Reaction of Nitrogen Chelates with the [Rh2]4+ Core:  Bis-Chelate Products and Demonstration of Reversible, Chelate-Based Reduction Processes

The preparations and properties are described of a series of [Rh2]4+ complexes possessing carboxylates and N-based chelates as ligands. Treatment of Rh2(O2CR)4(MeOH)2 (R = Me (1), Et (2), Ph (3) or CF3 (4)) with 2 equiv of 2,2‘-bipyridine (bpy) in refluxing MeCN leads to the [Rh2(O2CR)2(bpy)2]2+ cation (in complexes 5−9). Reaction of 1 with 1,10-phenanthroline (phen), 4,4‘-dimethyl-2,2‘-bipyridine (Me2bpy), 4,4‘-diphenyl-2,2‘-bipyridine (Ph2bpy), and 4,7-diphenyl-1,10-phenanthroline (Ph2phen) leads to the analogous cations of complexes 10−13. Complex 6a, [Rh2(OAc)2(bpy)2(MeCN)2](PF6)2·2MeCN, crystallizes in monoclinic space group P21/a with the following cell parameters at −172 °C: a = 14.433(2) Å, b = 12.810(1) Å, c = 22.78(3) Å, β = 104.42(3)°, Z = 4, and V = 3971.2 Å3. Complex 9, [Rh2(O2CCF3)4(bpy)2]·Me2CO, crystallizes in triclinic space group P1̄ with the following cell parameters at 20 °C: a = 14.260(4) Å, b = 15.375(4) Å, c = 9.709(2) Å, α = 105.98(2)°, β = 97.49(2)°, γ = 70.32(2)°, Z = 2, and V = 1925.2 Å3. Both 6a and 9 contain a singly-bonded [Rh2]4+ unit with two cis bridging RCO2 - groups and two syn-bpy chelate groups in a near-eclipsed conformation about the Rh−Rh vector. Reaction of [Rh2(tpy)2(MeCN)4](BF4)4 (15) (tpy = 2,2‘:6‘,2‘‘-terpyridine) with NBun 4(O2CPh) gives [Rh2(O2CPh)(tpy)2(MeCN)2](BF4)3·MeCN (16). Complex 16 crystallizes in triclinic space group P1̄ with the following cell parameters at −168 °C: a = 11.684(4) Å, b = 20.373(8) Å, c = 10.451(3) Å, α = 93.47(2)°, β = 110.53(1)°, γ = 100.03(2)°, Z = 2, and V = 2274.1 Å3. The cation of 16 consists of a [Rh2]4+ unit with a bridging PhCO2 - group and two chelating tpy groups. The solution 1H NMR properties of complexes 6−13 and 16 show the complexes to retain their solid-state structures on dissolution. The electrochemical properties of 6−13 and 16 were investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) in MeCN. The bis(bpy) and -(phen) complexes 6−8, 10, and 11 show a reversible, one-electron reduction in the range −0.83 to −0.91 V vs ferrocene and an additional, irreversible reduction and an irreversible oxidation; the Ph2bpy (12) and Ph2phen (13) complexes show a two-electron reversible reduction at −0.61 to −0.76 and a reversible one-electron oxidation at 0.68−0.89 V. Consideration of the potentials as a function of carboxylate and chelate identity leads to the conclusion that the reductions are ligand (chelate)-based. This is supported by an extend