1,3-Bis(trimethylsilyl)-2-phenyl-1-aza-3-phosphapropenide Anions as Bidentate Ligands for the Alkaline Earth Metals Magnesium, Calcium, Strontium, and Barium

The tetrahydrofuran complexes of the alkaline earth metal
bis[bis(trimethylsilyl)phosphanides] react with 2
equiv
of benzonitrile to the
bis[1,3-bis(trimethylsilyl)-2-phenyl-1-aza-3-phosphapropenides]
of magnesium (1), calcium
(2), strontium (3), and barium (4).
Whereas 1 and 2 crystallize as
bis(tetrahydrofuran) adducts, the heavier
homologous derivatives 3 and 4 precipitate as
tris(tetrahydrofuran) complexes. The bidentate
trans/trans-isomeric
1,3-bis(trimethylsilyl)-2-phenyl-1-aza-3-phosphapropenide ligands
of all these compounds display very similar
spectroscopic data; however, the influence of the alkaline earth metal
is observed in the silicon NMR. The
nitrogen atoms of these anions have trigonal planar geometry, whereas
the phosphorus atoms are coordinated
trigonal pyramidal. Crystallographic data:  1,
monoclinic, P21/c, a =
1505.3(3) pm, b = 1259.0(2) pm, c
=
2278.4(5) pm, β = 91.91(2)°, Z = 4, wR2
= 0.1279; 2, monoclinic,
P21/c, a =
1522.8(7) pm, b = 1279.2(5) pm,
c = 2314.0(10) pm, β = 92.53(2)°,
Z = 4, wR2 = 0.1513; 3, triclinic,
P1̄, a = 1144.8(3) pm, b =
1579.4(6) pm,
c = 1670.6(6) pm, α = 98.02(3)°, β =
109.67(3)°, γ = 104.18(3)°, Z = 2, wR2
= 0.1745; 4, triclinic, P1̄, a
=
1245.2(4) pm, b = 1907.1(4) pm, c =
2212.3(7) pm, α = 76.46(2)°, β = 76.45(2)°,
γ = 89.31(2)°, Z = 4, wR2
= 0.1409.