Formal Double Bond Research Articles

Relative stability of multiple bonds between silicon and bismuth. A theoretical study

Abstract Substituent effects on the potential energy surface of XSiBi (X = H, Li, Na, BeH, MgH, BH 2 , AlH 2 , CH 3 , SiH 3 , NH 2 , PH 2 , OH, SH, F, and Cl) were investigated by using B3LYP/Def2-TZVP, B3PW91/Def2-TZVPP, and CCSD(T) methods. The isomers include structures with formal double (Si BiX) and triple (XSi Bi) bonds to silicon–bismuth, so a direct comparison of these types of species is possible. Our model calculations indicate that electropositively substituted Si BiX species are thermodynamically and kinetically more stable than their isomeric XSi Bi molecules. Moreover, the theoretical findings suggest that F, OH, NH 2 , and CH 3 substitution prefer to shift the double bond (Si BiX) by forming a triple bond (XSi Bi).

Revealing Electron Delocalization through the Source Function

The source function (SF) introduced in late 90s by Bader and Gatti quantifies the influence of each atom in a system in determining the amount of electron density at a given point, regardless of the atom's remote or close location with respect to the point. The SF may thus be attractive for studying directly in the real space somewhat elusive molecular properties, such as "electron conjugation" and "aromaticity", that lack rigorous definitions as they are not directly associated to quantum-mechanical observables. In this work, the results of a preliminary test aimed at understanding whether the SF descriptor is capable to reveal electron delocalization effects are corroborated by further examination of the previously investigated benzene, 1,3-cyclohexadiene, and cyclohexene series and by extending the analysis to some benchmark organic systems with different unsaturated bond patterns. The SF can actually reveal, order, and quantify π-electron delocalization effects for formal double, single conjugated, and allylic bonds, in terms of the influence of distant atoms on the electron density at given bond critical points. In polycyclic aromatic hydrocarbons, the SF neatly reveals the mutual influence of the benzenoid subunits. In naphthalene it provides a rationale for the changes observed in the local aromatic character of one ring when the other is partially hydrogenated. The SF analysis describes instead biphenyl as made up by two weakly interacting benzene rings, only slightly perturbed by the combination of mutual steric and electronic effects. Eventually, a new SF-based indicator of local aromaticity is introduced, which shows excellent correlation with the aromatic index developed by Matta and Hernández-Trujillo, based on the delocalization indices. At variance with this latter and other commonly employed quantum-mechanical (local) aromaticity descriptors, the SF-based indicator does not require the knowledge of the pair density, nor the system wave function, being therefore promising for applications to experimentally derived charge density distributions.

Highly Unsaturated Binuclear Butadiene Iron Carbonyls: Quintet Spin States, Perpendicular Structures, Agostic Hydrogen Atoms, and Iron-Iron Multiple Bonds

The highly unsaturated binuclear butadiene iron carbonyls (C4H6)2Fe2(CO)n (n = 2, 1) have been examined using density functional theory. For (C4H6)2Fe2(CO)n (n = 2, 1), both coaxial and perpendicular structures are found. The global minima of (C4H6)2Fe2(CO)n (n = 2, 1) are the perpendicular structures 2Q-1 and 1Q-1, respectively, with 17- and 15-electron configurations for the iron atoms leading to quintet spin states. The Fe=Fe distance of 2.361 Å (M06-L) in the (C4H6)2Fe2(CO)2 structure 2Q-1 suggests a formal double bond. The Fe≡Fe bond distance in the (C4H6)2Fe2(CO) structure 1Q-1 is even shorter at 2.273 Å (M06-L), suggesting a triple bond. Higher energy (C4H6)2Fe2(CO)n (n = 2, 1) structures include structures in which a bridging butadiene ligand is bonded to one of the iron atoms as a tetrahapto ligand and to the other iron atom through two agostic hydrogen atoms from the end CH2 groups. Singlet (C4H6)2Fe2(CO) structures with formal Fe–Fe quadruple bonds of lengths ∼2.05 Å were also found but at very high energies (∼47 kcal/mol) relative to the global minimum.

