Qiong Luo

Bonding of Seven Carbonyl Groups to a Single Metal Atom: Theoretical Study of M(CO)n (M = Ti, Zr, Hf; n = 7, 6, 5, 4)

The equilibrium geometries, thermochemistry, and vibrational frequencies of the homoleptic metal-carbonyls of the group 4 elements, M(CO)n (M = Ti, Zr, Hf; n = 7, 6, 5, 4) were predicted using density functional theory. Analogous M(CO)n structures were found for all three metals. The global minima for the 18-electron M(CO)7 molecules are all singlet C(3v) capped octahedra. The global minima for the 16-electron M(CO)6 species are triplet M(CO)6 structures distorted from O(h) symmetry to D(3d) symmetry. However, the corresponding singlet M(CO)6 structures lie within 5 kcal/mol of the triplet global minima. The global minima for M(CO)n (n = 5, 4) are triplet structures derived from the D(3d) distorted octahedral structures of M(CO)6 by removal of one or two CO groups, respectively. Quintet D(3h) trigonal bipyramidal structures for M(CO)5 and singlet T(d) tetrahedral structures for M(CO)4 are also found, as well as higher energy structures for M(CO)6 and M(CO)7 containing a unique CO group bonded to the metal atom through both M-C and M-O bonds. The dissociation energies M(CO)7 --> M(CO)6 + CO are substantial, indicating no fundamental problem in bonding seven CO groups to a single metal atom.

Electron affinities of the radicals derived from cytosine

Theoretical studies have been carried out to investigate the electron affinities (EAs) of five radicals derived from cytosine by removing a hydrogen atom (C(-H)*), using carefully calibrated computational methods. Significant structural differences are predicted between cytosine, its five radicals and the five corresponding anions. The largest bond distance differences between the radical and its analogous anion are 0.05 angstroms. The theoretical EAs (predicted using carefully calibrated methods) for the five cytosine radicals range from 2.22 eV to 3.00 eV. Among these the N-centred radical 2 has the largest adiabatic electron affinity (EA(ad) = 3.00 eV), and the carbon-centred radical 8 has the smallest (2.22 eV). These values are much larger than EA(ad) for the neutral closed-shell cytosine molecule. The largest EA(ad) value predicted for the cytosine radical isomers (C(-H)*) is comparable to those for radicals related to other nucleoside bases. The ordering for the four DNA bases is EA(T(-H)*) = 3.46 > EA(A)-(-H)*) = 3.26 > EA(C(-H)*)= 3.00 > EA(G(-H)*) = 2.99 eV.

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 (Science2005, 310, 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)n (n = 9, 8) structures frees up d(xy) and d(x(2)-y(2)) orbitals for dπ→pπ* back-bonding to the carbonyl groups. The lowest energy Nb2(CO)n structures (n = 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 single Nb-Nb bond of length ∼3.1 Å and two four-electron donor η(2)-μ-CO groups, thereby giving each niobium atom only a 16-electron configuration.

Distinct Stepwise Reduction of a NickelNickel‐Bonded Compound Containing an α‐Diimine Ligand: From Perpendicular to Coaxial Structures

A nickel-nickel-bonded complex, [{Ni(μ-L(.-))}2] (1; L=[(2,6-iPr2C6H3)NC(Me)]2), was synthesized from reduction of the LNiBr2 precursor by sodium metal. Further controllable reduction of 1 with 1.0, 2.0 and 3.0 equiv of Na, respectively, afforded the singly, doubly, and triply reduced compounds [Na(DME)3]·[{Ni(μ-L(.-))}2] (2; DME=1,2-dimethoxyethane), [Na(Et2O)]Na[(L(.-))Ni-NiL(2-)] (3), and [Na(Et2O)]2Na[L(2-)Ni-NiL(2-)] (4). Here L represents the neutral ligand, L(.-) denotes its radical monoanion, and L(2-) is the dianion. All of the four compounds feature a short Ni-Ni bond from 2.2957(6) to 2.4649(8) Å. Interestingly, they display two different structures: the perpendicular (1 and 2) and the coaxial (3 and 4) structure, in which the metal-metal bond axis is perpendicular to or collinear with the axes of the α-diimine ligands, respectively. The electronic structures, Ni-Ni bonding nature, and energetic comparisons of the two structure types were investigated by DFT computations.

