Binuclear Lantern Complexes of All First-Row 3d Metals Scandium to Zinc: Metal-Metal Bond Lengths and Bond Orders, with Measures of Intrinsic Metal-Metal Bond Strength.

The B3LYP and M06-L functionals with the cc-pVTZ basis set are used to study lantern-type binuclear complexes of all the first-row (3d block) metals scandium to zinc in various low-energy spin states, out of which the ground states are predicted. These complexes are studied as models using mostly the unsubstituted formamidinate ligand. For each metal, metal-metal (MM) bond lengths are related to the formal MM bond orders (zero to five), derived by MO analysis and by electron counting. The predicted ground-state spin multiplicities and MM bond lengths of the model complexes generally agree fairly well with available experimental results on substituted analogues. Finally, values of the formal shortness ratio and Wiberg index for the MM bonds in all of these complexes in all spin states studied are categorized into ranges according to the MM bond orders 0 to 5 in steps of 0.5. The trends shown validate their use in estimating intrinsic metal-metal bond strength regardless of the metal.

Lantern-type dinickel complexes: An exploration of possibilities for nickel-nickel bonding with bridging bidentate ligands.

Many binuclear nickel complexes have NiNi distances suggesting NiNi covalent bonds, including lantern-type complexes with bridging bidentate ligands. This DFT study treats tetragonal, trigonal, and digonal lantern-type complexes with the formamidinate, guanidinate, and formate ligands, besides some others. Formal bond orders (ranging from zero to two) are assigned to all the NiNi bonds on the basis of MO occupancy considerations. A VB-based electron counting approach assigns plausible resonance structures to the dinickel cores. Model tetragonal complexes with the dimethylformamidinate and the dithioformate ligands have singlet ground states whose non-covalently bonded NiNi distances are close to those in their experimentally known counterparts. Trigonal dinickel complexes are unknown, but are predicted to have quartet ground states with NiNi bonds of order 0.5. The model digonal complexes are predicted to have triplet ground states, but the predicted NiNi bond lengths are longer than those found in their experimentally known counterparts. This could owe to inadequate treatment of electron correlation by DFT in these short NiNi bonds with their multiconfigurational character. All the NiNi bond distances here are categorized into ranges according to the NiNi bond orders of 0, 0.5, 1, 1.5, and 2, no NiNi bonds of order higher than two being identified. The NiNi bonds of given order in these lantern-type complexes are consistently shorter than the corresponding NiNi bonds in dinickel complexes having carbonyl ligands, attributable to the metalmetal bond lengthening effect of CO ligands.

Lantern-Type Divanadium Complexes with Bridging Ligands: Short Metal-Metal Bonds with High Multiple Bond Orders.

Vanadium forms binuclear complexes with a variety of ligands often containing V≡V triple bonds. Many tetragonal divanadium paddlewheel complexes with bridging bidentate ligands have been experimentally characterized. This research exhaustively treats model tetragonal, trigonal, and digonal paddlewheel-type divanadium complexes V2 Lx (L=formamidinate, guanidinate, and carboxylate; x=2, 3, 4), each in the three lowest-energy spin states. The V-V formal bond orders are obtained from metal-metal MO diagrams for representative structures. A number of short V-V multiple bonds of order 3, 3.5, and 4 are found in these model complexes. The short V≡V triple bonds and singlet ground state predicted here for the model tetragonal complexes correspond well with the limited experimental results for the series of known tetragonal paddlewheels. Digonal divanadium lanterns with very short V-V quadruple bonds are predicted as interesting synthetic targets. The V-V bond distances are categorized into distinct ranges according to the formal bond order values from 0.5 to 4. These bond length ranges are compared with the ranges compiled for other divanadium complexes including carbonyl complexes.

Stability, singlet-triplet splitting, and structure of didehydrogenated quinolines and isoquinolines.

Two DFT methods are used to study the stability, singlet-triplet splitting, and geometry of all didehydrogenated quinolines and isoquinolines in their lowest lying singlet and triplet states. Here, the relative stability within an isomeric series is differentiated from the biradical stabilization energy, which pertains to the propensity for radical abstraction reactions. Analysis of relative stability orders for these hetarynes points to the influence of the basic hetaryne type (ortho, meta, peri, para, and other), the effects of bond alternation in the bicycle, and the position of the ring nitrogen atom with respect to the nearest radical center. Singlet hetaryne stability tends to follow the order ortho > meta > (para and peri) and the reverse order for the triplet species. Among ortho-hetarynes, the singlet state is stabilized, and the triplet state destabilized when the hetaryne bond is shorter, and vice versa. The effects of the nitrogen atom on the relative stability depend upon (a) the hetaryne spin state, (b) whether the nitrogen is adjacent to or one atom removed from a radical center, and (c) the distance between the heteroatom and the nearest radical center for hetarynes with more widely separated radical centers. The singlet-triplet splitting is, in most cases, more dependent upon singlet stability than triplet stability. The biradical stabilization energy does not, in general, correlate with relative stability but furnishes predictions concerning capacity for reactions like hydrogen atom abstraction. Geometries of the singlet hetarynes (notably the ortho- and meta-hetarynes) present greater departures from the parent hetarene structure than do the triplet geometries.

Novel H-bonded base dimers as repeat units for information-bearing self-associative duplexes: a B3LYP/6-31G* search.

Density functional theory at the B3LYP/6-31G* level with counterpoise correction has been employed to study six sets of nitrogenous bases for the capacity of each to form H-bonded dimers restricted to a chosen pairing configuration. These results are augmented by MP2/6-311++G(d,p) single point calculations on the B3LYP/6-31G* optimized geometries. Each set has two bases, including substituted azoles, imidazoles, pyrimidines, and fused ring systems. This study aims to determine the suitability of each set to furnish H-bonded base pairs which may serve as repeat units for self-associative H-bonded macromolecular duplexes with the capacity to store and replicate information at the molecular level. Out of the various possibilities tested here, a set of two substituted pyrimidines best satisfies the prescribed criteria and may be put forward as a good candidate to yield isomorphic repeat units for designing such synthetic information-bearing macromolecular duplexes. The optimized configurations of these chosen base pairs as calculated at the B3LYP/6-31G* level compare well with those calculated at the B3LYP/6-31++G(d,p) and MP2/6-31G(d,p) levels, and indicate that isomorphism of the two base pairs is independent of method used. Assuming a one-to-one correspondence for encoding information in the macromolecule, such a set of two bases can allow the macromolecule to encode up to 8 types of encrypted species.