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1.
Chemistry ; 2020 Aug 02.
Artigo em Inglês | MEDLINE | ID: mdl-32743817

RESUMO

Quantum chemical studies using density functional theory and ab initio methods have been carried out for the molecules L-C3-L with L = PPh3 (1),  NHCMe (2) and cAACMe (3). The calculations predict that 1 and 2 have equilibrium geometries where the ligands are bonded with rather acute bonding angles at the linear C3 moiety. The phosphine adduct 1 has a synclinal (gauche) conformation whereas 2 exhibits a trans conformation of the ligands. In contrast, the compound 3 possesses a nearly linear arrangement of the carbene ligands at the C3 fragment. The bond dissociation energies of the ligands have the order 1 < 2 < 3. The bonding analysis using charge- and energy decomposition methods suggests that 3 is best described as a cumulene with electron-sharing double bonds between neutral fragments (cAACMe)=C3=(cAACMe) whereas 1 and 2 possess electron-sharing and dative bonds between positively charged ligands [(PPh3)2]+ and [(NHCMe)2]+ and negatively charged [C3]-.

2.
Nat Commun ; 11(1): 3441, 2020 Jul 10.
Artigo em Inglês | MEDLINE | ID: mdl-32651366

RESUMO

The concept of odd-electron σ-bond was first proposed by Linus Pauling. Species containing such a bond have been recognized as important intermediates encountered in many fields. A number of radicals with a one-electron or three-electron σ-bond have been isolated, however, no example of a diradical based odd-electron σ-bonds has been reported. So far all stable diradicals are based on two s/p-localized or π-delocalized unpaired electrons (radicals). Here, we report a dication diradical that is based on two Se∴Se three-electron σ-bonds. In contrast, the dication of sulfur analogue does not display diradical character but exhibits a closed-shell singlet.

3.
Chemistry ; 2020 Jul 14.
Artigo em Inglês | MEDLINE | ID: mdl-32666598

RESUMO

Quantum chemical calculations of the oxides AeO, imides AeNH and dihydrides AeH 2 of the alkaline earth atoms Ae = Be, Mg, Ca, Sr, Ba and the calcium cluster Ca 6 H 9 (N(SiMe 3 ) 2 ) 3 (PMDTA) 3 have been carried out using density functional theory. The analysis of the electronic structures using charge and energy partitioning methods suggests that the valence orbitals of the lighter atoms Be and Mg are the (n)s and (n)p orbitals. In contrast, the valence orbitals of the heavier atoms Ca, Sr and Ba comprise the (n)s and the (n-1)d orbitals. The alkaline earth metals beryllium and magnesium built covalent bonds like typical main-group elements whereas calcium, strontium and barium covalently bind like transition metals. The results not only shed new light on the covalent bonds of the heavier alkaline earth metals, but are also very important for understanding and designing experimental studies.

4.
Dalton Trans ; 2020 Jun 16.
Artigo em Inglês | MEDLINE | ID: mdl-32542268

RESUMO

The chemistry of coinage metal ions with Fe(CO)5, [Mn(CO)5]- and [Fe(CO)4CN]- has been explored using Mes3P and N-heterocyclic carbene supporting ligands. A comparison of [(SIPr)Au-Fe(CO)5][SbF6], [(Et2CAAC)Au-Fe(CO)5][SbF6] and [(Mes3P)Au-Fe(CO)5][SbF6] shows that the ligand donor strength towards Au(i) follows the order Mes3P > Et2CAAC > SIPr. These Fe(CO)5 complexes show significant blue shifts in [small nu, Greek, macron]CO bands relative to those observed for free Fe(CO)5 as a result of it serving as a net electron donor to Au(i). Au(i) is a much stronger acceptor in (SIPr)Au-Mn(CO)5 compared to Ag(i) in (SIPr)Ag-Mn(CO)5. The structural details of Mes3PAu-Mn(CO)5 are also presented. [Fe(CO)4CN]- afforded CN bridged coinage metal complexes with (IPr*)Au+, (SIPr)Ag+ and (SIPr)Cu+ moieties, rather than molecules with direct Fe/coinage metal bonds. The computed total interaction energies indicate that both [Mn(CO)5]- and [Fe(CO)4CN]- are stronger donors toward Au(i) than Fe(CO)5. A detailed analysis of the bonding interactions between the coinage metal ions and Fe(CO)5, [Mn(CO)5]- and [Fe(CO)4CN]- suggests that the largest contribution comes from electrostatic attraction, while the covalent component follows the Dewar-Chatt-Duncanson model. The σ-donor interactions of these organometallic ligands with coinage metal ions are considerably stronger than the π-backbonding from the coinage metal ions.