Binuclear Methylborole Iron Carbonyls: Iron−Iron Multiple Bonds and Perpendicular Structures

Methylborole iron tricarbonyl, (η(5)-C(4)H(4)BCH(3))Fe(CO)(3), is known experimentally and is a potential source of binuclear (C(4)H(4)BCH(3))(2)Fe(2)(CO)(n) (n = 5, 4, 3, 2, 1) derivatives through reactions such as photolysis. In this connection the lowest energy (C(4)H(4)BCH(3))(2)Fe(2)(CO)(5) structures are predicted theoretically to have a single bridging carbonyl group and Fe-Fe distances consistent with formal single bonds. The lowest energy (C(4)H(4)BCH(3))(2)Fe(2)(CO)(4) structures have two bridging carbonyl groups and Fe═Fe distances suggesting formal double bonds. Analogously, the lowest energy (C(4)H(4)BCH(3))(2)Fe(2)(CO)(3) structures have three bridging carbonyl groups and very short Fe≡Fe distances suggesting formal triple bonds. The tetracarbonyl (C(4)H(4)BCH(3))(2)Fe(2)(CO)(4) is predicted to be thermodynamically unstable toward disproportionation into (C(4)H(4)BCH(3))(2)Fe(2)(CO)(5) + (C(4)H(4)BCH(3))(2)Fe(2)(CO)(3), whereas the tricarbonyl is thermodynamically viable toward analogous disproportionation. The lowest energy structures of the more highly unsaturated methylborole iron carbonyls (C(4)H(4)BCH(3))(2)Fe(2)(CO)(n) (n = 2, 1) have hydrogen atoms bridging an iron-carbon bond. In addition, the lowest energy (C(4)H(4)BCH(3))(2)Fe(2)(CO) structures are "slipped perpendicular" structures with bridging methylborole ligands, a terminal carbonyl group, and agostic CH(3)→Fe interactions involving the methyl hydrogens. Thus, in these highly unsaturated systems the methyl substituent in the methylborole ligand chosen in this work is not an "innocent bystander" but instead participates in the metal-ligand bonding.

Trifluorophosphine as a Bridging Ligand in Homoleptic Binuclear Nickel Complexes†

The stable tetrahedral derivative Ni(PF(3))(4) is reported to generate binuclear Ni(2)(PF(3))(n) ions in its mass spectrum emerging from ion-molecule reactions. Theoretical studies on such binuclear complexes indicate Ni(2)(PF(3))(7) to be energetically unfavorable with respect to decomposition into Ni(PF(3))(4) + Ni(PF(3))(3). However, viable rather unsymmetrical structures with two PF(3) bridges for Ni(2)(PF(3))(n) (n = 6, 5, 4) are reported here as well as a triply semibridged Ni(2)(PF(3))(5) structure. The nickel-nickel distances in these Ni(2)(PF(3))(n) derivatives suggest a formal double bond (2.51 A), a formal triple bond (2.26 A), and a formal double bond (2.34 A) for n = 6, 5, and 4, respectively. The mononuclear Ni(PF(3))(n) derivatives are predicted to have tetrahedral, trigonal planar, and bent (139 degrees ) structures for n = 4, 3, and 2, respectively.

ChemInform Abstract: Raman Spectroscopic Investigation and Coordination Behavior of the Polyimido SVI Anions [RS(NR)3]- and [S(NR)4]2-.

On the basis of Raman spec- troscopic experiments and the computa- tional assignment of the SN vibrations to small wavenumbers (640 - 920 cm ˇ1 ), the bonding in sulfur triimides is inter- preted to be mainly electrostatic (> S +ˇN 7ˇ) rather than covalent (S(aNR)3). The coordination of the S- alkyldiiminosulfonamide (RS(NR')3) ˇ and tetraazasulfate (S(NR)4) 2ˇ to bari- um is elucidated. In the anions present- ed here the central sulfur atom adopts the oxidation state vi. They can be readily synthesized from sulfur tri- imides, by nucleophilic addition of orga- nolithium species or lithium amides to the SaN formal double bond. Although three or four nitrogen donor centers are present, tripodal coordination to barium was not observed in any case. In the (S(NtBu)3) 2ˇ dianion, tripodal coordina- tion is facilitated by all tert-butyl groups pointing towards the lone pair of the sulfur atom, leaving all lone pairs of the nitrogen atoms pointing in the opposite direction. However, in ((thf)2Ba- {(NtBu)3SMe}2 )( 4) the steric demand of the S-methyl group in the (MeS- (NtBu)3) ˇ ion forces one tert-butyl group to the open site of the anion, and the third nitrogen atom is blocked. Within the (S(NtBu)4) 2ˇ ion in ((thf)4Ba2{N- (SiMe3)2}2{(NtBu)4S}) (7), tripodal coor- dination to barium is precluded by the bent SNtBu arrangement. In the known isoelectronic complex (Li2{(N- tBu)4W}2) with a linear WNtBu group tripodal coordination was observed.

