W₂Cl₄(NR₂)₂(PR′ ₃)₂ Molecules. 8. Synthesis and Structural Characterization of Both Cis Isomers of Stoichiometry W₂Cl ₄(NHR)₂(PMe₃)₂, R = Et, Pr ⁿ, and Buⁿ. Direct Evidence Favoring an Internal Flip Mechanism for Cis-Trans Isomerization

The triply-bonded ditungsten(III) complexes of stoichiometry W2Cl4(NHR)2(PMe3)2 (R = Et, Prn, Bun) are formed by a two-step, one-pot, synthesis from the reaction of W2Cl6(THF)4 with the appropriate alkylamines NH2R (R = Et, Prn, Bun), followed by phosphine substitution. The dinuclear species W2Cl4(NHR)2(NH2R)2 are the probable intermediate complexes for such reactions. In each case, three isomeric forms have been detected in solution by 31P{1H}NMR spectroscopy, and they are formulated as the trans isomers and the two types of cis isomers possessing Ci and C2 symmetries. It has also been shown that the trans isomers are the initial (kinetic) products irrespective of the R group. The X-ray crystallographic characterization of the Ci isomer of cis-W2Cl4(NHEt)2(PMe3)2 (1), the C2 isomer of cis-W2Cl4(NHPrn)2(PMe 3)2 (2), and the C2 isomer of cis-W2Cl4(NHBun)2(PMe 3)2 (3a,b) has been accomplished. Crystallographic data are as follows: compound 1, triclinic space group P1̄, a = 7.4366(2) Å, b = 8.8618(6) Å, c = 9.352(1) Å, α = 105.262(6)°, β= 109.20(1)°, γ = 94.423(9)°, Z = 1; compound 2, monoclinic space group I2/a, a = 13.1268(8) Å, b = 10.871(1) Å, c = 17.106(1) Å, β= 92.872(5)°, Z = 4; compound 3a, monoclinic space group P21/c, a = 22.058(9) Å, b = 8.562(1) Å, c = 29.768(8) Å, β= 105.642(1)°, Z = 8; compound 3b, P21/n, a = 8.5429(8) Å, b = 18.393(2) Å, c = 16.957(3) Å, β = 90.264(5)°, Z = 4. Complex 1 is the first structurally characterized compound with a W2Cl4N2P2 core of Ci symmetry. The midpoint of the W-W unit in 1 resides on an inversion center and compound 1 is therefore nonchiral. For 2 and 3 (3a,b), the molecules are chiral since each has only C2 symmetry. The W-W distances for 1, 2, 3a, and 3b are 2.3066(6), 2.3170(6), 2.3162(9), and 2.3147(5) Å, respectively. In these molecules there are two intramolecular hydrogen bonds across the W-W triple bond, thus ensuring an essentially eclipsed geometry in each case. Each of them has the P and N atoms on each W atom cis to each other. The observation and, in one case, 1, the isolation of the unprecedented cis isomers of Ci symmetry provide support for our earlier proposal that the isomerization processes occur by an "internal flip" of the W2 unit within the quasi-cubic set of coordinated ligand atoms.