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

Abstract

The preparations and properties are described of a series of [Rh(2)](4+) complexes possessing carboxylates and N-based chelates as ligands. Treatment of Rh(2)(O(2)CR)(4)(MeOH)(2) (R = Me (1), Et (2), Ph (3) or CF(3) (4)) with 2 equiv of 2,2'-bipyridine (bpy) in refluxing MeCN leads to the [Rh(2)(O(2)CR)(2)(bpy)(2)](2+) cation (in complexes 5-9). Reaction of 1 with 1,10-phenanthroline (phen), 4,4'-dimethyl-2,2'-bipyridine (Me(2)bpy), 4,4'-diphenyl-2,2'-bipyridine (Ph(2)bpy), and 4,7-diphenyl-1,10-phenanthroline (Ph(2)phen) leads to the analogous cations of complexes 10-13. Complex 6a, [Rh(2)(OAc)(2)(bpy)(2)(MeCN)(2)](PF(6))(2).2MeCN, crystallizes in monoclinic space group P2(1)/a with the following cell parameters at -172 degrees C: a = 14.433(2) Å, b = 12.810(1) Å, c = 22.78(3) Å, beta = 104.42(3) degrees, Z = 4, and V = 3971.2 Å(3). Complex 9, [Rh(2)(O(2)CCF(3))(4)(bpy)(2)].Me(2)CO, crystallizes in triclinic space group P&onemacr; with the following cell parameters at 20 degrees C: a = 14.260(4) Å, b = 15.375(4) Å, c = 9.709(2) Å, alpha = 105.98(2) degrees, beta = 97.49(2) degrees, gamma = 70.32(2) degrees, Z = 2, and V = 1925.2 Å(3). Both 6a and 9 contain a singly-bonded [Rh(2)](4+) unit with two cis bridging RCO(2)(-) groups and two syn-bpy chelate groups in a near-eclipsed conformation about the Rh-Rh vector. Reaction of [Rh(2)(tpy)(2)(MeCN)(4)](BF(4))(4) (15) (tpy = 2,2':6',2"-terpyridine) with NBu(n)(4)(O(2)CPh) gives [Rh(2)(O(2)CPh)(tpy)(2)(MeCN)(2)](BF(4))(3).MeCN (16). Complex 16 crystallizes in triclinic space group P&onemacr; with the following cell parameters at -168 degrees C: a = 11.684(4) Å, b = 20.373(8) Å, c = 10.451(3) Å, alpha = 93.47(2) degrees, beta = 110.53(1) degrees, gamma = 100.03(2) degrees, Z = 2, and V = 2274.1 Å(3). The cation of 16 consists of a [Rh(2)](4+) unit with a bridging PhCO(2)(-) group and two chelating tpy groups. The solution (1)H 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 Ph(2)bpy (12) and Ph(2)phen (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 extended Hückel calculation on the model compound [Rh(2)(O(2)CH)(2)(bpy)(2)(HCN)(2)](2+), which shows the LUMO to be a bpy-based orbital comprising in-phase, sigma overlap of two bpy pi orbitals, one on each of the syn-bpy groups. In contrast, complex 16 shows no reversible reduction processes. Complex 6b, [Rh(2)(OAc)(2)(bpy)(2)(MeCN)(2)](BF(4))(2), in MeCN may be reduced with sodium acenaphthylenide in THF to give deep blue [Rh(2)(OAc)(2)(bpy)(2)(MeCN)(2)](BF(4)) (14) in high yield. The cation of 14 may also be generated by controlled potential electrolysis of 6a in MeCN at -0.99 V and by heating of a solution of 6a or 6b in alcohol.