Ligand properties of boryl ligands in bis-boryl rhodium(III) complexes: a case study.

The oxidative addition of five diborane(4) derivatives, symmetrical and unsymmetrical, to [Rh(PMe3)3Cl] was studied. Only for the more electron poor diboron derivatives, B2cat2, B2pin2 and catB-Bpin the resulting octahedral bis-boryl complexes [(PMe3)3Rh(boryl)2Cl] were obtained, while for the more electron rich congeners only the equilibrium oxidative addition (catB-Bdmab) or no significant reaction (pinB-Bdmab) was observed (pin = (OCMe2)2, cat = 1,2-O2C6H4, dmab = 1,2-(NMe)2C6H4). By abstraction of the chlorido ligand with NaBArF (BArF = tetrakis-[3,5-bis-(trifluormethyl)-phenyl]-borat) in the presence of a neutral ligand (L = PMe3, MeCN, MeNC) the corresponding cationic octahedral complexes [(PMe3)3Rh(boryl)2L]+ were obtained. All isolated complexes were fully characterised including single crystal X-ray diffraction and heteronuclear, temperature dependent NMR spectroscopy. Whilst the complexes [(PMe3)3Rh(boryl)2Cl] and [(PMe3)3Rh(boryl)2L]+ show many similarities, their detailed structural and spectroscopic properties depend crucially on the properties of both boryl ligands.

N-H deprotonation of a diaminodialkoxido diborane(4) - a structural study on bifunctional Lewis acids/bases and their dimerisation to B(sp2)2B(sp3)2N2 six membered rings.

The N-H deprotonation of the diaminodialkoxido diborane(4) pinB-Bdab (1) (pin: (OCMe2)2, dab: 1,2-(NH)2C6H4), is crucial for the electrophilic N-functionalisation towards unsymmetrical diborane(4) reagents. An N-H deprotonated diborane(4) comprises Lewis basic nitrogen atoms and at the same time Lewis acidic boron atoms. This bifunctionality governs its reactivity and structural chemistry. Whilst bases such as Na(hmds), tBuLi or Li(tmp) readily effect a single deprotonation of 1, the second deprotonation is less straightforward and cleanly only achieved with Li(tmp) as a strong but little nucleophilic base. The N-H deprotonated diborane(4) derivatives readily dimerise to give B(sp2)2B(sp3)2N2 six-membered ring Lewis base adducts. The structural chemistry of this class of compounds was studied in detail in the solid state by single crystal X-ray diffraction as well as in solution by NMR spectroscopy.

Terminal versus Bridging Boryl Coordination in N-Heterocyclic Carbene Copper(I) Boryl Complexes: Syntheses, Structures, and Dynamic Behavior.

The B-B bond activation of the diborane(4) derivatives B2cat2 with the copper(I) alkoxido complex [(SIDipp)Cu-O tBu] delivers, depending on the solvent, either the linear boryl complex [(SIDipp)Cu-Bcat] from PhMe or the µ-boryl complex [((SIDipp)Cu)2Bcat][cat2B] from THF. The relevant conversion of the linear boryl complex to the µ-boryl complex occurs in the polar solvent via formal boryl anion abstraction by the Lewis acid catB-O tBu, concomitantly formed during the B-B activation. With Lewis acids such as BPh3 or [CPh3][BArF] (reversible), boryl abstraction from the linear complexes [(SIDipp)Cu-Bcat] or [(SIDipp)Cu-Bdmab] occurs and results in the µ-boryl complexes [((SIDipp)Cu)2Bcat/dmab][Ph3B-Bcat/dmab] and [((SIDipp)Cu)2Bcat][BArF]. The formation of [((SIDipp)Cu)2Bcat][cat2B] is generally accompanied by the concomitant formation of the µ-hydrido complex [((SIDipp)Cu)2H][cat2B]. The spiroborate [cat2B]- is formed from the initially formed Lewis acid/base adduct [catB-B(O tBu)cat]- presumably in a process that involves the glass surface of the reaction vessel. All complexes are thoroughly characterized structurally as well as spectroscopically, in particular with respect to the dynamic behavior of the µ-boryl complexes in solution.