Pathways to the Polymerization of Boron Monoxide Dimer To Give Low-Density Porous Materials Containing Six-Membered Boroxine Rings

Density functional theory has been used to examine the key mechanistic details of the polymerization of boron monoxide (BO) via its OB–BO dimer to give ultimately low-density porous polymeric (BO)_<i>n</i> materials. The structures of such materials consist of planar layers of six-membered boroxine (B₃O₃) rings linked by boron–boron bonds. Initial cyclooligomerization of B₂O₂ leads to a B₄O₄ dimer with a four-membered B₂O₂ ring, a B₆O₆ trimer containing a six-membered B₃O₃ (boroxine) ring, a B₈O₈ tetramer containing an eight-membered B₄O₄ ring, and even a B₁₀O₁₀ pentamer containing a ten-membered B₅O₅ ring. However, an isomeric B₁₀O₁₀ structure containing two boroxine rings linked by a B–B bond is a much lower energy structure by ∼31 kcal/mol owing to the special stability of the aromatic boroxine rings. Rotation of the boroxine rings around the central B–B bond in this B₁₀O₁₀ structure has a low rotation barrier suggesting that further oligomerization to give products containing either perpendicular or planar orientations of the B₃O₃ rings is possible. However, the planar oligomers are energetically more favorable since they have fewer high-energy external BO groups bonded to the network of boroxine rings. The pendant boronyl groups are reactive sites that can be used for further polymerization. Mechanistic aspects of the further oligomerization of (BO)_<i>x</i> systems to give a B₂₄O₂₄ oligomer with a naphthalene-like arrangement of boroxine rings and a B₈₄O₈₄ structure with a coronene-like arrangement of boroxine rings have been examined. Further polymerization of these intermediates by similar processes is predicted to lead ultimately to polymers consisting of planar networks of boroxine rings. The holes between the boroxine rings in such polymers suggests that they will be porous low-density materials. Applications of such materials as absorbents for small molecules are anticipated

The Quest for Metal–Metal Quadruple and Quintuple Bonds in Metal Carbonyl Derivatives: Nb₂(CO)₉ and Nb₂(CO)₈

The synthesis by Power and co-workers of the first metal–metal quintuple bond (<i>Science</i> <b>2005</b>, <i>310</i>, 844) is a landmark in inorganic chemistry. The 18-electron rule suggests that Nb₂(CO)₉ and Nb₂(CO)₈ are candidates for binary metal carbonyls containing metal–metal quadruple and quintuple bonds, respectively. Density functional theory (MPW1PW91 and BP86) indeed predicts structures having very short Nb–Nb distances of ∼2.5 Å for Nb₂(CO)₉ and ∼2.4 Å for Nb₂(CO)₈ as well as relatively large Nb–Nb Wiberg bond indices supporting these high formal Nb–Nb bond orders. However, analysis of the frontier molecular orbitals of these unbridged structures suggests formal NbNb triple bonds and 16-electron metal configurations. This contrasts with an analysis of the frontier orbitals in a model chromium­(I) alkyl linear CH₃CrCrCH₃, which confirms the generally accepted presence of chromium–chromium quintuple bonds in such molecules. The presence of NbNb triple bonds rather than quadruple or quintuple bonds in the Nb₂(CO)_<i>n</i> (<i>n</i> = 9, 8) structures frees up d­(<i>xy</i>) and d­(<i>x</i>²–<i>y</i>²) orbitals for dπ→pπ* back-bonding to the carbonyl groups. The lowest energy Nb₂(CO)_<i>n</i> structures (<i>n</i> = 9, 8) are not these unbridged structures but structures having bridging carbonyl groups of various types and formal Nb–Nb orders no higher than three. Thus, the two lowest energy Nb₂(CO)₉ structures have NbNb triple bond distances of ∼2.8 Å and three semibridging carbonyl groups, leading to a 16-electron configuration rather than an 18-electron configuration for one of the niobium atoms. The lowest energy structure of the highly unsaturated Nb₂(CO)₈ is unusual since it has a formal <i>single</i> Nb–Nb bond of length ∼3.1 Å and two four-electron donor η²-μ-CO groups, thereby giving each niobium atom only a 16-electron configuration

Construction of the Tetrahedral Trifluorophosphine Platinum Cluster Pt₄(PF₃)₈ from Smaller Building Blocks

