Daniel Franz

Systematic Investigation of the Ring-Expansion Reaction of N-Heterocyclic Carbenes with an Iminoborane Dihydride

Conversions of iminoboranes with an N-heterocyclic carbene (NHC) result in borane dihydride formation (BDF) concomitant with dihydrogenated NHC. The iminoborane dihydrides are prone to a hydride-mediated ring-expansion reaction (RER) at elevated temperature, that is, the insertion of the boron atom into the adjacent CN bond of the NHC to yield boracycles. Upon conversion of a saturated-backbone NHC with respective iminoborane precursors RER yet occurs at ambient temperature to yield the ring-expanded products. When a less bulky iminoborane is brought in contact with sterically unhindered NHC neither the iminoborane dihydride is stable at room temperature nor is the RER observed to take place upon heating. The conversions of iminoboranes with very bulky NHC do not show BDF at ambient temperature and only in the case of the less hindered borane precursor the RER is found to proceed in a controlled fashion upon heating.

Activation of elemental sulfur by aluminum dihydride: isolation of mono- and bis(hydrogensulfide) complexes of aluminum.

The conversions of the dimeric imidazolin-2-iminato aluminum dihydride {μ-L(Dipp)AlH2}2 (1) with ¼ or ½ equivalents of S8 gave the elusive hydrogensulfide {μ-L(Dipp)Al(H)SH}2 (2) and the bisthiol {μ-L(Dipp)Al(SH)2}2 (3), respectively (L(Dipp)=NC(N(Dipp)CH)2, Dipp=2,6-diisopropylphenyl). Notably, these transformations proceeded in the absence of a promoter. The reaction of {μ-L(Mes)AlH2}2 (4), a less bulky congener of 1, with phenyl disulfide produces the phenyl sulfide {μ-L(Mes)Al(H)SPh}2 (5; L(Mes)=NC(N(Mes)CH)2, Mes=2,4,6-trimethylphenyl). The hitherto unknown compounds 2-5 were characterized by using spectroscopic methods and single-crystal X-ray diffraction analysis.

Isolation and Structure of Germylene-Germyliumylidenes Stabilized by N-Heterocyclic Imines

The ditopic germanium complex FGe(NIPr)2 Ge[BF4 ] (3[BF4 ]; IPr=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) is prepared by the reaction of the amino(imino)germylene (Me3 Si)2 NGeNIPr (1) with BF3 ⋅OEt2 . This monocation is converted into the germylene-germyliumylidene 3[BAr(F) 4 ] [Ar(F) =3,5-(CF3 )2 -C6 H3 ] by treatment with Na[BAr(F) 4 ]. The tetrafluoroborate salt 3[BF4 ] reacts with 2 equivalents of Me3 SiOTf to give the novel complex (OTf)(GeNIPr)2 [OTf] (4[OTf]), which affords 4[BAr(F) 4 ] and 4[Al(OR(F) )4 ] [R(F) =C(CF3 )3 ] after anion exchange with Na[BAr(F) 4 ] or Ag[Al(OR(F) )4 ], respectively. The computational, as well as crystallographic study, reveals that 4(+) has significant bis(germyliumylidene) dication character.

A monotopic aluminum telluride with an Al=Te double bond stabilized by N-heterocyclic carbenes

Aluminum chalcogenides are mostly encountered in the form of bulk aluminum oxides that are structurally diverse but typically consist of networks with high lattice energy in which the chalcogen atoms bridge the metal centres. This makes their molecular congeners difficult to synthesize because of a pronounced tendency for oligomerization. Here we describe the isolation of the monotopic aluminum chalcogenide (LDipN)AlTe(LEt)2 (LDip=1,3-(2,6-diisopropylphenyl)-imidazolin-2-imine, LEt=1,3-diethyl-4,5-dimethyl-imidazolin-2-ylidene). Unique features of (LDipN)AlTe(LEt)2 are the terminal position of the tellurium atom, the shortest aluminum–tellurium distance hitherto reported for a molecular complex and the highest bond order reported for an interaction between these elements, to the best of our knowledge. At elevated temperature (LDipN)AlTe(LEt)2 equilibrates with dimeric {(LDipN)AlTe(LEt)}2 in which the chalcogen atoms assume their common role as bridges between the metal centres. These findings demonstrate that (LDipN)AlTe(LEt)2 comprises the elusive Al=Te double bond in the form of an N-heterocyclic carbene-stabilized species.

Isolation of a Three‐Coordinate Boron Cation with a Boron–Sulfur Double Bond

The reaction of the bulky bis(imidazolin-2-iminato) ligand precursor (1,2-(L(Mes)NH)2-C2H4)[OTs]2 (1(2+)  2[OTs](-); L(Mes) = 1,3-dimesityl imidazolin-2-ylidene, OTs = p-toluenesulfonate) with lithium borohydride yields the boronium dihydride cation (1,2-(L(Mes)N)2-C2H4)BH2[OTs] (2(+)  [OTs](-)). The boronium cation 2(+)  [OTs](-) reacts with elemental sulfur to give the thioxoborane salt (1,2-(L(Mes)N)2-C2H4)BS[OTs] (3(+)  [OTs](-)). The hitherto unknown compounds 1(2+)  2[OTs](-), 2(+)  [OTs](-), and 3(+)  [OTs](-) were fully characterized by spectroscopic methods and single-crystal X-ray diffraction. Moreover, DFT calculations were carried out to elucidate the bonding situation in 2(+) and 3(+). The theoretical, as well as crystallographic studies reveal that 3(+) is the first example for a stable cationic complex of three-coordinate boron that bears a B=S double bond.