Carbonyl migration from phosphorus to the metal in binuclear phosphaketenyl metal carbonyl complexes to give bridging diphosphido complexes

Alkali metal salts of the 2-phosphaethynolate anion PCO− synthesized from reactions of CO with NaPH2 or K3P7 have recently become available in quantities for the synthesis of transition metal complexes of the potentially ambidentate PCO ligand (Angew. Chem., Int. Ed., 2013, 38, 10064). This is exemplified by the recently reported rhenium carbonyl complex (triphos)Re(CO)2(PCO) (triphos = MeP(CH2PPh2)3). Density functional theory studies on the related manganese carbonyl complexes Mn(CO)n(PCO) (n = 5, 4, 3) and Mn2(CO)n(PCO)2 (n = 8, 7, 6, 5) are now reported. For the binuclear systems the low-energy Mn2(CO)8(PCO)2 structures are singlet spin state structures having two bridging P-bonded phosphaketenyl μ-PCO ligands without a direct Mn–Mn bond. Carbonyl loss from Mn2(CO)8(μ-PCO)2 is predicted to lead to migration of CO groups from phosphorus to manganese resulting in Mn2(CO)n+2(μ-P2) structures with bridging diphosphido groups as the lowest energy Mn2(CO)n(PCO)2 isomers (n = 7, 6, 5). Isomeric Mn2(CO)6(PCO)2 structures with dihapto bridging η2-μ-PCO ligands at ∼30 kcal mol−1 above the global minimum are also found representing intermediates in the migration of CO groups from phosphorus to manganese. For the mononuclear systems the P-bonded Mn(CO)n(PCO) (n = 5, 4) phosphaketenyl structures are found to lie 20 to 28 kcal mol−1 in energy below the isomeric O-bonded Mn(CO)n(PCO) phosphaethynoxy isomers consistent with previously reported results by Grutzmacher and coworkers on R3E(PCO)–R3E(OCP) systems (R = iPr, Ph; E = Si, Sn, Ge, Pb). The lowest energy structure for the tricarbonyl Mn(CO)3(PCO) is a singlet structure with an unusual trihapto η3-PCO ligand. However, higher energy isomeric Mn(CO)3(PCO) structures with P-bonded phosphaketenyl or O-bonded phosphaethynoxy ligands and tetrahedral Mn coordination are also found.

Easy chairs: the conformational preferences of polyfluorocyclohexanes

Polyfluorocyclohexanes present an interesting challenge to our current understanding of fundamental organic chemistry. In part to improve molecular mechanics methods and facilitate drug design, a systematic survey of cyclohexanes with up to six fluorine substituents has been carried out, using theoretical methods. The preferred conformers are determined by delocalization effects, such as hyperconjugation, and do not necessarily follow the common assumption that substituents prefer the equatorial position. Thus, accurate ab intio results, which can capture electronic effects, are required. The lowest energy conformations of fluorocyclohexane, difluorocyclohexanes (six structural isomers), trifluorocyclohexanes (nine structural isomers), tetrafluorocyclohexanes (seventeen structural isomers), pentafluorocyclohexane (ten structural isomers), and hexafluorocyclohexanes (seven structural isomers) have been determined; relative energies, geometries, dipole moments, and population distributions are reported. We present a model for predicting the relative energies of polyfluorocyclohexane conformers based on the number of 1,2; 1,3; and 1,4 interactions present. The model is based on the energies from the difluorocyclohexanes; the correlation coefficient (R2) between computed relative energies and model relative energies is 0.967.

Substantial Dissociation Energies for the Recently Synthesized NC-Ag-NH3 and Br-Ag-NH3 Molecules and Their Isovalent Family Members M(CN)XY3 and M(Br)XY3 (M = Cu, Ag, Au; X = N, P; Y = H, F).

