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Activation by multiple internal hydrogen bonds is necessary
to catalyze the reaction, and the reversible nature of the Fe–N
dative bonds allows self-selection of the most thermo-dynamically
favorable products. While the alcoholic reactant favors a single
isomer upon assembly, greater steric bulk is added upon reaction,
forcing isomerization to a statistical mixture of M2L3 products
during the reaction process.
Acknowledgement is made to the National Science Founda-
tion (CHE-1151773) as well as UC Riverside for support of this
research.
Fig. 4 (a) Characterization of 3: (a) predicted and observed ESI-MS of
parent ion [Fe2ÁL3Á(ClO4)3ÁNaÁCH3CN]2+; (b) cartoon representation of the
observed isomers of 3 in solution.
Notes and references
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ability of the urethane group introduced upon reaction.
Attempted recovery of the urethane-containing bis-imine ligand
by hydrolysis of 3 in 50/50 D2O–CD3CN led to cleavage of the
complex, imine, and urethane, and no product was observed.
As all the products of the post-synthetic modification are M2L3
mesocates, it must be concluded that introduction of larger groups
on the mesocate interior disfavours the in3 isomer and a mixture of
conformers at C–O (i.e. in3, in2Áout, inÁout2 and out3, see Fig. 4b) is
formed. Even though the in3 isomer of 2 was predominant, this is
controlled by sterics and packing of the ligand around the metal.
After reaction, greater steric bulk is added to the ligand, disfavouring
the in3 isomer and causing isomerisation to a statistical mixture of
1
M2L3 products. H, 13C and DOSY NMR analysis (see ESI†) corro-
borates the assignment: the four urethane NH peaks (B9.0 ppm,
Fig. 3b) provide the clearest evidence. Integration shows that
while the populations of the isomers are similar, they are
present in different amounts, and the four peaks do not
correspond to diastereotopic protons in a single assembly.
Analysis of the 1H spectrum of 3 at temperatures ranging from
À40 1C to +75 1C shows the populations vary slightly with
temperature, corroborating that different isomers are formed.
The other alkylisocyanate products 4 and 6 show similar 1H NMR
spectra to 3, and the four possible isomers are observed. For
a-methybenzyl derivative 5, however, the newly added group
is so large it completely disfavours formation of any of the
in isomers, giving only the out3 isomer (see ESI† for illustrative
molecular modeling).
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In conclusion, we have shown that self-assembled meso-
cates can promote their own post-synthetic modification.
1380 | Chem. Commun., 2014, 50, 1378--1380
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