5.
Chemistry ; 2020 Jun 08.
Artigo em Inglês | MEDLINE | ID: mdl-32515082

RESUMO

Following the recent discovery of stable octa-coordinated alkaline earth metals with N2 and CO, the role of group II metals in the catalytic reduction of these ligands by means of density functional theory (DFT) calculations and conceptual DFT-based reactivity indices is investigated. Cubic group IV and octahedral group VI transition metal complexes as well as the free ligands are computed for reference. The outer and most accessible atoms of N2 and CO become much more nucleophilic and electrophilic in all complexes, relevant for N2 fixation, as probed by the Fukui function and local softness. Within one row of the periodic table, the alkaline earth complexes often show the strongest activation. On the contrary, the electrostatic character is found to be virtually unaffected by complexation. Trends in the soft frontier orbital and hard electrostatic character are in agreement with calculated proton affinities and energy decomposition analyses of the protonated structures, demonstrating the dominance of the soft (HOMO-LUMO) orbital interactions.

6.
Artigo em Inglês | MEDLINE | ID: mdl-32583528

RESUMO

The reactions of laser-ablated beryllium atoms with dinitrogen and carbon monoxide mixtures form the end-on bonded NNBeCO and side-on bonded (η2 -N2 )BeCO isomers in solid argon, which are predicted by quantum chemical calculations to be almost isoenergetic. The end-on bonded complex has a triplet ground state while the side-on bonded isomer has a singlet electronic ground state. The complexes rearrange to the energetically lowest lying NBeNCO isomer upon visible light excitation, which is characterized to be an isocyanate complex of a nitrene derivative with a triplet electronic ground state. A bonding analysis using a charge- and energy decomposition procedure reveals that the electronic reference state of Be in the NNBeCO isomers has an 2s0 2p2 excited configuration and that the metal-ligand bonds can be described in terms of N2 →Be←CO σ donation and concomitant N2 ←Be→CO π backdonation. The results demonstrate that the activation of N2 with the N-N bond being completely cleaved can be achieved via coupling with carbon monoxide mediated by a main group atom.

7.
Artigo em Inglês | MEDLINE | ID: mdl-32463527

RESUMO

Double deprotonation of the diamine 1,1'-(tBuCH2 NH)-ferrocene (1-H2 ) by alkaline-earth (Ae) or EuII metal reagents gave the complexes 1-Ae (Ae=Mg, Ca, Sr, Ba) and 1-Eu. 1-Mg crystallized as a monomer while the heavier complexes crystallized as dimers. The Fe⋅⋅⋅Mg distance in 1-Mg is too long for a bonding interaction, but short Fe⋅⋅⋅Ae distances in 1-Ca, 1-Sr, and 1-Ba clearly support intramolecular Fe⋅⋅⋅Ae bonding. Further evidence for interactions is provided by a tilting of the Cp rings and the related 1 H NMR chemical-shift difference between the Cp α and ß protons. While electrochemical studies are complicated by complex decomposition, UV/Vis spectral features of the complexes support Fe→Ae dative bonding. A comprehensive bonding analysis of all 1-Ae complexes shows that the heavier species 1-Ca, 1-Sr, and 1-Ba possess genuine Fe→Ae bonds which involve vacant d-orbitals of the alkaline-earth atoms and partially filled d-orbitals on Fe. In 1-Mg, a weak Fe→Mg donation into vacant p-orbitals of the Mg atom is observed.