Photoisomerization of Model Retinal Chromophores: Insight from Quantum Monte Carlo and Multiconfigurational Perturbation Theory

We present a systematic investigation of the structural relaxation in the excited state of model retinal chromophores in the gas phase using the complete-active-space self-consistent theory (CASSCF), multiconfigurational second-order perturbation theory (CASPT2), quantum Monte Carlo (QMC), and coupled cluster (CC) methods. In contrast to the CASSCF photoisomerization mechanism of bond inversion followed by torsion around formal double bonds, we find that the other approaches predict an initial skeletal relaxation which does not lead to bond inversion but to a rather flexible retinal chromophore with longer bonds and with the bond-length pattern of the ground state being partly preserved. The relaxation proceeds then preferentially via partial torsion around formal single bonds and does not reach a conical intersection region. Our findings are compatible with solution experiments which point to the existence of multiple minima and relaxation pathways, some of which are nonreactive, do not lead to photoproducts via conical intersection, and are dominant in solution. Our results also demonstrate the importance of a balanced description of dynamical and static correlation in the excited-state gradients and raise serious concerns on the common use of the CASSCF method to investigate structural properties of photoexcited retinal systems.

Major Difference between the Isoelectronic Fluoroborylene and Carbonyl Ligands: Triply Bridging Fluoroborylene Ligands in Fe3(BF)3(CO)9 Isoelectronic with Fe3(CO)12

The structure of Fe(3)(BF)(3)(CO)(9) is predicted to be very different than that of any of the isoelectronic homoleptic M(3)(CO)(12) derivatives (M = Fe, Ru, Os). Thus the lowest energy Fe(3)(BF)(3)(CO)(9) structure by approximately 19 kcal/mol has mu(3)-BF groups bridging the top and bottom of the Fe(3) triangle with a third edge-bridging BF group in addition to nine terminal carbonyl groups. No analogous M(3)(CO)(12) structures are found with mu(3)-CO groups bridging the M(3) triangle. Higher energy Fe(3)(BF)(3)(CO)(9) structures with two edge-bridging mu-BF groups and one terminal BF group are also found, analogous to the experimentally known Fe(3)(CO)(12) structure. However, these structures are transition states leading to local minima with one unsymmetrical face-bridging mu(3)-BF group, one edge-bridging mu-BF group, and one terminal BF group. No Fe(3)(BF)(3)(CO)(9) structures are found with exclusively terminal BF and CO groups analogous to the known structures of M(3)(CO)(12) (M = Ru, Os). These studies suggest that the BF group has a significantly greater tendency than the CO group to bridge two or three metal atoms, probably owing to the reluctance of the fluorine of the BF ligands to be a part of a formal double or triple bond.

Binuclear Nickel Carbonyl Thiocarbonyls: Metal−Metal Multiple Bonds versus Four-Electron Donor Thiocarbonyl Groups

