigate the reaction pathway for the formation of 2, we carried out the reaction of [(BH2)4(CH2S2)2], 3, with 1 that yielded compound 2. To the best of our knowledge, it appears that compound 2 is the first example of a ruthenium diborinane complex where the central six-membered ring [CB2S3] adopts the chair conformation. Furthermore, room temperature reaction of 1 with [BH3·thf] resulted in the isolation of agostic-bis(σ-borate) complex, [Cp*Ru(µ-H)2BH(S-CH═S)], 4. Thermolysis of 4 with trace amount of tellurium powder led to formation of bis(bridging-boryl) complex, [{Cp*Ru(µ,η2-HBS2CH2)}2], 5, via dimerization of 4 followed by dehydrogenation. Compound 5 can be considered as a bis(bridging-boryl) species, in which the boryl units are connected to two ruthenium atoms. Theoretical studies and chemical bonding analyses demonstrate the reason for exceptional reactivity and stability of these complexes.

Stabilization of Classical [B2 H5 ]- : Structure and Bonding of [(Cp*Ta)2 (B2 H5 )(µ-H)L2 ] (Cp*=η5 -C5 Me5 ; L=SCH2 S).

The room-temperature reaction of [Cp*TaCl4 ] with LiBH4 ⋅THF followed by addition of S2 CPPh3 results in pentahydridodiborate species [(Cp*Ta)2 (µ,η2 :η2 -B2 H5 )(µ-H)(κ2 ,µ-S2 CH2 )2 ] (1), a classical [B2 H5 ]- ion stabilized by the binuclear tantalum template. Theoretical studies and bonding analysis established that the unusual stability of [B2 H5 ]- in 1 is mainly due to the stabilization of sp2 -B center by electron donation from tantalum. Reactions to replace the hydrogens attached to the diborane moiety in 1 with a 2 e {M(CO)4 } fragment (M=Mo or W) resulted in simple adducts, [{(Cp*Ta)(CH2 S2 )}2 (B2 H5 )(H){M(CO)3 }] (6: M=Mo and 7: M=W), that retained the diborane(5) unit.

Trimetallic Cubane-Type Clusters: Transition-Metal Variation as a Probe of the Roots of Hypoelectronic Metallaheteroboranes.

In an effort to synthesize chalcogen-rich metallaheteroborane clusters of group 5 metals, thermolysis of [Cp*TaCl4] (Cp* = η5-C5Me5) with thioborate ligand Li[BH2S3] was carried out, affording trimetallic clusters [(Cp*Ta)3(µ-S)3(µ3-S)3B(R)], 1-3 (1, R = H; 2, R = SH; and 3, R = Cl). Clusters 1-3 are illustrative examples of cubane-type organotantalum sulfido clusters in which one of the vertices of the cubane is missing. In parallel to the formation of 1-3, the reaction also yielded tetrametallic sulfido cluster [(Cp*Ta)4(µ-S)6(µ3-S)(µ4-O)], 6, having an adamantane core structure. Compound 6 is one of the rarest examples containing the µ4-oxo unit with a heavier early transition metal, i.e., tantalum. In an effort to isolate selenium analogues of clusters 1-3, we have isolated the trimetallic cluster [(Cp*Ta)3(µ-Se)3(µ3-Se)3B(H)], 4, from the thermolytic reaction of [Cp*TaCl4] and Li[BH2Se3]. In contrast, the thermolysis of [Cp*TaCl4] with Li[BH2Te3] under the same reaction conditions yielded tantalum telluride complex [(Cp*Ta)2(µ-Te)2], 5. Compounds 1-4 are hypo-electronic clusters with an electron count of 50 cluster valence electrons. All these compounds have been characterized by 1H, 11B{1H}, and 13C{1H} NMR spectroscopy; infrared spectroscopy; mass spectrometry; and single-crystal X-ray crystallography. The density functional theory calculations were also carried out to provide insight into the bonding and electronic structures of these molecules.

Synthesis, Chemistry, and Electronic Structures of Group 9 Metallaboranes.