8.
Artigo em Inglês | MEDLINE | ID: mdl-32249989

RESUMO

We challenge the interpretation of the chemical bond in NaBH3 - proposed by Liu et al. We argue that NaBH3 - has an electron-sharing Na-BH3 - covalent bond rather than a dative bond Na- →BH3 .

9.
Artigo em Inglês | MEDLINE | ID: mdl-32196126

RESUMO

The preparation and spectroscopic identification of the complexes NNBe(η2 -N2 ) and (NN)2 Be(η2 -N2 ) and the energetically higher lying isomers Be(NN)2 and Be(NN)3 are reported. NNBe(η2 -N2 ) and (NN)2 Be(η2 -N2 ) are the first examples of covalently side-on bonded N2 adducts of a main-group element. The analysis of the electronic structure using modern methods of quantum chemistry suggests that NNBe(η2 -N2 ) and (NN)2 Be(η2 -N2 ) should be classified as π complexes rather than metalladiazirines.

10.
Chemistry ; 2020 Mar 19.
Artigo em Inglês | MEDLINE | ID: mdl-32191361

RESUMO

Homoleptic Group 4 metal carbonyl cation and neutral complexes were prepared in the gas phase and/or in solid neon matrix. Infrared spectroscopy studies reveal that both zirconium and hafnium form eight-coordinate carbonyl neutral and cation complexes. In contrast, titanium forms only the six-coordinate Ti(CO)6 + and seven-coordinate Ti(CO)7 . Titanium octacarbonyl Ti(CO)8 is unstable as a result of steric repulsion between the CO ligands. The 20-electron Zr(CO)8 and Hf(CO)8 complexes represent the first experimentally observed homoleptic octacarbonyl neutral complexes of transition metals. The molecules still fulfill the 18-electron rule, because one doubly occupied valence orbital does not mix with any of the metal valence atomic orbitals. Zr(CO)8 and Hf(CO)8 are stable against the loss of one CO because the CO ligands encounter less steric repulsion than Zr(CO)7 and Hf(CO)7 . The heptacarbonyl complexes have shorter metal-CO bonds than that of the octacarbonyl complexes due to stronger electrostatic and covalent bonding, but the significantly smaller repulsive Pauli term makes the octacarbonyl complexes stable.

11.
Dalton Trans ; 2020 Feb 05.
Artigo em Inglês | MEDLINE | ID: mdl-32022052

RESUMO

We present the syntheses of trigonal planar coordinated Fe(ii) carbodiphosphorane (CDPR) complexes, starting from iron(ii)-bis(trimethylsilylamide) [Fe{N(SiMe3)2}2] and hexaphenyl-(CDPPh) and sym-dimethyltetraphenyl-carbodiphosphoranes (CDPMe), respectively. Both complexes [CDPPh-Fe{N(SiMe3)2}2] (1) and [CDPMe-Fe{N(SiMe3)2}2] (2) were examined in solution and in the solid state. 1 shows a dissociation equilibrium in solution which we monitored by variable temperature 1H-NMR spectroscopy. Magnetic measurements of 1 and 2 yielded a high spin configuration (S = 2) for both complexes. Quantum chemical calculations were performed to analyze the bonding situation in compound 1.

12.
Phys Chem Chem Phys ; 22(9): 5377-5379, 2020 Mar 07.
Artigo em Inglês | MEDLINE | ID: mdl-32077870

RESUMO

We challenge the statement of Koch et al. that the M → CO charge transfer and the decrease of the CO stretching frequency in metal carbonyl complexes do not depend on the metal d orbitals. The approach of the authors is severely flawed and leads to misleading conclusions.

