Mixed chloride/amine complexes of dimolybdenum(II,II). 6. Stepwise substitution of amines by tertiary phosphines and vice versa: Stereochemical hysteresis

The substitution reactions of primary amines by tertiary phosphines in quad ruply bonded dimolybdenum(II,II) complexes Mo2Cl4(NH2R)(4) have been studie d. The exchange reaction has been shown to result at room temperature in di substituted species Mo2Cl4(NH2R)(2)(PR3)(2) (PR3 = PMe3, NH2R = NH2Prn (1a) , NH2But (2a), NH2Cy (3a); PR3 = PMe2Ph, NH2R = NH2Cy (4a)), while heating is needed to obtain fully substituted complexes Mo2Cl4-(PR3)(4) The crystal structure of disubstituted products has been investigated by X-ray crystal lography and revealed that they all belong to the alpha-isomer, having both phosphine groups at the same Mo atom. Crystal data are as follows: for la, tetragonal space group I4(1)/a with a 17.737(2) Angstrom, c 15.6915(6) Ang strom, and Z = 8; for 3a, monoclinic space group P2(1) with a 10.963(3) Ang strom, b = 10.117(2) Angstrom, c 13.323(4) A, beta = 90.05(2)degrees, and Z = 2; for 4a, triclinic space group P1 with a = 9.329(3) Angstrom, b = 10.2 06(2) Angstrom, 18.975(3) Angstrom, a = 85.45(2)degrees, beta = 87.10(1)deg rees, gamma = 80.88(1)degrees, and Z = 2. The substitution processes for th e direct and reverse reactions have been monitored by P-31 NMR. They both p roceed in a stepwise manner; however, a stereochemical hysteresis is taking place, i.e., the back reaction, the substitution of phosphines by amines, goes through another isomer of Mo2Cl4(NH2R)(2)(PR3)(2), having phosphine li gands on different Mo atoms. This beta-isomer is more thermodynamically sta ble and can be obtained by thermal conversion of the alpha-form. All chemic al equilibria studied in the paper have been explained as governed by a hig her trans effect of PR3 groups compared to NH2R groups.