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Iminoether Complexes of the Type, fac ‐[Re(CO) 3 L{HNC(CH 3 )OCH 3 }]BF 4 (L = Bipyridine or a Substituted Bipyridine): Synthesis and Properties
Author(s) -
Perera Theshini,
Abhayawardhana Pramuditha,
Fronczek Frank R.,
Marzilli Patricia A.,
Marzilli Luigi G.
Publication year - 2012
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/ejic.201100768
Subject(s) - chemistry , steric effects , ligand (biochemistry) , acetonitrile , stereochemistry , bipyridine , substituent , crystallography , proton nmr , nuclear magnetic resonance spectroscopy , medicinal chemistry , crystal structure , receptor , biochemistry , chromatography
The methyl acetimidate (an iminoether) complexes, fac ‐[Re(CO) 3 L{HNC(CH 3 )OCH 3 }]BF 4 [where L = 2,2′‐bipyridine (bipy), 4,4′‐Me 2 bipy, 5,5′‐Me 2 bipy, or 6,6′‐Me 2 bipy], were formed when fac ‐[Re(CO) 3 (CH 3 CN) 3 ]BF 4 in acetonitrile/methanol was treated with 2,2′‐bipyridine (bipy) or the dimethyl‐2,2′‐bipyridines (4,4′‐Me 2 bipy, 5,5′‐Me 2 bipy, or 6,6′‐Me 2 bipy). Structural analysis of the four fac ‐[Re(CO) 3 L{HNC(CH 3 )OCH 3 }]BF 4 complexes revealed that all of the complexes crystallize with the iminoether ligand in the Z configuration and that distortions in L that involved the two pyridyl rings are minor except for 6,6′‐Me 2 bipy, which is highly distorted. This distortion of the 6,6′‐Me 2 bipy ligand is reflected in the NMR‐spectroscopic data. Upon complex formation, the methyl group signal in the 13 C NMR spectrum shifted downfield significantly for 6,6′‐Me 2 bipy, but this did not occur in the 13 C NMR spectrum of the 5,5′‐Me 2 bipy complex. The reaction times for forming the respective fac ‐[Re(CO) 3 L{HNC(CH 3 )OCH 3 }]BF 4 complexes for these two ligands were comparable, which indicated that the differences in the distortion of L and the methyl substituent position have little influence on the ease of iminoether formation. However, a higher steric bulk of the alcohol (methanol, ethanol, and2‐propanol) decreases the ease of fac ‐[Re(CO) 3 (5,5′‐Me 2 bipy){HNC(CH 3 )OR}]BF 4 formation. Isopropanol did not form the iminoether complex after two days of heating the reaction mixture at reflux. The low reactivity of the alcohols tested versus the amines that were tested previously suggests that selective bioconjugation to the fac ‐{ 99m Tc(CO) 3 } + core through an amidine linkage is feasible for biomolecules that have both amine and hydroxy groups. Finally, a comparison of the C≡N bond lengths that were obtained for the fac ‐[Re(CO) 3 (CH 3 CN) 3 ]BF 4 /PF 6 complexes with those values obtained for other rhenium acetonitrile complexes revealed that there is no significant difference between the C≡N bond lengths and that there is no correlation to the acetonitrile reactivity. It is possible that the fac ‐{Re(CO) 3 } + core stabilizes the transition state for amine or alcohol addition.