Zinc(II) and Cadmium(II) Monohydroxide Bridged, Dinuclear Metallacycles: A Unique Case of Concerted Double Berry Pseudorotation

The
reactions of M­(ClO4)2·6H2O [M
= Zn­(II), Cd­(II)] and the ligands m-bis­[bis­(1-pyrazolyl)­methyl]­benzene, Lm, or m-bis­[bis­(3,5-dimethyl-1-pyrazolyl)­methyl]­benzene, Lm*, in the presence of a
base yield the hydroxide bridged dinuclear metallacycles [M2(μ-OH)­(μ-L)2]­(ClO4)3, L = Lm, M = Zn­(II) (1); L = Lm*, M =
Zn­(II) (2), Cd­(II) (3). In the solid state,
the coordination environment of the metals is distorted trigonal bipyramidal
with the bridging hydroxide in an equatorial position and M-O-M angles
greater than 161°. The observation of two equal intensity resonances
for each type of pyrazolyl-ring hydrogen in the 1H NMR
for all three complexes coupled with the determination of the hydrodynamic
radius based on the diffusion coefficient of 1 that matches
that observed in the crystal structure, demonstrate this structure
is retained in solution. Additional proof of the dinuclear structures
in solution is given by the 113Cd NMR spectrum of [Cd2(μ-OH)­(μ-Lm*)2]­(ClO4)3 showing 111/113Cd satellites (J111Cd‑113Cd = 173 Hz). Complex 1 is dynamic in solution, with the resonances for each type
of pyrazolyl-ring hydrogen broadening and averaging at higher temperatures.
Detailed variable temperature studies show that ΔGpz⧧ = 15.2(±0.2) kcal/mol, ΔHpz⧧ = 6.6(±0.1) kcal/mol,
and ΔSpz⧧ = −28.8(±0.4)
cal/mol·K at 25 °C for this process. The same ΔG⧧ value for the dynamic process was also
determined by saturation transfer experiments. The most plausible
mechanism for this dynamic process, which exchanges the axial and
equatorial positions of the pyrazolyl rings in the trigonal bipyramidal
arrangement, involves Berry pseudorotation at both metal sites using the bridging oxygen atom as the pivot ligand, coupled with
the ring flip of the ligand’s phenylene spacer by 180°,
a rearrangement process we termed the “Columbia Twist and Flip”.
This process was shown to be influenced by trace amounts of water
in the solvent, with a linear relationship between the water concentration
and ΔGpz⧧; increasing
the water concentration lowers ΔGpz⧧. Spin saturation transfer experiments demonstrated
the exchange of the hydrogens between the water in the solvent and
the bridging hydroxide group, with ΔGOH⧧ = 16.8(±0.2) kcal/mol at 25 °C, a value
larger than the barrier of ΔGpz⧧ = 15.2(±0.2) kcal/mol for the “Columbia
Twist and Flip”. Compounds 2 and 3 do not show dynamic behavior involving the pyrazolyl-rings in solution
because of steric crowding caused by the methyl group substitution,
but do show the exchange between the water in the solvent and the
bridging hydroxide group.