REACTION OF 2,2'-BIPYRIDINE (BPY) WITH DIRHODIUM CARBOXYLATES - MONO-BPY PRODUCTS WITH VARIABLE CHELATE BINDING MODES AND INSIGHTS INTO THE REACTION-MECHANISM

Synthetic procedures are described for the preparation of mono-2,2'-bipyridine (bpy) complexes possessing the Rh24+ core. Reaction of Rh2(OAc)4(MeOH)2 (1) with 1 equiv of bpy in Me2CO produces Rh2(OAc)4(bpy) (3) in excellent yield. The compound crystallizes in the triclinic space group P1BAR with the following cell parameters at -172-degrees-C: a = 9.883(3), b = 13.078(5), c = 8.323(2) angstrom; a = 97.02(2), beta = 106.90(1), gamma = 94.29(2)-degrees; Z = 2; V = 1015 A3. The structure was solved by direct methods (MULTAN) and refined to values of conventional indices R (R(w)) of 4.72% (4.50%) using 2636 unique reflections with F(o) > 3sigma(F(o)). The two Rh2+ centers are bridged by three eta1:eta1:mu2 AcO-groups over a Rh-Rh bond distance of 2.475(2) angstrom. The coordination spheres of Rh(1) and Rh(2) are completed by chelating bpy and OAc-groups, respectively. The chelating OAc- is asymmetrically bound as a consequence of the axial/equatorial positioning of its two oxygen atoms O(27) and O(29). Compound 3 undergoes a reaction on dissolution in MeOH to give [Rh2(OAC)3(bpy)(MeOH)](OAc).2MeOH (4) as determined by H-1 NMR (1D- and 2D-homonuclear J-resolved spectroscopy), elemental analysis, conductivity measurements, and reactivity studies. Reaction of Rh2(O2CCF3)4(Me2CO)2 (2) with 1 equiv of bpy in CH2Cl2, followed by recrystallization of the product from THF/H2O, produces crystals of Rh2(O2CCF3)4(bpy)(THF)(H2O).THF (5). Compound 5 crystallizes in the monoclinic space group Pc with the folowing cell parameters at -105-degrees-C: a = 10.683(8), b = 9.170(8), c = 17.783(1) angstrom; alpha = 90.00, beta = 103.52(7), gamma = 90.00-degrees; Z = 2, V = 1693 angstrom.3 The structure was solved by direct methods (MITHRIL) and refined to values of R (R(w)) of 5.1% (5.8%) using 2493 unique reflections with F > 3sigma(F). The two Rh2+ centers are bridged by two eta1:eta1:mu2 O2CCF3- ligands across a Rh-Rh separation of 2.520(3) A. Coordination about Rh(1) is completed by two terminal eta1-O2CCF3- groups in equatorial sites with the axial site being occupied by a water molecule. Rh(2) is ligated by a chelating bpy group in the equatorial plane, with axial ligation being provided by a THF molecule. Reaction mixtures comprising [Rh2(OAc)2(MeCN)6] (BF4)2 (6) and 1 equiv of bpy in MeCN lead to isolation of [Rh2(OAc)2(bpy)(MeCN)4](BF4)2.MeCN (7). Compound 7 crystallizes in the triclinic space group P1BAR with the following cell parameters at -170-degrees-C: a = 12.381(2), b = 14.301(2), c = 12.246(2) angstrom; alpha = 94.60(1), beta = 105.84(1), gamma = 117.42(1)-degrees; Z = 2; V = 1797 angstrom3. The structure was solved by direct methods (MULTAN) and refined to values of R (R(w)) of 4.92% (5.74%) using 4723 unique reflections with F > 3sigma(F). The two metals are bridged by two cis eta1:eta1:mu2-OAc-groups that span a Rh-Rh distance of 2.5395(8) angstrom. Coordination at Rh(1) is completed by a chelating bpy in the equatorial plane and an axial MeCN ligand, while the coordination sphere of Rh(2) is completed by two equatorial MeCN molecules and an axial MeCN molecule.