Magnesium, five-coordinate

The molecular structures in the solid state of 118 and its 1,4-dimethyl derivative were determined by X-ray crystallography and appeared to be isostructural. In 118 the magnesium atom is bound to C(9) and C(10) with rather long bond distances [2.25(1) and 2.33(1) A] (Figure 57). Due to the loss of aromaticity in the central ring of the anthracene skeleton, the molecule is folded (26.6°). As the result of three additional coordinating THF molecules [Mg-0 2.059(7), 2.028(8) and 2.091(8) A], the magnesium atom is five-coordinate. 

The magnesium porphyrin radical, Mg(tetraphenylporphyrin)C104 (136), has been used as a model for the structural and stereochemical consequences of loss of an electron in photosynthetic chromophores.521 The primary photosynthetic reaction in plants and bacteria consists of a transfer of an electron, in the picosecond time domain, from the chlorophyll phototrap to nearby acceptors yielding chlorophyll -cation radicals. The structure of (136) shows a five-coordinated Mg2+ cation which is not quite symmetrically sited in the porphyrin ring but has metal-ligand distances similar to those found in the previous structures. The perchlorate anion is tightly bound (Mg—O = 2.01 A) in a monodentate mode. It was concluded that the porphyrin can act as an electron sink and that no major effects are found in the bond lengths, or on the stereochemistry, of the macrocycle. 

The bacteriochlorophyll a protein from the green phot osynthetic bacterium Prosthecochloris aestuarii has been determined at 2.8 A resolution.389 It is made up of three identical subunits, tightly packed around a three-fold symmetry axis. Each subunit consists of a core of seven bacteriochlorophyll a molecules enclosed within a bag of protein. There are extensive contacts between the phytyl chains of the seven bacteriochlorophylls within each subunit. These tails form an inner hydrophobic core. The seven magnesiums appear to be five-coordinate. In five cases the fifth ligand is a histidine side-chain, in one case a protein backbone carbonyl oxygen, and in the other case a water molecule. It appears that the removal of one or more of the bacteriochlorophyll molecules would destabilize the protein. It is unlikely that the chlorophyll can be reversibly removed from the protein. 

Phenazi ne-/V. A -( MgBr (THF),)i] [ MgBr, (THF)j] (monomeric bisbromomagnesiodihydrophenuzine with five-coordinate magnesium atoms)... 

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