The experimentally known but structurally uncharacterized Pt₄(PF₃)₈ is predicted to have an <i>S</i>₄ structure with a central distorted Pt₄ tetrahedron having four short PtPt distances, two long Pt–Pt distances, and all terminal PF₃ groups. The structures of the lower nuclearity species Pt­(PF₃)_<i>n</i> (<i>n</i> = 4, 3, 2), Pt₂(PF₃)_<i>n</i> (<i>n</i> = 7, 6, 5, 4), and Pt₃(PF₃)₆ were investigated by density functional theory to assess their possible roles as intermediates in the formation of Pt₄(PF₃)₈ by the pyrolysis of Pt­(PF₃)₄. The expected tetrahedral, trigonal planar, and linear structures are found for Pt­(PF₃)₄, Pt­(PF₃)₃, and Pt­(PF₃)₂, respectively. However, the dicoordinate Pt­(PF₃)₂ structure is bent from the ideal 180° linear structure to approximately 160°. Most of the low-energy binuclear Pt₂(PF₃)_<i>n</i> (<i>n</i> = 7, 6, 5) structures can be derived from the mononuclear Pt­(PF₃)_<i>n</i> (<i>n</i> = 4, 3, 2) structures by replacing one of the PF₃ groups by a Pt­(PF₃)₄ or Pt­(PF₃)₃ ligand. In some of these binuclear structures one of the PF₃ groups on the Pt­(PF₃)_<i>n</i> ligand becomes a bridging group. The low-energy binuclear structures also include symmetrical [Pt­(PF₃)_<i>n</i>]₂ dimers (<i>n</i> = 2, 3) of the coordinately unsaturated Pt­(PF₃)_<i>n</i> (<i>n</i> = 3, 2). The four low-energy structures for the trinuclear Pt₃(PF₃)₆ include two structures with central equilateral Pt₃ triangles and two structures with isosceles Pt₃ triangles and various arrangements of terminal and bridging PF₃ groups. Among these four structures the lowest-energy Pt₃(PF₃)₆ structure has an unprecedented four-electron donor η²-μ₃-PF₃ group bridging the central Pt₃ triangle through three Pt–P bonds and one Pt–F bond. Thermochemical studies on the aggregation of these Pt-PF₃ complexes suggest the tetramerization of Pt­(PF₃)₂ to Pt₄(PF₃)₈ to be highly exothermic regardless of the mechanistic details

Cyclization of Thiocarbonyl Groups in Binuclear Homoleptic Nickel Thiocarbonyls To Give Ligands Derived from Sulfur Analogues of Croconic and Rhodizonic Acids

The sulfur analogue of the well-known Ni­(CO)₄, namely, Ni­(CS)₄, has been observed spectroscopically in low temperature matrices but is not known as a stable species under ambient conditions. Theoretical studies show that Ni­(CS)₄ with monomeric CS ligands and tetrahedrally coordinated nickel is disfavored by ∼17 kcal/mol relative to unusual isomeric Ni­(C₂S₂)₂ structures. In the latter structures the CS ligands couple pairwise through C–C bond formation to give dimeric SCCS ligands, which bond preferentially to the nickel atom through their CS bonds rather than their CC bonds. Coupling of CS ligands in the lowest energy binuclear Ni₂(CS)_<i>n</i> (<i>n</i> = 7, 6, 5) structures results in cyclization to give remarkable C_<i>n</i>S_<i>n</i> (<i>n</i> = 5, 6) ligands containing five- and six-membered carbocyclic rings. Such ligands, which are the sulfur analogues of the well-known croconate (<i>n</i> = 5) and rhodizonate (<i>n</i> = 6) oxocarbon ligands, function as bidentate ligands to the central Ni₂ unit. Higher energy Ni₂(CS)_<i>n</i> (<i>n</i> = 7, 6, 5) structures contain dimeric C₂S₂ ligands, which can bridge the central Ni₂ unit. Dimeric C₂S₂ ligands rather than tetrathiosquare C₄S₄ ligands are found in the lowest energy Ni₂(CS)₄ structures

Binuclear Cyclopentadienylmetal Methylene Sulfur Dioxide Complexes of Rhodium and Iridium Related to a Photochromic Metal Dithionite Complex

The photochromic dithionite complex Cp*₂Rh₂(μ-CH₂)₂(μ-O₂SSO₂) (Cp* = η⁵-Me₅C₅) is of interest because it undergoes an unusual fully reversible unimolecular photochemical rearrangement to the isodithionite complex Cp*₂Rh₂(μ-CH₂)₂(μ-O₂SOSO). In order to obtain more insight into these systems, a comprehensive density functional theory study has been carried out on isomeric Cp₂M₂(CH₂)₂(SO₂)₂ (M = Rh, Ir) derivatives. The experimentally observed rhodium complexes with coupled sulfur dioxide (SO₂) units to give dithionite or isodithionite ligands are surprisingly high-energy kinetic isomers in our analysis, reflecting the need for dithionite rather than SO₂ for their synthesis. Many isomeric structures containing two separate SO₂ ligands are found to lie at lower energies than these dithionite and isodithionite complexes. In the lowest-energy Cp₂M₂(CH₂)₂(SO₂)₂ isomers, the two methylene groups couple to form an ethylene ligand that can be either terminal or bridging. In slightly higher energy structures, a formal hydrogen shift is predicted to occur within the ethylene ligand to give a methylcarbene CH₃CH ligand. Isomers with a bridging methylcarbene ligand are energetically preferred over isomers with a terminal methylcarbene ligand. Generation of the lower-energy Cp₂Rh₂(CH₂)₂(SO₂)₂ isomers containing separate SO₂ ligands should be achievable through reactions of SO₂ with more highly reduced cyclopentadienylrhodium methylene complexes such as Cp*₂Rh₂(μ-CH₂)₂