Formation of an Imino‐Stabilized Cyclic Tin(II) Cation from an Amino(imino)stannylene

The novel amino(imino)stannylene 1 was prepared by conversion of HNIPr (NIPr = bis(2,6-diisopropylphenyl)imidazolin-2-imino) with one equivalent of Lapperts tin reagent (Sn[N(SiMe3)2]2). Treatment of 1 with DMAP (4-dimethylaminopyridine) yields its Lewis acid-base adduct 2. The reaction of 1 with one equivalent of trimethylsilyl azide results in replacement of the amino group at the tin center by an N3 substituent with concomitant elimination of N(SiMe3)3 to afford dimeric [N3SnNIPr]2 (3). Remarkably, the reaction of 1 with B(C6F5)3 produces the novel tin(II) monocation 4(+)[MeB(C6F5)3](-) comprising a four-membered stannacycle through methyl-abstraction from the trimethylsilyl group.

A Tin Analogue of Carbenoid: Isolation and Reactivity of a Lithium Bis(imidazolin‐2‐imino)stannylenoid

The lithium bis(imino)stannylenoid (NIPr)2 Sn(Li)Cl (1; NIPr=bis(2,6-diisopropylphenyl)imidazolin-2-imino) was prepared by the reaction of LiNIPr with 0.5 equiv of SnCl2 ⋅diox (diox=1,4-dioxane) and the ambiphilic character of the compound was demonstrated by investigations into its reactivity. Treatment of 1 with I2 or MeI yielded the oxidative addition products (NIPr)2 SnI2 and (NIPr)2 Sn(Me)I, respectively. In contrast, the reaction of 1 with one equivalent of Me3 SiCl resulted in the formation of Me3 SiNIPr and the chlorostannylene dimer [NIPrSnCl]2 . Moreover, the substitution reaction of compound 1 with MeLi led to the formation of the methyl-substituted stannate (NIPr)2 Sn(Li)Me.

The twinned crystal structure of tripotassium benzene-1,3,5-tris(trifluoroborate).

The title compound, 3K(+).C(6)H(3)B(3)F(9)(3-), crystallizes as discrete anions and cations which are connected by K...F and K...pi interactions. Two of the -BF(3) residues attached to the aromatic ring adopt a conformation with all F atoms out of the plane of the aromatic ring, whereas the third residue has an almost synperiplanar conformation for one of the F-B-C-C torsion angles. It is remarkable that only one of the K(+) cations interacts with the arene ring and that only one side of the aromatic ring coordinates to a K(+) cation. As a result, a sandwich structure does not occur. All K(+) ions show a coordination mode that cannot be conveniently described with a polyhedron. The anions are located in the (102) planes with the K(+) cations located between these planes. The investigated crystal was a nonmerohedral twin with the fractional contribution of the minor twin component being 0.405 (4). The title compound is the first example of a structure containing a benzene ring substituted with three -BF(3) groups. Only eight other structures have been reported in which a benzene ring carries at least one -BF(3) group. Just five of these contain a K(+) ion, but in none of these is the K(+) ion coordinated to the aromatic ring.

Three‐Coordinate Boron(III) and Diboron(II) Dications

The enhanced electron-donor properties of the bulky bisimino ligand 1,2-(LMes N)2 -C2 H4 (1; LMes =1,3-bis(mesityl)-imidazolin-2-ylidene), mesityl=2,4,6-trimethylphenyl) were exploited for the stabilization of elusive electron-deficient and low-coordinate boron dication species. The reaction of 1 with PhBBr2 or (B(Cl)NMe2 )2 afforded a dicationic mononuclear boron(III) complex or a dicationic dinuclear boron(II) complex, respectively (42+ , 62+ ). The bonding situations of 42+ and 62+ were examined by means of single crystal X-ray diffraction analysis, as well as theoretical methods. Significant allocation of positive charge density into the ligand system was diagnosed for both dications. However, the metalloid-centered Lewis acidity of the dications was confirmed via hydride transfer reactions.

Tetrakis(μ3-9-oxa-10-boraanthracen-10-olato)tetrakis[(diethyl ether)lithium]

The title compound, [Li4O4(C12H8BO)4(C4H10O)4], features a Li4O4 cube. Each Li atom in the cube is additionally coordinated by a diethyl ether molecule and each O atom in the cube carries a 9-oxa-10-boraanthracene residue. The crystal studied was a non-merohedral twin [twin law (-1 0 0 / 0 0 1 / 0 1 0); the contribution of the major twin component refined to 0.553 (3)] emulating apparent tetragonal symmetry, whereas the actual crystal system is just orthorhombic.