Susanne Klein

Borylene complexes (BH)L2 and nitrogen cation complexes (N+)L2: isoelectronic homologues of carbones CL2.

Quantum chemical calculations using DFT (BP86, M05-2X) and ab initio methods (CCSD(T), SCS-MP2) have been carried out on the borylene complexes (BH)L(2) and nitrogen cation complexes (N(+))L(2) with the ligands L=CO, N(2), PPh(3), NHC(Me), CAAC, and CAAC(model). The results are compared with those obtained for the isoelectronic carbones CL(2). The geometries and bond dissociation energies of the ligands, the proton affinities, and adducts with the Lewis acids BH(3) and AuCl were calculated. The nature of the bonding has been analyzed with charge and energy partitioning methods. The calculated borylene complexes (BH)L(2) have trigonal planar coordinated boron atoms which possess rather short B-L bonds. The calculated bond dissociation energies (BDEs) of the ligands for complexes where L is a carbene (NHC or CAAC) are very large (D(e) =141.6-177.3 kcal mol(-1)) which suggest that such species might become isolated in a condensed phase. The borylene complexes (BH)(PPh(3))(2) and (BH)(CO)(2) have intermediate bond strengths (D(e) =90.1 and 92.6 kcal mol(-1)). Substituted homologues with bulky groups at boron which protect the boron atom from electrophilic attack might also be stable enough to become isolated. The BDE of (BH)(N(2))(2) is much smaller (D(e) =31.9 kcal mol(-1)), but could become observable in a low-temperature matrix. The proton affinities of the borylene complexes are very large, particularly for the bulky adducts with L=PPh(3), NHC(Me), CAAC(model) and CAAC and thus, they are superbases. All (BH)L(2) molecules bind strongly AuCl either η(1) (L=N(2), PPh(3), NHC(Me), CAAC) or η(2) (L=CO, CAAC(model)). The BDEs of H(3)B-(BH)L(2) adducts which possess a hitherto unknown boron→boron donor-acceptor bond are smaller than for the AuCl complexes. The strongest bonded BH(3) adduct that might be isolable is (BH)(PPh(3))(2)-BH(3) (D(e) =36.2 kcal mol(-1)). The analysis of the bonding situation reveals that (BH)-L(2) bonding comes mainly from the orbital interactions which has three major contributions, that is, the donation from the symmetric (σ) and antisymmetric (π(||)) combination of the ligand lone-pair orbitals into the vacant MOs of BH L→(BH)←L and the L←(BH)→L π backdonation from the boron lone-pair orbital. The nitrogen cation complexes (N(+))L(2) have strongly bent L-N-L geometries, in which the calculated bending angle varies between 113.9° (L=N(2)) and 146.9° (L=CAAC). The BDEs for (N(+))L(2) are much larger than those of the borylene complexes. The carbene ligands NHC and CAAC but also the phosphane ligands PPh(3) bind very strongly between D(e) =358.4 kcal mol(-1) (L=PPh(3)) and D(e) =412.5 kcal mol(-1) (L=CAAC(model)). The proton affinities (PA) of (N(+))L(2) are much smaller and they bind AuCl and BH(3) less strongly compared with (BH)L(2). However, the PAs (N(+))L(2) for complexes with bulky ligands L are still between 139.9 kcal mol(-1) (L=CAAC(model)) and 168.5 kcal mol(-1) (L=CAAC). The analysis of the (N(+))-L(2) bonding situation reveals that the binding interactions come mainly from the L→(N(+))←L donation while L←(N(+) )→L π backdonation is rather weak.

Donor-acceptor-stabilized silicon analogue of an acid anhydride.

A stable silicon analogue of an acid anhydride {PhC(Bu(t)N)(2)}Si{═O·B(C(6)F(5))(3)}O-Si(H){═O·B(C(6)F(5))(3)}{(NBu(t))(HNBu(t))CPh} (4) with a O═Si-O-Si═O core has been prepared by treating monochlorosilylene PhC(Bu(t)N)(2)SiCl (1) with H(2)O·B(C(6)F(5))(3) in the presence of NHC (NHC = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene). Compound 4 has been characterized by elemental analysis and multinuclear NMR spectroscopic investigations. The molecular structure of 4 has been established by single-crystal X-ray diffraction studies, and DFT calculations support the experimental results.

N-Heterocyclic carbenes versus transition metals for stabilizing phosphinyl radicals

It is shown that vanadium-iminato ligands are more efficient than imidazolidin-2-iminato substituents to delocalize the spin density from a phosphorus nucleus. However, the latter is stabilizing enough to allow for the isolation and characterization in the liquid and solid states of a neutral phosphinyl radical.

Carbodiylides C(ECp*)2 (E=B–Tl): Another Class of Theoretically Predicted Divalent Carbon(0) Compounds†

It was recently recognized that there are organic compounds with the general formula CL2 where the carbon atom retains its four valence electrons as two lone pairs and where the chemical bonding to the donor ligands L takes place through donor–acceptor interactions L!C !L.[1] The first example for which this bonding mode was proposed is the carbodiphosphorane (CDP) C(PPh3)2, which was synthesized in 1961 and structurally characterized by an X-ray analysis in 1978. After realizing the particular bonding situation in CDPs, we carried out quantum-chemical calculations of the hitherto unknown carbodicarbenes C(NHC)2 (NHC = Nheterocyclic carbene), which possess unusual C!C donor– acceptor bonds where a divalent carbon(II) atom acts as s donor while the divalent carbon(0) atom is a s acceptor. Carbodicarbenes have since been synthesized and structurally characterized by Bertrand et al., and they were extensively studied by F rstner and co-workers. New carbodicarbenes and related compounds have recently been calculated in a theoretical study that showed that other divalent carbon(0) compounds had already been previously synthesized, but the donor–acceptor bonds had not been identified. It was suggested that, in the light of recent theoretical and experimental findings, there should be a rethinking regarding the bonding of carbon. The name “carbone” was coined for compounds CL2, which, owing to the existence of two lone pairs, are s and p donors, whereas carbenes CR2, which have one lone pair at carbon, are s donors and (weak) p acceptors. Herein we present quantum-chemical calculations that suggest that there is another class of stable carbones CL2, where L is a Group 13 diyl ligand ECp* (E = B–Tl). Transition metal complexes with ligands ECp* have been the subject of extensive experimental and theoretical investigations since the first stable complex [(CO)4Fe-AlCp*] was isolated and characterized by X-ray analysis in 1997 by Fischer et al. Further work was reported with Group 13 homologues [(CO)4Fe-ECp*] where E = B, Ga. [8b,c] Numerous other Group 13 complexes with ligands ER, where R is either a strong p donor or a very bulky substituent, have since been reported. Very recently, the first homoleptic complex with an ECp* substituent [Mo(GaCp*)6] has been synthesized. [9]