A comparison of the coordination behaviour of R2PCH2BMe2 (R = Me vs. Ph) ambiphilic ligands with late transition metals

A new synthesis that avoids the use of Me2PH is reported for (Me2PCH2BMe2)2, and this method was extended to the synthesis of (Ph2PCH2BMe2)2. The ligand precursor (Me2PCH2BMe2)2 did not react with [{M(μ-Cl)(cod)}2] (cod = 1,5-cyclooctadiene; M = Ir and Rh) or [PtCl2(cod)] at room temperature. However, after 12-48 hours at 65-70 °C, these reactions afforded (a) [Ir(cod)(μ-Cl)(Me2PCH2BMe2)] (1), (b) an equilibrium mixture of (Me2PCH2BMe2)2, [{Rh(μ-Cl)(cod)}2] and [Rh(cod)(μ-Cl)(Me2PCH2BMe2)] (2), and (c) cis-[Pt(μ-Cl)2(Me2PCH2BMe2)2] (3), respectively. By contrast, reactions between the phenyl-substituted analogue, (Ph2PCH2BMe2)2, and [{M(μ-Cl)(cod)}2] (cod = 1,5-cyclooctadiene; M = Ir and Rh) proceeded over the course of 1 hour at 20 °C to generate [M(cod)(μ-Cl)(Ph2PCH2BMe2)] (M = Ir (4) and Rh (5)), indicative of room temperature (Ph2PCH2BMe2)2 dissociation. Room temperature reactions of (Ph2PCH2BMe2)2 with [{Rh(μ-Cl)(coe)2}2] (coe = cyclooctene) using a 1 : 1 or 3 : 1 stoichiometry also afforded [{Rh(coe)(μ-Cl)(Ph2PCH2BMe2)}2] (6) or [RhCl(Ph2PCH2BMe2)3] (7), respectively, where the latter is a borane-appended analogue of Wilkinson's catalyst, and reactions of (Ph2PCH2BMe2)2 with [PtX2(cod)] (X = Cl or Me) yielded cis-[Pt(μ-Cl)2(Ph2PCH2BMe2)2] (8) and cis-[PtMe2(Ph2PCH2BMe2)2] (9). Compounds 1-9, (Me2PCH2BMe2)2 and (Ph2PCH2BMe2)2 were crystallographically characterized. In compounds 1-5 and 8, each chloride co-ligand is coordinated by the borane of an R2PCH2BMe2 ligand. Additionally, in the solid state structure of 6, each bridging chloride ligand interacts weakly with a pendent borane, and in 7, the chloride ligand is tightly coordinated to the borane of one Ph2PCH2BMe2 ligand and weakly coordinated to the borane of a second Ph2PCH2BMe2 ligand. By contrast, both boranes in 9 (and one of the three boranes in 7) are non-coordinated.

New Insights in Frustrated Lewis Pair Chemistry with Azides

The geminal frustrated Lewis pair (FLP) tBu2 PCH2 BPh2 (1) reacts with phenyl-, mesityl-, and tert-butyl azide affording, respectively, six, five, and four-membered rings as isolable products. DFT calculations revealed that the formation of all products proceeds via the six-membered ring structure, which is thermally stable with an N-phenyl group, but rearranges when sterically more encumbered Mes-N3 and tBu-N3 are used. The reaction of 1 with Me3 Si-N3 is believed to follow the same course, yet subsequent N2 elimination occurs to afford a four-membered heterocycle (5), which can be considered as a formal FLP-trimethylsilylnitrene adduct. Compound 5 reacts with hydrochloric acid or tetramethylammonium fluoride and showed frustrated Lewis pair reactivity towards phenylisocyanate.

Unsaturated vicinal frustrated phosphane/borane Lewis pairs as ligands in gold(i) chemistry

The P/B FLP 6, formed by the 1,1-hydroboration of dimesitylphosphino(trimethylsilyl)acetylene with HB(C6F5)2 forms the Au(i)X complexes 9a (X: Cl) and 10a (X: NTf2), respectively. Both show marked AuB interactions. The P/B FLP isomer 8, featuring the bulky SiMe3 substituent at the carbon adjacent to boron forms gold complexes 9b and 10b, both of which show weaker AuB interactions. Complex 10a was employed in the catalytic hydroamination of a series of alkynes with p-toluidine. Complexes 9 and 10 were characterized by X-ray diffraction.

Gold(I) Complexes of the Geminal Phosphinoborane tBu2PCH2BPh2

In this work, we explored the coordination properties of the geminal phosphinoborane tBu₂PCH₂BPh₂ (2) toward different gold(I) precursors. The reaction of 2 with an equimolar amount of the sulfur-based complex (Me₂S)AuCl resulted in displacement of the SMe₂ ligand and formation of linear phosphine gold(I) chloride 3. Using an excess of ligand 2, bisligated complex 4 was formed and showed dynamic behavior at room temperature. Changing the gold(I) metal precursor to the phosphorus-based complex, (Ph₃P)AuCl impacted the coordination behavior of ligand 2. Namely, the reaction of ligand 2 with (Ph₃P)AuCl led to the heterolytic cleavage of the gold-chloride bond, which is favored over PPh₃ ligand displacement. To the best of our knowledge, 2 is the first example of a P/B-ambiphilic ligand capable of cleaving the gold-chloride bond. The coordination chemistry of 2 was further analyzed by density functional theory calculations.