The structures of the mononuclear derivatives Ni(CS)(CO)(n) (n = 3, 2, 1, 0) and the binuclear derivatives Ni(2)(CS)(2)(CO)(n) (n = 5, 4, 3, 2) have been optimized by density functional theory for comparison with the corresponding structures of Ni(CO)(n+1) and Ni(2)(CO)(n+2), respectively. In the lowest energy structures for Ni(CS)(CO)(n) (n = 3, 2, 1), the nickel atom has approximate tetrahedral (n = 3), trigonal planar (n = 2), or bent coordination (n = 1) corresponding to 18-, 16-, and 14-electron metal configurations, respectively. The six lowest energy Ni(2)(CS)(2)(CO)(5) structures all have a single CE (E = S, O) bridge and a formal Ni-Ni single bond of length approximately 2.6 to approximately 2.7 A analogous to the lowest energy Ni(2)(CO)(7) structure. The Ni(2)(CS)(2)(CO)(5) structures with a bridging CS group are of lower energies than similar structures with a bridging CO group. Higher energy Ni(2)(CS)(2)(CO)(5) structures with a linear bridging CE group (E = O, S) and no Ni...Ni bond are also found with tetrahedral coordination for both nickel atoms. The lowest energy Ni(2)(CS)(2)(CO)(4) structures are doubly bridged structures with only two-electron donor CO and CS groups and Ni=Ni distances of approximately 2.5 A suggesting the formal double bond needed to give both nickel atoms the favored 18-electron configuration. For Ni(2)(CS)(2)(CO)(3) the structures with one four-electron donor bridging eta(2)-mu-CS group and a formal Ni=Ni double bond of approximately 2.4 to approximately 2.5 A are energetically preferred over triply bridged structures with a shorter Ni[triple bond]Ni distance of approximately 2.2 A, corresponding to a formal triple bond and similar to the lowest energy Ni(2)(CO)(5) structure. The lowest energy Ni(2)(CS)(2)(CO)(2) structures are generally derived from Ni(2)(CS)(2)(CO)(3) structures by removal of a carbonyl group. No formal quadruple bonds are found in any of the Ni(2)(CS)(2)(CO)(2) structures, as indicated by the absence of ultrashort Ni(-4)Ni distances.

Binuclear manganesecarbonyl thiocarbonyls: metal–metal multiple bonds versus four-electron donorthiocarbonyl groups

Density functional theory (DFT) studies on Mn2(CS)2(CO)8 using the B3LYP and BP86 methods show that no less than eight different unbridged structures are of significantly lower energies than the lowest energy doubly bridged structure. The Mn–Mn single bonds in these Mn2(CS)2(CO)8 structures range from 2.99 ± 0.02 A for the four structures with staggered equatorial CO/CS groups to 3.12 ± 0.04 A for the four structures with eclipsed equatorial CO/CS groups. The six lowest energy Mn2(CS)2(CO)7 structures all have four-electron donor bridging η2-μ-CE groups (E = S, O) and formal Mn–Mn single bonds of lengths 2.95 ± 0.01 A, rather than only two-electron donor CO and CS groups and formal MnMn double bonds. The Mn2(CS)2(CO)7 structures with an η2-μ-CS group are of lower energy than those with an η2-μ-CO group. These Mn2(CS)2(CO)7 structures are similar to the lowest energy structure for Mn2(CO)9 predicted previously as well as (Ph2PCH2PPh2)2Mn2(CO)4(η2-μ-CO), which has been synthesized and structurally characterized by X-ray diffraction. The lowest energy Mn2(CS)2(CO)6 structures are predicted have a single four-electron donor bridging η2-μ-CS group and a formal MnMn double bond. However, at only slightly higher energies, Mn2(CS)2(CO)6 structures are found with two η2-μ-CS groups and a formal Mn–Mn single bond. A formal MnMn triple bond of length 2.36 ± 0.03 A is found in an even higher energy unbridged Mn2(CS)2(CO)6 structure, similar to the lowest energy Mn2(CO)8 structure found in a previous theoretical study. The lowest energy structures for Mn2(CS)2(CO)5 have two η2-μ-CS groups and a formal MnMn double bond of length 2.57 ± 0.03 A.

Neutral homoleptic tetranuclear iron carbonyls: why haven’t they been synthesized as stable molecules?

