From: Starcrown (stch8002$##$bureau.ucc.ie)
Date: Thu Jun 17 1999 - 14:23:30 EDT
zabicky$##$bgumail.bgu.ac.il wrote:
>Hello Fergus,
>
>Your narrative of the problem is somehow unclear. When you say "with a
>large transition metal organometallic 'substituent' attached to the central
>(i.e. 3-) carbon" is that an assumption, or did you prove the presence of a
>C-metal bond by whatever method?
OK - I was economical with the background details because what I *really*
wanted was suggestions for very strong H-bond acceptors - on which topic,
alas, the list has been utterly silent 8-( (Surely SOMEONE out there in
orglist-land must have ideas on this?)
So let's see if I can make the overall problem clearer - but this, I'm
afraid, will require a little patience from the reader:
Our starting point is a 3-substituted-pentane-2,4-dione aka
3-substituted-acacH. In this case the 3-'substituent' is an octahedral
organomolybdenum structure that can be represented as Tp'(CO)2Mo(triple
bond)C- where Tp' is a tridentate chelating ligand and CO is carbon
monoxide. The whole organomolybdenum structure is attached to the acacH
'backbone' via the free valence on the carbon of the Mo(triple bond)C-
(i.e. metal carbyne) fragment. From this description it is, I hope, clear
that the Mo atom is remote from the acacH moiety and geometrically
positioned so that it couldn't be chelated by the latter - even were it
not the case that the other ligands on Mo ensure that there are no vacant
coordination sites available in any case.
We have also made an analogous derivative of methylacetoacetate with the
same organomolybdenum 'substituent' on the carbon atom between the ketonic
and ester carbonyls.
There is no ambiguity as to the nature of these species - the acacH
derivative has been characterised by microanalysis, IR, H and C NMR and we
have a good X-ray crystal structure. We have similar data - apart from an
X-ray structure - on the methylacetoacetate derivative. Both compounds
behave, for the most part, just like acacH itself in that we can
deprotonate them and make bimetallic complexes in which the second metal IS
chelated by the beta-diketonate structure.
Our real 'problem' began with the serendipitou discovery that quenching
(i.e. reprotonating) DMSO solutions of the conjugate anion of either the
substituted acac or acetoacetate compounds under a variety of conditions
yielded the expected neutral stable acacH or acetoacetate derivative
together with, in each case, a second labile compound which can be
separated from the normal 'stable' compounds by differential solubility.
Quenching with water alone will produce the 'labile' compounds - but in
rather poor yield. Much better results are obtaind by quenching with
aqueous triethylamine. Here, in each case, H NMR shows that the 'labile'
compounds produced by this method contain Et3N. With the acacH derivative
the amount of amine present varies from preparation to preparation but with
the acetoacetate derivative it is reproducibly equimolar (microanalysis and
H NMR). However, in the latter case, repeated extraction of the solid
'labile' material with cyclohexane removes Et3N until the amine-substrate
ratio stabilises at ca. 1:3 but without any other significant change in the
spectroscopic properties of the species.
Samples of the 'labile' materials produced by different methods have very
similar IR and NMR spectra. The most obvious differences between the
'labile' and 'stable' acacH compounds are the presence of a strong
'organic' carbonyl band in the IR of the 'labile' material that is not
present in the spectrum of the 'stable' compound and the disappearence of
the H NMR AcacH enolic proton resonance in the 'labile' material.
In both cases the new materials are very readily converted back to the
'normal' compounds - e.g. by shaking a dichloromethane solution with dilute
aqueous HCl or by filtering it through silica or alumina.
The possible structures that we have considered for the 'labile' materials are:
(1) Relatively stable diketo isomers - pretty unlikely, I would think.
(2) Relatively stable trans-enolic isomers - again, not very likely -
unless there is some special structural feature that enhances the stability
of the trans-enol. H-bonding of the enolic OH to the electron-rich Mo-C
triple bond of the metal carbyne structure or to O of a CO ligand might be
considered.
(3) Addition or conjugate addition of a nucleophile (amine, water?) to the
acac or acetoacetate structures or ....
(4) Hydrogen bonding between a Lewis base (amine, water?) and the enolic
proton the acac or acetoacetate structures. Neither of options (3) or (4)
are easily reconciled with the fact that the Et3N-substrate ratio of the
'labile' acetoacetate compound can be reduced to ca. 1:3 without any other
significant change in the spectroscopic properties of the species.
At present we have no evidence that strongly indicates any one of these
options although the evident - but ambiguous - involvement of Et3N (and
some other Lweis bases not discussed here) does seems to favour option (3)
or (4) (or some similar solution) over (1) and (2).
If hydrogen bonding IS involved a very strong H-bonding acceptor might
yield an adduct stable enough to be unambiguously characterised. Hence my
original request for suggestions for just such an acceptor.
Any helpful suggestions will be most gratefully received.
TIA -
Fergus Lalor
--------------------------------------------------
Dr. Fergus Lalor, Chemistry Dept., University
College, Cork, IRELAND.
Telephone: 353-(0)21-902317. Fax: 353-(0)21-274097
E-mail: STCH8002$##$BUREAU.UCC.IE
dein goldenes Haar Margarete
dein aschenes Haar Shulamith
'Todesfuge', Paul Celan (1920-1970).
but also ....
Mairidh Gaol is Ceol
and ...
'I hear the ancient footsteps like the motion of the sea
Sometimes I turn, there's someone there - other times it's only me ...'
(Bob Dylan - 'Every Grain of Sand')
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