Macrocyclization of the semiclathrochelate o-carboranylboronate and n-butylboronate iron(II) oximehydrazonates: synthesis and structure of clathrochelate products and unexpected allosteric effect of the apical substituent

Abstract

The template condensation of diacetylmonooxime hydrazone (HDXO) with n butylboronic acid and dimethyl ester of o carboranyldiboronic acid on the iron(II) ion matrix afforded the [Fe(DXO) 3 (ВBu)](BF 4 ) and [Fe(DXO) 3 (Вo carb)](BF 4 ) semiclathrochelates. The Н + ion catalyzed macrocyclization of these precursors with an excess of formaldehyde and triethyl orthoformate (TOF) resulted in the corresponding macrobicyclic complexes. In the case of o carboranylboronate semiclathrochelate, the macrocyclization with TOF gave the clathro chelate with the previously unknown syn,syn,anti orientation of the ethoxy substituents rela tive to the 1,3,5 triazacyclohexane capping fragment. The complexes obtained were character ized using elemental analysis, IR, UV Vis, MALDI TOF mass, 1 H, 11 B, and 13 C NMR, and 57 Fe Mössbauer spectroscopies and X ray crystallography.A number of mono and binuclear semiclathrochelate iron(II) and cobalt(II,III) tris oxime hydrazonates with bo ron , tin , antimony, and germanium containing cross linking apical groups have been obtained up to date; their Н + catalyzed condensation with triethyl orthoformate (TOF) gave in each case a sole isomer of the clathrochelate complex. In the case of boron containing clathrochelates, the ethoxy groups and the semiclathrochelate fragment are syn,syn,syn oriented with respect to the mean plane of the 1,3,5 triazacyclohexane ring, 1 whereas in the tin , antimony , and germanium containing complexes, the ethoxy groups have the anti,anti,anti orientation. 2-4 This difference was explained 5 by the change in the geometry of the semiclathrochelate precursor from trigonal pris matic (TP) for boron containing compounds to trigonal antiprismatic (TAP) for other types of cross linking groups.In this study, we planned to synthesize the binuclear clathrochelate oximehydrazonates using an о carboranyl diboronic acid derivative and to obtain previously un known n butylboronate iron(II) semiclathrochelate and to perform its macrocyclization with TOF. Results and DiscussionOnly few о carborane containing organoboron com pounds with the С-B bond, obtained by reactions of boron trichloride with appropriate С lithium derivatives, are described in the literature. 6 We synthesized methyl ester of о carboranyldiboronic acid (1) by a similar pro cedure starting from о carborane with chlorodimethoxy borane as a borylating agent (Scheme 1). According to 11 B{ 1 H} NMR spectrum of the reaction mixture, the de gree of conversion was ~75%. Apart from signals of the boron atoms of the symmetrically substituted carborane polyhedron, this spectrum contained the characteristic * Dedicated to Academician G. A. Abakumov on the occasion of his 70th birthday.

Synthesis and structure of the ferrocenylboron-capped clathrochelate iron(ii) oximehydrazonates

“…At first we attempted to obtain [4](BF 4 ) by the condensation of the [1](BF 4 ) complex with TOF in the presence of catalytic amounts of trifluoroacetic anhydride as described earlier 11 for the alkylborate and o carbonylborate oxime hydrazonate iron(II) clathrochelates. The red orange col oration of the reaction mixture has changed to dark red, but only partial dissolution of the initial [1](BF 4 ) semi clathrochelate was observed.…”

Synthesis and Structure of an o‐Carboranyl‐Substituted Three‐Coordinate Borane Radical Anion

“…Close‐range conjugation of a carboranyl moiety to a 3‐coordinate boron has rarely been studied. Examples include o ‐carboranyl‐substituted boron dihalides or acids for use in materials [79,99–101] and more recent, crystallographic studies of C‐isopropyl‐ o ‐carborane‐organoboron derivatives with solid state geometries similar to 3‐coordinate boranes [102] . o ‐Carboranes substituted with C‐benzodiazaborolyl donor moieties were examined by photophysical, electrochemical, and spectroelectrochemical methods showing large Stokes shifts due to a substituent‐to‐cage charge transfer (CT) transition [8,103–105] .…”

Iron(II) Clathrochelate with Terminal Triple C≡C Bond and Its Carboranoclathrochelate Derivative with a Flexible Linker between the Polyhedral Cages: Synthesis and X‐Ray Structure

“…Hybrid ditopic carboranoclathrochelates seem to be very prospective for the design of antiviral and antitumor prodrugs, including those for a binary drug -radio-therapy. A series of the iron(II) carboranoclathrochelates (Scheme 1) has been obtained using a Lewis acidity of ortho-carboranylboronic acids and their derivatives [29][30][31] and by a metal-catalyzed carboranylation of the corresponding halogenoclathrochelate precursor [32] as well, but in all the cases, the molecules of the obtained hybrid compounds contained an inherent carboranyl substituent(s).…”