13.
J Am Chem Soc ; 142(10): 4560-4564, 2020 Mar 11.
Artigo em Inglês | MEDLINE | ID: mdl-32088963

RESUMO

The alkaline-earth elements (Be, Mg, Ca, Sr, and Ba) strongly favor the formation of diamagnetic compounds in the +2 oxidation state. Herein we report a paramagnetic beryllium radical cation, [(CAAC)2Be]+• (2) [CAAC = cyclic (alkyl)(amino)carbene], prepared by oxidation of a zero-valent beryllium complex with 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO). Compound 2 was characterized by EPR spectroscopy, elemental analysis, X-ray crystallography, and DFT calculations. Notably, the isolation of 2 represents the first s-block charged radical and the first crystalline beryllium radical.

14.
J Phys Chem A ; 2020 Feb 03.
Artigo em Inglês | MEDLINE | ID: mdl-31951409

RESUMO

Quantum chemical calculations using ab initio methods at the CCSD(T) level with large basis sets and DFT calculations using the BP86 functional have been carried out for O22+ and N2. An energy decomposition analysis of the chemical bonds suggests that the shorter bond in O22+ compared with isoelectronic N2 is due to the weaker Pauli repulsion in the dication, which overcompensates the weakening of attractive interactions that are operative in N2. At the equilibrium distance of N2, the orbital (covalent) bonding in O22+ is weaker than in N2, and the attractive Coulomb interactions in the neutral diatomic system become repulsive in the dication, but the weakening of the Pauli repulsion caused by the shrinking of the orbitals in O22+ compensates for these forces and leads to a shortening of the bond. The results also show that the bond dissociation energy is not a reliable indicator for the strength of bond, which is more faithfully given by the (local) force constant.

15.
Chem Commun (Camb) ; 56(14): 2167-2170, 2020 Feb 18.
Artigo em Inglês | MEDLINE | ID: mdl-31970366

RESUMO

One-electron oxidation of 1,3-digerma-2,4-dipnictacyclobutadiene [LHGeE]2 (LH = CH[CHNDipp]2, dipp = 2,6-iPr2C6H3; 1: E = P; 2: E = As) with Ag[Al(ORF)4] (RF = C(CF3)3) afforded the stable radical cation salts 1˙+·[Al(ORF)4]- and 2˙+·[Al(ORF)4]-, respectively. The radical cation salts have been fully characterized, in conjunction with theoretical calculations. The EPR spectroscopic studies and DFT calculations reveal that the spin density mainly resides at the heavy pnictogen atoms, rather than delocalizes over the Ge2E2 ring. They represent the first structurally characterized cyclic radical species composed of both heavy group 14 and 15 elements.

16.
Angew Chem Int Ed Engl ; 59(10): 3850-3854, 2020 Mar 02.
Artigo em Inglês | MEDLINE | ID: mdl-31816143

RESUMO

N-Heterocyclic carbene (NHC)- and cyclic (alkyl)(amino)carbene (CAAC)-stabilized borafluorene radicals have been isolated and characterized by elemental analysis, single-crystal X-ray diffraction, UV/Vis absorption, cyclic voltammetry (CV), electron paramagnetic resonance (EPR) spectroscopy, and theoretical studies. Both the CAAC-borafluorene radical (2) and the NHC-borafluorene radical (4) have a considerable amount of spin density localized on the boron atoms (0.322 for 2 and 0.369 for 4). In compound 2, the unpaired electron is also partly delocalized over the CAAC ligand carbene C and N atoms. However, the unpaired electron in compound 4 mainly resides throughout the borafluorene π-system, with significantly less delocalization over the NHC ligand. These results highlight the Lewis base dependent electrostructural tuning of materials-relevant radicals. Notably, this is the first report of crystalline borafluorene radicals, and these species exhibit remarkable solid-state and solution stability.