The tetranuclear iron carbonyls Fe4(CO)n (n = 16, 15, 14) have been investigated using density functional theory (DFT). Low-energy structures are predicted for Fe4(CO)16, Fe4(CO)15, and Fe4(CO)14 in which the Fe4 units are rhombi, planar butterflies, and tetrahedra with four, five, and six Fe–Fe single bonds, respectively, to give all four iron atoms the favored 18-electron configuration. However, for Fe4(CO)14 an alternative low energy structure is predicted with a planar Fe4 butterfly and one of the five iron–iron distances short enough to correspond to a formal FeFe double bond, thereby also leading to the favored 18-electron configuration. The dissociation energy of Fe4(CO)16 into Fe(CO)5 + Fe3(CO)11 or Fe3(CO)12 + Fe(CO)4 is predicted to be very low (<6 kcal mol−1). Similarly, the dissociation energy of Fe4(CO)15 into Fe(CO)5 + Fe3(CO)10 is predicted to be very low at ∼5 kcal mol−1. These low predicted dissociation energies of Fe4(CO)16 and Fe4(CO)15 into smaller iron carbonyl fragments are consistent with the fact that neither of these molecules have been synthesized or detected spectroscopically, even in low temperature matrices. However, Fe4(CO)14 is predicted to be considerably more stable towards dissociation into smaller fragments with a dissociation energy of 23.2 kcal mol−1 (B3LYP) or 39.2 kcal mol−1 (BP86) into Fe(CO)5 + Fe3(CO)9. This is consistent with the detection of Fe4(CO)14 by infrared spectroscopy in the reaction product of mass selected Fe4+ with CO in low temperature matrices.

How long are the ends of polyene chains?

In this work we study conjugation in all-trans polyene chains H[Single Bond](HC[Double Bond]CH)(n)[Single Bond]H with a view to establishing the length scale for the interaction between conjugated double bonds. As a polyene oligomer is made longer, bond length alternation between formal carbon-carbon single and double bonds diminishes toward the middle of the chain, eventually reaching a constant value characteristic of an "infinite" chain. However those bonds near the end of the chain continue to be influenced by the end, even in the long-chain limit. We have determined optimized geometries for polyene oligomers with up to n=11 repeat units at the MP2/cc-pVTZ level. At this length the central-most bonds are almost converged to the long chain limit, for which we estimate R(C[Double Bond]C)=1.3652 A and R(C[Single Bond]C)=1.4238 A. In contrast, the endmost double bond has a length of 1.3442 A and the endmost single bond has a length of 1.4425 A. We find that a given bond is significantly influenced by conjugation paths through up to six neighboring conjugated double bonds. End effects can also be monitored by examining the energy increment per added monomer as the oligomer length is increased. This analysis also indicates that significant conjugation effects extend out through approximately six neighboring double bonds. From the energy per monomer of the longest chains we extract a value of about 8 kcal/mol for the extra stabilization energy per monomer due to conjugation in long chains.

Structure and Reactivity of New Iridium Complexes with Bis(Oxazoline)-Phosphonito Ligands

The synthesis and characterization of novel iridium(I) complexes bearing a neutral bis(oxazoline)phosphonite ligand, NOPON(Me(2)) (I), are reported. Numerous Ir(I) complexes have been isolated in high yields and characterized by spectroscopy and X-ray diffraction. [Ir(mu-Cl)(cod)](2) (cod = 1,5-cyclooctadiene) reacted with I to give the air-sensitive complex [IrCl(cod)(NOPON(Me(2)))] (1), which shows broad (1)H and (13)C{(1)H} NMR signals due to dynamic exchange equilibria involving the cod and the NOPON(Me(2)) ligands. Reaction between a solution of 1 and CO afforded the carbonyl complex [IrCl(CO)(NOPON(Me(2)))] (2), whose solid-state structure has been determined by X-ray diffraction. Cationic complexes have been obtained by using NaBAr(F) (BAr(F) = B[3,5-(CF(3))(2)C(6)H(3)](4)) as a chloride abstractor. The complex [Ir(cod)(NOPON(Me(2)))]BAr(F) (3) displays a mononuclear structure in the solid state with ligand I acting as a bidentate P,N chelating ligand. This complex is a precatalyst for the hydrogenation of alkenes. Oxidative addition of H(2) to 3 occurred either in solution or in the solid-state and this reaction allowed the isolation of the 32 electron, dinuclear dihydrido-bridged iridium(III) complex [IrH(mu-H)(NOPON(Me(2)))](2)(BAr(F))(2) (4), in which the NOPON(Me(2)) ligands exhibit a facial coordination mode. It contains only hydrides as bridging ligands and the Ir(2)(mu-H)(2) unit can be viewed as containing a formal Ir-Ir double bond or two 3c-2e bonds. Complex 3 has also been reacted with CO in solution and in the solid state, and this yielded the dicarbonyl derivative [Ir(CO)(2)(NOPON(Me(2)))]BAr(F) (5). A transmetalation reaction between 3 and [PdCl(2)(NCPh)(2)] afforded the cationic Pd(II) complex [PdCl(NOPON(Me(2)))]BAr(F) (6), which has been structurally characterized.

