FAST ORTHO-METALATION REACTIONS IN BINUCLEAR DIRHODIUM COMPOUNDS - SYNTHESES AND MOLECULAR-STRUCTURES OF A MONOMETALLATED COMPOUND AND 2 DOUBLY METALATED COMPOUNDS WITH HEAD-TO-HEAD CONFIGURATIONS

The reactions of Rh2(O2CCH3)4(MeOH)2 and triphenylphosphine, in 1:4, 1:2, and 1:1 molar ratios, have been spectroscopically studied using different reaction conditions. Very reactive intermediate products, Rh2(O2CCH3)3[(C6H4)PPh2](HO2CCH3)2 (1(HO2CCH3)2) and Rh2(O2CCH3)3[(C6H4)PPh2][PPh3](HO2CCH3) (2(HO2CCH3)) have been isolated. The structure of 1(HO2CCH3)2 has been determined by X-ray diffraction. Crystal data: space group P1BAR, a = 9.807 (6) angstrom, b = 19.822 (10) angstrom, c = 8.476 (5) angstrom, alpha = 80.10 (2)-degrees, beta = 111.02 (2)-degrees, gamma = 93.67 (2)-degrees, Z = 2, 3374 reflections, R = 0.0289. 1(HO2CCH3)2 readily reacts with an excess of triphenylphosphine at room temperature to form the already reported doubly metalated compound Rh2(O2CCH3)2[(C6H4)PPh2]2(PPh3)2 (3(PPh3)2) with a head-to-tail structure. However under thermal conditions 1(HO2CCH3)2 reacts with excess of triphenylphosphine giving in addition to 3(PPh3)2, Rh2(O2CCH3)2[(C6H4)PPh2]2(PPh3)(HO2CCH3) (4-(PPh3,HO2CCH3)), a doubly metalated compound with the orthometalated ligands in a head-to-head configuration. The structure of the closely related bis(acetic acid) adduct Rh2(O2CCH3)2[(C6H4)PPh2]2(HO2CCH3)2 (4(HO2CCH3)2) has been determined by X-ray diffraction. Crystal data: space group C2/c, a = 23.443 (9) angstrom, b = 19.868 (8) angstrom, c = 20.158 (9) angstrom, beta = 106.60 (2)-degrees, Z = 8, 4873 reflections, R = 0.0451. The same mixture of compounds is formed from the reaction of Rh2(O2CCH3)4(MeOH)2 and a 4-mol excess of PPh3 in refluxing acetic acid. Para-substituted triarylphosphines P(p-XC6H4)3 (X = CH3, Cl) behave in a similar way, yielding a mixture of compounds with head-to-tail and head-to-head configurations. The structure of the compound Rh2(O2CCH3)2[(ClC6H3)P(p-ClC6H4)2]2(HO2CCH3)2 (8(HO2CCH3)2), having a head-to-head configuration, has been determined by X-ray diffraction. Crystal data: space group C2/c, a = 36.063 (9) angstrom, b = 16.558 (6) angstrom, c = 23.531 (8) angstrom, beta = 125.34 (2)-degrees, Z = 8, 4316 reflections, R = 0.0525. Two different reaction pathways are proposed to justify this chemical behavior.