Chippindale et al. have recently synthesized the unique molecules (NC)Ag(NH3) and BrAg(NH3) and shown the heavy atom skeletal structures to be linear. Here, a theoretical study is reported of 12 members each of the two isovalent series of molecules. For (NC)Ag(NH3) and BrAg(NH3), the theoretical structures agree well with those determined by X-ray crystallography. Structures for the 22 yet unknown compounds should be similarly reliable. The dissociation energies for a loss of NH3 from the two known compounds are significant (34 and 31 kcal/mol), confirming their viability. For the other systems, the ligand dissociation energies are highly variable, ranging from 9 kcal/mol (BrAg-NF3) to 44 kcal/mol (BrAu-PH3). The bond dissociation energies for the different metals follow the irregular order Au > Cu > Ag. For the XY3 ligands, the dissociation energies follow the order NH3 > PH3 > PF3 > NF3, except for the BrAu-XY3 complexes. Electronic structure insights are gained via Natural Bond Orbital (NBO) analyses.

Addition of hydrogen atom/hydride anion to the double bonds of cytosine tautomers: radical and anion structures and energetics

The structures, energetics, and electron affinities of 18 H-addition cytosine radical isomers (C + H)· are predicted using carefully calibrated density functional methods. Radical rO1 in which H is attached at the N3 site of amino-oxo cytosine, has the lowest energy. The lowest energy anion is aO2, in which, qualitatively, H− is attached to the C6 site of amino-oxo cytosine. The theoretical adiabatic electron affinities (AEAs) for the 18 radicals range from −0.20 to 2.59 eV. Radical rO4, where the additional H atom is at the C4 site has the largest AEA. In contrast, when H is attached to the N3 site of the trans amino-hydroxy form, the resulting radical rA1 has a negative AEA, −0.20 eV. The radical rB1, in which H is appended on the N1 atom of the cis amino-hydroxy form, also has a negative AEA value, −0.16 eV. Generally, the AEA values for the cytosine H-addition radicals are rather smaller than those of the H-abstraction cytosine radicals studied earlier, which range from 2.22 to 3.00 eV.

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 S4 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)n (n = 4, 3, 2), Pt2(PF3)n (n = 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)n (n = 7, 6, 5) structures can be derived from the mononuclear Pt(PF3)n (n = 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)n ligand becomes a bridging group. The low-energy binuclear structures also include symmetrical [Pt(PF3)n]2 dimers (n = 2, 3) of the coordinately unsaturated Pt(PF3)n (n = 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 η(2)-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.

Major differences between trifluorophosphine and carbonyl ligands in binuclear cyclopentadienyliron complexes

The cyclopentadienyliron trifluorophosphine hydride CpFe(PF3)2H, in contrast to CpFe(CO)2H, is a stable compound that can be synthesized by reacting Fe(PF3)5 with cyclopentadiene. Theoretical studies on the binuclear Cp2Fe2(PF3)n (n = 5, 4, 3, 2) derivatives derived from CpFe(PF3)2H indicate the absence of viable structures having PF3 ligands bridging Fe–Fe bonds solely through the phosphorus atom. This contrasts with the analogous Cp2Fe2(CO)n systems for which the lowest energy structures have two (for n = 4 and 2) or three (for n = 3) CO groups bridging an iron–iron bond. Higher energy singlet Cp2Fe2(PF3)3 structures have a novel four-electron donor bridging η2-μ-PF3 ligand bonded to one iron atom through its phosphorus atom and to the other iron atom through a fluorine atom. Other higher energy triplet and singlet Cp2Fe2(PF3)2 structures are of the Cp2Fe2F2(μ-PF2)2 type having terminal fluorine atoms and bridging μ-PF2 ligands. The lowest energy Cp2Fe2(PF3)5 structure is actually Cp2Fe2(PF3)3(PF4)(μ-PF2) with a bridging PF2 group and a terminal PF4 group. Such structures are derived from a Cp2Fe2(PF3)4(μ-PF3) precursor by migration of a fluorine atom from the bridging PF3 group to a terminal PF3 group with a low activation energy barrier.