Reduction–oxidation properties of organotransition-metal complexes. Part 30. Triazenido-bridged carbonylrhodium bipyridyl complexes with [Rh₂]²⁺, [Rh₂]³⁺, and [Rh₂]⁴⁺cores, and the X-ray structures of [Rh₂(CO)₂(bipy)(µ-RNNNR)₂][BF₄]·CH₂Cl₂and [{Rh₂I(CO)(bipy)(µ-RNNNR)₂}₂][PF₆]₂·2.5CH₂Cl₂(R =p-tolyl)

Abstract

The reaction of [{Rh(CO)2(µ-RNNNR)}2](1; R =p-tolyl) with 2,2′-bipyridyl (bipy) in boiling n-heptane gives [Rh2(CO)2(bipy)(µ-RNNNR)2](2) which undergoes two one-electron oxidations at a platinum-bead electrode in CH2Cl2. Chemical oxidation of (2) with [Fe(η-C5H5)2]+ gives [Rh2(CO)2(bipy)(µ-RNNNR)2]+(2+) the X-ray structure of which, as a dichloromethane solvate of the [BF4]– salt, shows a Rh–Rh separation of 2.646(1)Å consistent with a formal metal–metal bond order of 0.5. The cation (2+) undergoes carbonyl substitution reactions with PPh3 or P(OPh)3 to give [Rh2(CO)L(bipy)(µ-RNNNR)2][PF6](3+), and with iodide ion to yield [Rh2l(CO)(bipy)(µ-RNNNR)2](4) which may also be prepared directly from (2) and iodine at –78 °C. The cyclic voltammogram of (4) in CH2Cl2 shows one reduction and three oxidation waves; chemical oxidation of (4) with [Fe(η-C5H5)2][PF6] gives the tetranuclear complex [{Rh2l(CO)-(bipy)(µ-RNNNR)2}2][PF6]2(5). X-ray structural studies on the CH2Cl2 solvate of (5) show that the dication is centrosymmetric, having two binuclear (4+) units [Rh–Rh distance within each, 2.544(1)Å] linked by asymmetric iodide bridges, Rh–laxial, 2.760(1)Å and Rh–lequatorial 2.670(1)Å. Complex (5) reacts with I– to give [Rh2l2(CO)(bipy)(µ-RNNNR)2](6), which may also be made directly from (2) and iodine, and with Na[S2CNMe2] to yield [Rh2(CO)(S2CNMe2)(bipy)-(µ-RNNNR)2][PF6](7). The complexes [3+; L = PPh3 or P(OPh)3] react with chloride ion in the presence of [Fe(η-C5H5)2][PF6] giving [Rh2Cl(CO)L(bipy)(µ-RNNNR)2][PF6][8; L = PPh3 or P(OPh)3]. Both (7) and (8) undergo one-electron oxidation and reduction at a platinum electrode. The mechanism of the oxidative addition of iodine to (2) to give (5) and the low-temperature e.s.r. spectra of the paramagnetic [Rh2]3+ complexes (2+), (3+), and (4) are discussed.