Tetraarylpentaborates, [B<sub>5</sub>O<sub>6</sub>Ar<sub>4</sub>]<sup>-</sup> (Ar = C<sub>6</sub>H<sub>4</sub>OMe-4, C<sub>6</sub>H<sub>3</sub>Me<sub>2</sub>-2,6): Their Formation from the Reaction of Arylboronic Acids with an Aryloxorhodium Complex, Structure, and Chemical Properties
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The reactions of (4-methoxyphenyl)boronic acid (1a) and of (2,6-dimethylphenyl)boronic acid (1b) with (PMe3)3Rh-(OC6H4Me-4) (2) in a 5:1 molar ratio result in the formation of cationic rhodium complexes with new tetraarylpentaborates [Rh(PMe3)4]+[B5O6Ar4]- (3a, Ar = C6H4OMe-4; 3b, Ar = C6H3Me2-2,6). The characterization of 3a is as follows: orthorhombic space group P212121, a = 14.7600(5) Å, b = 17.1675(5) Å, c = 19.8654(5) Å; V = 5033.7(3) Å3; Z = 4. The characterization of 3b is as follows: orthorhombic space group Pnma, a = 23.704(6) Å, b = 17.254(8) Å, c = 13.304(2) Å; V = 5441(2) Å3; Z = 4. An intermediate complex, [Rh(PMe3)4]+[Ph3B3O3(OC6H4Me-4)]- (4), was isolated from the reaction of phenylboroxine, (PhBO)3, with 2. The tetraarylpentaborates smoothly undergo hydrolysis to give [Rh(PMe3)4]+[B5O6(OH)4]- (5).
