Crystals Containing Conformers: A Rare Case in Which a Solid Closely Reflects a Solution Equilibrium Mixture

The factors influencing the structural, energetic and
dynamic chemistry of complexes with coordinated aromatic
N-donor ligands (L) are not well understood. Complexes of the type
cis,cis,cis-RuCl2(Me2SO)2(L1)(L2)
with L1
= L2 and L1 ≠ L2 (L1 trans to Cl, L2 trans to
Me2SO) have afforded highly informative results.
Here we report
studies on two such complexes. One complex provides a rare example
of a compound in which the solid state
closely reflects the solution state, both in the structure of species
involved in a conformational equilibrium and
in the position of that equilibrium. In effect, the components of
a fluxional equilibrium in solution are trapped,
almost unchanged, in the solid state. Specifically, the complex
cis,cis,cis-RuCl2(Me2SO)2(3,5-lut)(1,2-Me2Im)
(2) (L1 = 3,5-lut = 3,5-lutidine; L2 =
1,2-Me2Im = 1,2-dimethylimidazole) exists as two
conformers (R1 and
R2) in solution and in the solid state (ratio =
0.63(2):0.37(2)); these rotamers differ by a 180° rotation
of 1,2-Me2Im about the Ru−N bond. The orientations of
3,5-lut and 1,2-Me2Im found in the solid state explain
the
solution data well, both in terms of 1H NMR chemical shift
dispersion and rotation rate of the N-donor ligands.
However, there is a displacement of the 1,2-Me2Im
toward the 3,5-lut in the less abundant conformer R2 in
the
solid (N(1)−Ru−N(2) angle 78.1(3)° in R2 vs
93.5(2)° in R1); this clear feature in the solid is a
type of result
that cannot normally be derived from solution investigations or from
the study of one conformer in the solid
state. Although
cis,cis,cis-RuCl2(Me2SO)2(1,2-Me2Im)(Me3Bzm)
(1) (Me3Bzm =
1,5,6-trimethylbenzimidazole)
exists as a mixture of dynamic rotamers in solution, the crystal
structure is typical and reveals the presence of
just the rotamer shown to be most abundant by NMR methods.
1 crystallizes in the triclinic space group
P1̄, Z
= 2, with a = 8.863(4) Å, b =
11.907(6) Å, c = 12.406(6) Å, α =
74.32(4)°, β = 83.27(4)°, and γ =
88.89(3)°;
2 in the orthorhombic space group Pbca,
Z = 8, with a = 12.587(2) Å,
b = 12.329(3) Å, and c = 28.382(5)
Å.
With the added results from this study, it is now clear that the
cis,cis-RuCl2(Me2SO)2
moiety maintains a relatively
fixed arrangement that appears also to dominate in solution. The
stability of this arrangement is sufficient that
the strain evident in the structures appears to be relieved by
displacement of the L ligands.