Formation of a four-electron donor carbonyl group in the decarbonylation of the unsaturated H 2C 2Fe 2(CO) 6 tetrahedrane as an alternative to an iron–iron triple bond

The unsaturated Fe 2C 2 tetrahedrane derivatives R 2C 2Fe 2(CO) 6 (R = Ph, t Bu) are among the many products obtained from reactions of the alkynes RC CR with iron carbonyls. In this connection theoretical studies have been performed on the simplest such compounds H 2C 2Fe 2(CO) n ( n = 6, 5) for comparison with the experimentally known structure of the t-butyl derivative t-Bu 2C 2Fe 2(CO) 6 and in order to predict the decarbonylation pathways for such (alkyne)Fe 2(CO) 6 derivatives. These theoretical studies predict an Fe 2C 2 tetrahedrane structure for H 2C 2Fe 2(CO) 6 with a formal Fe Fe double bond very similar to the experimental structure for t-Bu 2C 2Fe 2(CO) 6. Decarbonylation of H 2C 2Fe 2(CO) 6 is predicted to give an H 2C 2Fe 2(CO) 5 isomer retaining the Fe 2C 2 tetrahedrane structure, with an Fe Fe double bond but with the unprecedented feature of a four-electron donor bridging carbonyl group in an M 2C 2 tetrahedrane structure. The formation of formal Fe Fe triple bonds appears to be avoided in even the higher energy H 2C 2Fe 2(CO) 5 structures. These include three triplet Fe 2C 2 tetrahedrane structures with formal Fe Fe double bonds as well as a coordinately unsaturated singlet structure, still with an Fe Fe double bond.

Why are trimethylsilyl groups asymmetrically coordinated? Gas-phase molecular structures of 1-trimethylsilyl-1,2,3-benzotriazole and 2-trimethylsilyl-1,3-thiazole

The structures of 1-trimethylsilyl-1,2,3-benzotriazole and 2-trimethylsilyl-1,3-thiazole have been determined by gas electron diffraction and computational methods. While 1-trimethylsilyl-1,2,3-benzotriazole shows a significant asymmetry in the way the SiMe(3) groups bonds to the ring system, the same is not true for 2-trimethylsilyl-1,3-thiazole. However, it has been shown that when the positions of formal single and double bonds in the rings systems are considered, the silyl groups in both compounds are displaced towards the neighbouring ring nitrogen atom. Calculated structures of a series of analogous compounds with different substituents on silicon show only minor variations in the extent of the distortion, although with hydrogen instead of a silyl group the displacement is significantly smaller.

Mononuclear and binuclear manganese carbonyl hydrides: the preference for bridging hydrogens over bridging carbonyls

The structures of the mononuclear derivatives HMn(CO)n (n=4 and 3) are shown by density functional theory (B3LYP and PB86) to derive from octahedral HMn(CO)5 by losses of various combinations of carbonyl groups with relatively little change in the C-Mn-C angles involving the remaining carbonyl groups. The binuclear H2Mn2(CO)n structures are predicted to have bridging hydrogen atoms in preference to bridging carbonyl groups. Thus, two structures are found for the binuclear H2Mn2(CO)9, isoelectronic with Fe2(CO)9, in which all nine of the carbonyl groups are terminal carbonyl groups. The lowest lying H2Mn2(CO)9 structure is the dihydrogen complex (OC)5Mn-Mn(CO)4(eta2-H2), in which one of the equatorial CO groups of Mn2(CO)10 is replaced by a dihydrogen ligand. A slightly higher energy H2Mn2(CO)9 structure by approximately 6 kcal mol(-1) has an Mn-Mn bond bridged by a single hydrogen and all terminal carbonyl groups as well as a single terminal hydrogen. The H2Mn2(CO)8 molecule is predicted to have a structure with a central Mn(micro-H)2Mn core related to diborane. In this structure the manganese-manganese bond of length 2.703 A (BP86) can be considered the diprotonated formal double bond required to give both manganese atoms the favoured 18-electron configuration. The more highly unsaturated H2Mn2(CO)n (n=7, 6) derivatives have similar structures derived from the H2Mn2(CO)8 structure by loss of one or two carbonyl groups. In many cases the Mn[triple bond, length as m-dash]Mn distance in the central Mn(micro-H)2Mn unit shortens to approximately 2.4 A suggesting a diprotonated triple bond.