17.
Dalton Trans ; 48(42): 16108-16114, 2019 Nov 14.
Artigo em Inglês | MEDLINE | ID: mdl-31620743

RESUMO

A set of complexes containing dative interactions between a rare-earth metal and carbon are reported. Complex 2, Br3Ce(CDP)(THF), with a Ce←C bond was synthesized by the reaction of CeBr3 with a carbon(0) ligand, carbodiphosphorane (CDP). More significantly, a trivalent cerium complex 3, [BrCe(CDP)2](BPh4)2, with two σ dative interactions C→Ce←C was also isolated, which represents an unusual example of two dative interactions formed with the same atom in a molecule. Furthermore, π donation by the second lone-pair electrons of the CDP ligand is rather weak. Single-crystal X-ray diffraction shows that the Ce-C bond lengths in these complexes are comparable with those in cerium(iii)-carbene species. Density functional theory calculations support the dative interaction formation in these complexes and the strength of σ-donation in 3 is stronger than that in 2.

18.
Angew Chem Int Ed Engl ; 58(48): 17365-17374, 2019 Nov 25.
Artigo em Inglês | MEDLINE | ID: mdl-31498532

RESUMO

We report the synthesis and spectroscopic identification of the trisbenzene complexes of strontium and barium M(Bz)3 (M=Sr, Ba) in low-temperature Ne matrix. Both complexes are characterized by a D3 symmetric structure involving three equivalent η6 -bound benzene ligands and a closed-shell singlet electronic ground state. The analysis of the electronic structure shows that the complexes exhibit metal-ligand bonds that are typical for transition metal compounds. The chemical bonds can be explained in terms of weak donation from the π MOs of benzene ligands into the vacant (n-1)d AOs of M and strong backdonation from the occupied (n-1)d AO of M into vacant π* MOs of benzene ligands. The metals in these 20-electron complexes have 18 effective valence electrons, and, thus, fulfill the 18-electron rule if only the metal-ligand bonding electrons are counted. The results suggest that the heavier alkaline earth atoms exhibit the full bonding scenario of transition metals.

19.
Science ; 365(6453)2019 08 09.
Artigo em Inglês | MEDLINE | ID: mdl-31395758

RESUMO

Landis et al claim in their comment that Ca does not bind like a transition metal in Ca(CO)8 We reject their statement, which is based on a misconception of bonding models and misleading application and interpretation of quantum chemical methods for analyzing chemical bonds.


Assuntos
Metais Alcalinoterrosos , Elementos de Transição
20.
Chemistry ; 25(50): 11772-11784, 2019 Sep 06.
Artigo em Inglês | MEDLINE | ID: mdl-31276242

RESUMO

The octacarbonyl cation and anion complexes of actinide metals [An(CO)8 ]+/- (An=Th, U) are prepared in the gas phase and are studied by mass-selected infrared photodissociation spectroscopy. Both the octacarbonyl cations and anions have been characterized to be saturated coordinated complexes. Quantum chemical calculations by using density functional theory show that the [Th(CO)8 ]+ and [Th(CO)8 ]- complexes have a distorted octahedral (D4h ) equilibrium geometry and a doublet electronic ground state. Both the [U(CO)8 ]+ cation and the [U(CO)8 ]- anion exhibit cubic structures (Oh ) with a 6 A1g ground state for the cation and a 4 A1g ground state for the anion. The neutral species [Th(CO)8 ] (Oh ; 1 A1g ) and [U(CO)8 ] (D4h ; 5 B1u ) have also been calculated. Analysis of their electronic structures with the help on an energy decomposition method reveals that, along with the dominating 6d valence orbitals, there are significant 5f orbital participation in both the [An]←CO σ donation and [An]→CO π back donation interactions in the cations and anions, for which the electronic reference state of An has both occupied and vacant 5f AOs. The trend of the valence orbital contribution to the metal-CO bonds has the order of 6d≫5f>7s≈7p, with the 5f orbitals of uranium being more important than the 5f orbitals of thorium.

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