Dimetallaoctaborane(12) of Ru, Co, and Rh have been well-characterized by a range of spectroscopic techniques and X-ray diffraction studies. Thus, reinvestigation of the Ir-system became of interest. As a result, a slight modification in the reaction conditions enabled us to isolate the missing Ir analogue of octaborane(12), [(Cp*Ir)2B6H10], 1. Compound 1 adapts a geometry similar to that of its parent octaborane(12) and Ru, Co, and Rh analogues. In [M2B6H10+x](M = Ru, x = 2; M = Co and Rh, x = 0), there exist two M-H-B protons. However, a significant difference observed in [(Cp*Ir)2B6H10] is the presence of two Ir-H instead of Ir-H-B protons that eventually controls the reactivity of this molecule. For example, unlike [M2B6H10](M = Co or Rh), the Ir-analogue does not react with metal carbonyl compounds or [Au(PPh3)Cl]. Along with 1, a closo trimetallic 8-vertex iridaborane [(Cp*Ir)3B5H4Cl], 2 was also isolated. Additionally, from another reaction, 12-vertex closo iridaboranes [(Cp*Ir)2B10Hy(OH)z], 3a and 3b (3a: y = 12, z = 0; 3b: y = 8, z = 2), have also been isolated. Further, density functional theory calculations were performed to gain useful insight into the structure and stability of these compounds.

Rule breaker boron clusters: a new class of hypoelectronic osmaborane clusters [(Cp*Os)2BnHn] (n = 6-10).

Since the pioneering electron counting rule for borane clusters was proposed by Wade, the structures and bonding of boron clusters and their derivatives have been elegantly rationalised. However, this rule and its modified versions faced problems explaining the electronic structures of less spherical deltahedra, unlike the core geometries of borate dianions [BnHn]2- (n = 6-12). Herein, we report the isolation of a series of osmaborane clusters [(Cp*Os)2BnHn], 1-5, (n = 6-10) by the thermolysis of an in situ generated intermediate, obtained from the rapid condensation of [Cp*OsBr2]2 and [LiBH4·THF], with [BH3·THF] or [BH3·SMe2]. Interestingly, all these clusters show unusual less spherical deltahedral shapes that can be generated from canonical [BnHn]2- (n = 8-12) shapes by doing diamond-square-diamond (DSD) rearrangements. The DSD rearrangements led to the generation of higher degree vertices, which are occupied by Os atoms and also generated Os-Os bonds in these clusters. Theoretical calculations revealed that these Cp*Os⋯OsCp* interactions in clusters 1-5 played a crucial role in their structural shape and electron count. These less spherical deltahedral clusters are rare, and most significantly, clusters 1-5 with (n-1) skeleton electron pairs (SEPs) do not follow Wade-Mingos electron counting rules and can be classified as hypoelectronic closo clusters.

Transition-metal-like coordination chemistry of dicoordinate borylenes with organic azides.

The photolytic or oxidative liberation of a cyclic (amino)(alkyl)carbene (CAAC)-stabilized arylborylene in the presence of organoazides yielded borylene-organoazide complexes (4a,b) has been achieved in a manner akin to the first step of the Staudinger reaction. Similarly, a CAAC-stabilized aminoborylene also afforded borylene-organoazide complexes (6a-c), which further undergo rearrangement to produce aminoborane triazene species (7a,b).

16-Vertex oblato-hypho-titanaborane [(Cp*Ti)2B14H18].

Although Lipscomb predicted in 1977 that supra-icosahedral boron clusters would be viable, their synthesis has been impeded by the unavailability of appropriate synthetic methodologies. Herein, we report the first examples of the open 16-vertex oblato-hypho-titanaborane clusters [(Cp*Ti)2B14H17R] (1: R = H; 2: R = Me) having a non-Wadean 19-skeletal-electron-pair count. Interestingly, these clusters show a six-membered [Ti2B4] open face, which could lead to closo-19-vertex clusters.

Heterometallic Triply-Bridging Bis-Borylene Complexes.

Triply-bridging bis-{hydrido(borylene)} and bis-borylene species of groups 6, 8 and 9 transition metals are reported. Mild thermolysis of [Fe2 (CO)9 ] with an in situ produced intermediate, generated from the low-temperature reaction of [Cp*WCl4 ] (Cp*=η5 -C5 Me5 ) and [LiBH4 ⋅THF] afforded triply-bridging bis-{hydrido(borylene)}, [(µ3 -BH)2 H2 {Cp*W(CO)2 }2 {Fe(CO)2 }] (1) and bis-borylene, [(µ3 -BH)2 {Cp*W(CO)2 }2 {Fe(CO)3 }] (2). The chemical bonding analyses of 1 show that the B-H interactions in bis-{hydrido (borylene)} species is stronger as compared to the M-H ones. Frontier molecular orbital analysis shows a significantly larger energy gap between the HOMO-LUMO for 2 as compared to 1. In an attempt to synthesize the ruthenium analogue of 1, a similar reaction has been performed with [Ru3 (CO)12 ]. Although we failed to get the bis-{hydrido(borylene)} species, the reaction afforded triply-bridging bis-borylene species [(µ3 -BH)2 {WCp*(CO)2 }2 {Ru(CO)3 }] (2'), an analogue of 2. In search for the isolation of bridging bis-borylene species of Rh, we have treated [Co2 (CO)8 ] with nido-[(RhCp*)2 (B3 H7 )], which afforded triply-bridging bis-borylene species [(µ3 -BH)2 (RhCp*)2 Co2 (CO)4 (µ-CO)] (3). All the compounds have been characterized by means of single-crystal X-ray diffraction study; 1 H, 11 B, 13 C NMR spectroscopy; IR spectroscopy and mass spectrometry.

