Bora-amidinate as a cooperative ligand in group 2 metal catalysis.

Syntheses and crystal structures of the monomeric bora-amidinate (bam) complexes DIPPNBN-Mg·(THF)3 and DIPPNBN-Ca·(THF)4 are presented; DIPPNBN = HB[N(2,6-iPr2-C6H3)]2. The simplicity of their 1H NMR spectra in THF-d8 suggest that their monomeric solid state structures are retained in solution. DIPPNBN-Mg·(THF)3 in C6D6, however, is in equilibrium with a dimeric species. Calculations (B3PW91/6-311++G**) reveal a very high localized negative charge (NPA: -1.103) on the N atoms in DIPPNBN-Mg. The strongly basic properties of the bam ligand are in agreement with catalytic activity of these complexes in the intramolecular alkene hydroamination. A mechanism is proposed in which the bam ligand is non-innocent and cooperative, playing an active role in substrate deprotonation and product protonation.

Thermal decomposition of mono- and bimetallic magnesium amidoborane complexes.

Complexes of the type [(DIPPnacnac)MgNH(R)BH(3)] have been prepared (DIPPnacnac=CH{(CMe)(2,6-iPr(2)C(6)H(3)N)}(2)). The following substituents R have been used: H, Me, iPr, DIPP (DIPP=2,6-diisopropylphenyl). Complexes [(DIPPnac- nac)MgNH(2)BH(3)].THF, [{(DIPPnac- nac)MgNH(iPr)BH(3)}(2)] and [(DIPPnacnac)MgNH(DIPP)BH(3)] were structurally characterised. The Mg amidoborane complexes decompose at a significantly higher temperature (90-110 degrees C) than the corresponding Ca amidoborane complexes (20-110 degrees C). The complexes with the smaller R substituents (H, Me) gave a mixture of decomposition products of which one could be structurally characterised as [{(DIPPnacnac)Mg}(2)(H(3)B-NMe-BH-NMe)].THF. [{(DIPP- nacnac)MgNH(iPr)BH(3)}(2)] cleanly decomposed to [(DIPPnacnac)MgH], which was characterised as a dimeric THF adduct. The amidoborane complex with the larger DIPP-substituent decomposed into a borylamide complex [(DIPPnacnac)MgN(DIPP)BH(2)], which was structurally characterised as its THF adduct. Bimetallic Mg amidoborane complexes decompose at lower temperatures (60-90 degrees C) and show a different decomposition pathway. The dinuclear Mg amidoborane complexes presented here are based on DIPPnacnac units that are either directly coupled through N-N bonding (abbreviated NN) or through a 2,6-pyridylene bridge (abbreviated PYR). Crystal structures of [PYR-{Mg(nBu)}(2)], [PYR-{MgNH(iPr)BH(3)}(2)], [NN-{MgNH(iPr)BH(3)}(2)]THF and the decomposition products [PYR-Mg(2)(iPrN-BH-iPrN-BH(3))] and [NN-Mg(2)(iPrN-BH-iPrN-BH(3))].THF are presented. The following conclusions can be drawn from these studies: i) The first step in the decomposition of a metal amidoborane complex is beta-hydride elimination, which results in formation of a metal hydride complex and R(H)N=BH(2), ii) depending on the nature of the metal, the metal hydride is either stable and can be isolated or it reacts further, iii) amidoborane anions with small R substituents decompose into the dianionic species (RN-BH-RN-BH(3))(2-), whereas large substituents result in formation of the borylamide RN==BH(2)(-) and iv) enforced proximity of two Mg amidoborane units results in decomposition at a significantly lower temperature and cleanly follows the BNBN pathway.

Peculiar binding modes of a ligand with two adjacent beta-diiminato binding sites in alkali and alkaline-earth metal chemistry.

Reaction of the diprotic ligand [Xanthdim]H(2) (a ligand system where two adjacent beta-dialdimine units are linked by a xanthyl backbone) with 2 equiv of potassium hydride or benzylcesium gave access to bimetallic alkali metal complexes. These complexes were structurally characterized by X-ray diffraction, which showed that the beta-diiminato units are orientated in a W-conformation. Treatment with 2 equiv of Mg[N(SiMe(3))(2)](2)(THF)(2) led to the formation of the heteroleptic complex [Xanthdim][MgN(SiMe(3))(2)(THF)](2), that crystallized as a highly strained monomer. The heteroleptic Mg complex is remarkably stable against ligand exchange but is not active in CO(2)/cyclohexene oxide copolymerization. Reaction with Ca[N(SiMe(3))(2)](2)(THF)(2) gave the homoleptic complex [Xanthdim][Ca(THF)]. Both alkaline-earth metal complexes display considerable distortions in their solid state structure.

Superbulky metallocene complexes of the heavier alkaline-earth metals strontium and barium.

Superbulky sandwich complexes with the cyclopentadienyl ligand (4-nBu-C(6)H(4))(5)Cp (abbreviated here as Cp(BIG)) have been prepared by reaction of Cp(BIG)H with benzylic strontium or barium reagents. Both metallocenes have been structurally characterized by single crystal X-ray diffraction. Even for the larger metallocene (Cp(BIG))(2)Ba a highly symmetric sandwich complex with parallel Cp rings was isolated (Cp(center)-Ba = 2.667(1) A). Both structures show evidence for a C-HC(pi) network between the Cp ligands. These attractive forces induce an inward out-of-plane bending of the aryl substituents (Sr 3.4(2) degrees; Ba 5.3(2) degrees). A linear correlation between this bending angle and metal size has been found.