Two derivatives of 1,5-disubstituted tetrazoles: 1-(4-nitrophenyl)-1H-tetrazol-5-amine and {(E)-[1-(4-ethoxyphenyl)-1H-tetrazol-5-yl]iminomethyl}dimethylamine

The geometric features of 1-(4-nitrophenyl)-1H-tetrazol-5-amine, C(7)H(6)N(6)O(2), correspond to the presence of the essential interaction of the 5-amino group lone pair with the pi system of the tetrazole ring. Intermolecular N-H...N and N-H...O hydrogen bonds result in the formation of infinite chains running along the [110] direction and involve centrosymmetric ring structures with motifs R(2)(2)(8) and R(2)(2)(20). Molecules of {(E)-[1-(4-ethoxyphenyl)-1H-tetrazol-5-yl]iminomethyl}dimethylamine, C(12)H(16)N(6)O, are essentially flattened, which facilitates the formation of a conjugated system spanning the whole molecule. Conjugation in the azomethine N=C-N fragment results in practically the same length for the formal double and single bonds.

Unsaturated Binuclear Cyclopentadienylrhenium Carbonyl Derivatives: Metal−Metal Multiple Bonds and Agostic Hydrogen Atoms

The cyclopentadienylrhenium carbonyls Cp 2Re 2(CO) n (Cp = eta (5)-C 5H 5; n = 5, 4, 3, 2) have been studied by density functional theory. The global minima for the Cp 2Re 2(CO) n ( n = 5, 4, 3, 2) derivatives are predicted to be the singly bridged structure Cp 2Re 2(CO) 4(mu-CO) with a formal Re-Re single bond; the doubly semibridged structure Cp 2Re 2(CO) 4 with a formal ReRe double bond; the triply bridged structure Cp 2Re 2(mu-CO) 3 with a formal ReRe triple bond; and the doubly bridged structure Cp 2Re 2(mu-CO) 2, respectively. The first three of these predicted structures have been realized experimentally in the stable compounds (eta (5)-C 5H 5) 2Re 2(CO) 4(mu-CO), (eta (5)-Me 5C 5) 2Re 2(CO) 4 and (eta (5)-Me 5C 5) 2Re 2(mu-CO) 3. In addition, structures of the type Cp 2Re-Re(CO) n with both rings bonded only to one metal and unknown in manganese chemistry are also found for rhenium but at energies significantly above the global minima. The unsaturated Cp 2Re-Re(CO) n structures ( n = 4, 3, 2) have agostic Cp hydrogen atoms forming C-H-Re bridges to the unsaturated Re(CO) n group with a Re-H distance as short as 2.04 A.

Synthesis of 2-(4′-R-benzylidene)-4-aza-1,3-indanediones and investigation of isomerization around the exocyclic double bond

2-Benzylidene-4-aza-1,3-indanediones have been synthesized and their structures have been investigated by NMR spectroscopy and quantum chemistry in the AM1 approximation. It was established that rotation about the exocyclic C(2)=C(10) formal double bond occurs in chloroform solutions of these compounds. The energy characteristics of this process have been determined experimentally and estimated theoretically.

Crystal Clear Structural Evidence for Electron Delocalization in 1,2-Dihydro-1,2-azaborines

The first examples of "pre-aromatic" 1,2-dihydro-1,2-azaborine heterocycles have been structurally characterized, enabling the direct comparison of delocalized bonds of 1,2-dihydro-1,2-azaborines to their corresponding formal double and single bonds in nonaromatic systems. The crystallographic data provide an unprecedented look into the structural changes that occur in six-membered BN-heterocycles on their road to aromaticity, and they establish with little ambiguity that 1,2-dihydro-1,2-azaborines possess delocalized structures consistent with aromaticity.