Chalcogen stabilized trimetallic clusters: synthesis, structures, and bonding of [(Cp*M)3(E)6+m(BH)n] (M = Nb or Ta; E = S or Se; m = 0 or 1 or 2; n = 0 or 1).

In an effort to isolate the chalcogen-rich niobium analogue of [(Cp*Ta)3(µ-S)3(µ3-S)3BH], the room temperature reaction of [Cp*NbCl4] (Cp* = η5-C5Me5) with Li[BH2S3] was carried out. Although the objective of isolating the niobium analogue was not achieved, the reaction yielded a homocubane-type cluster [(Cp*Nb)3(µ-S)3(µ3-S)3(µ-S)BH], 1, and a hexa-sulfido cluster [(Cp*Nb)3(µ-S)6], 2. Cluster 1 is a notable example of a homocubane-type cluster in which one of the vertices of the homocubane is missing. Compound 1 may be considered as a hypo-electronic cluster with an electron count of 64 cve (cve = cluster valence electrons), whereas compound 2 shows the presence of two doubly bridging η1-S around each Nb-Nb bond. On the other hand, the room temperature reaction of [Cp*TaCl4] with selenaborate ligand, [LiBH2Se3], led to the formation of [(Cp*Ta)3(µ-Se)4{µ-Se2(Se2)}], 3. Compound 3 is one of the rarest examples having a Ta3Se6 core structure with a unique diselenide bridging fragment. The presence of a short Se-Se bond of this diselenide unit makes this molecule of further interest. All these compounds were characterized by 1H, 11B{1H} and 13C{1H} NMR spectroscopy, infrared spectroscopy, mass spectrometry, and single-crystal X-ray crystallography. Density functional theory (DFT) calculations were carried out to provide insight into the bonding and electronic structures of these chalcogen-rich trimetallic clusters.

[(Cp2M)2B9H11] (M = Zr or Hf): early transition metal 'guarded' heptaborane with strong covalent and electrostatic bonding.

Among the series of stable closo-borate dianions, [B n H n ]2-, the X-ray crystallographic structure of [B7H7]2- was determined only in 2011. To explore its chemistry and stability, we have isolated and structurally characterized two new transition metal complexes of the heptaborane, [(Cp2M)2B9H11] (Cp = η5-C5H5; M = Zr or Hf). The structures of [(Cp2M)2B9H11] contain a pentagonal bipyramidal B7 core, coordinated by two {Cp2M} and two {BH2} units equatorially. Structural and spectroscopic characterizations and DFT calculations show that [(Cp2M)2B9H11] complexes are substantially more stable than the parent dianion, in either [B7H7]2- or ( n Bu4N)2[B7H7]. Our theoretical study and chemical bonding analyses reveal that the surprising stability of the two new heptaborane metal complexes is due to multi-center covalent bonds related to the two exo-{Cp2M} units, as well as electrostatic interactions between the {Cp2M} units and the B7 core. The facile syntheses of the heptaborane metal-complexes will allow further exploration of their chemistry.

Intravascular magnetic resonance/radiofrequency may enhance gene therapy for prevention of in-stent neointimal hyperplasia.

RATIONALE AND OBJECTIVES: We evaluated the potential of using intravascular magnetic resonance (MR)/radiofrequency (RF) to enhance vascular endothelial growth factor (VEGF) gene therapy of in-stent neointimal hyperplasia. MATERIALS AND METHODS: By using a catheter-based approach, VEGF/lentivirus was locally transferred into 10 (five paired) bilateral femoral-iliac arteries of five hypercholesterolemic pigs, whereas the right arteries were heated up to approximately 41 degrees C by using an intravascular MR/RF system. Then, identical stents were placed immediately into the bilateral VEGF-targeted arteries to create in-stent neointimal hyperplasia. At day 60 after gene/stent interventions, the targeted arteries were harvested for histological correlation. RESULTS: X-Ray angiography-detectable in-stent stenoses were found in three of the arteries treated with VEGF genes only, whereas there were no in-stent stenoses in arteries treated by using MR/RF-heated VEGF genes. Correlative histological examination confirmed a 138% reduction in average thickness of neointimal hyperplasia in VEGF/RF-treated arteries compared with VEGF-only-treated arteries (P < .01). CONCLUSION: We report a potential method of using an intravascular MR/RF heating technique to enhance gene therapy of in-stent restenosis.

In vivo MR imaging of bone marrow cells trafficking to atherosclerotic plaques.

PURPOSE: To develop a magnetic resonance imaging (MRI)-based method to monitor in vivo trafficking of bone marrow (BM) cells to atherosclerotic lesions. MATERIALS AND METHODS: BM cells from LacZ-transgenic mice were labeled with a superparamagnetic iron oxide (Feridex) and then transplanted into ApoE(-/-) recipient mice that were fed an atherogenic diet. Twenty-four ApoE(-/-) mice were divided into three study groups: 1) group I with Feridex-labeled BM transplantation (BMT) cells (N = 9), 2) group II with unlabeled BMT cells (N = 10), and 3) group III with no BMT cells (N = 5). Migrated Feridex/LacZ-BM cells to atherosclerotic aortic walls were monitored in vivo using a 4.7T MR scanner and correlated with histopathological findings. RESULTS: In group I with Feridex-BMT cells, histology examination displayed plaques in five of nine animals. In four of these five animals, in vivo MRI showed large MR signal voids of the aorta walls (due to the "blooming" effect of migrated Feridex-BM cells in plaques), which were correlated with Feridex- and/or LacZ-positive cells detected in the atherosclerotic lesions. No signal voids could be visualized in the two control animal groups (groups II and III). CONCLUSION: This study demonstrates the potential use of in vivo MRI to monitor the trafficking of magnetically labeled BM cells to atherosclerotic lesions.

MRI of intravenously injected bone marrow cells homing to the site of injured arteries.

The aim of this study was to test the feasibility of using MRI to detect magnetically labeled, intravenously injected bone marrow (BM) cells homing to injured arteries. In the first phase, BM cells from LacZ-transgenic or green fluorescent protein (GFP)-transgenic mice were transplanted into eight recipient mice. The left femoral arteries of recipient mice were injured using a cuff-constriction or endothelium-damage approach, and the right femoral arteries were uninjured to serve as controls. The location and distribution of migrated LacZ-BM or GFP-BM cells were confirmed with histology. In the second phase, BM-derived cells from LacZ-transgenic mice were labeled with superparamagnetic iron oxide (Feridex) and then transplanted into eight recipient mice with cuff-induced injuries in the left femoral arteries. Migrated Feridex/LacZ-BM cells were monitored in vivo using a 4.7 T MR scanner. Subsequently, high-resolution ex vivo MRI was performed on 9.4 T and 11.7 T. LacZ-positive or GFP-positive cells in the thickened adventitia of the injured arteries were evident on histology. Both in vivo and ex vivo MRI showed larger regions of hypointensity with Feridex-labeled cells at the sites of the injured arteries compared with control arteries (P < 0.01). This study provides initial evidence that may support the potential use of MRI to detect homing of intravenously injected BM cells to injured arteries.

Development of a 0.014-inch magnetic resonance imaging guidewire.

The purpose of this study was to develop a standard 0.014-inch intravascular magnetic resonance imaging guidewire (MRIG), a coaxial cable with an extension of the inner conductor, specifically designed for use in the small vessels. After a theoretical analysis, the 0.014-inch MRIG was built by plating/cladding highly electrically conductive materials, silver or gold, over the inside and outside of the coaxial conductors. The conductors were made of superelastic, nonmagnetic, biocompatible materials, Nitinol or MP35N. Then, in comparison with a previously designed 0.032-inch MRIG, the performance of the new 0.014-inch MRIG in vitro and in vivo was successfully evaluated. This study represents the initial work to confirm the critical role of highly conductive and superelastic materials in building such small-size MRIGs, which are expected to generate high-resolution MR imaging of vessel walls/plaques and guide endovascular interventional procedures in the small vessels, such as the coronary arteries.