Pyridine complexes with boron metals

The dipotassium adduct (9) was prepared from potassium metal and pentaphenylborole (5) in THF solution, as a dark red solid in 91 /o yield. Since siloles (12) also form dimetallo adducts readily (Equation (2)), the independent reaction of pyridine and 4-(dimethylamino)pyridine complexes of (5) with potassium metal to give the dipotassium adduct (9) confirms the participation of the boron 2/jj-orbital in the reaction (Scheme 2). The absence of pyridine from the dipotassium adduct (9) proves that the anionic charge in the borole displaces the coordinated pyridine in order to utilize the 2p -orbital on boron. 

The series of neutral B-containing carbocycles as complex ligands within the theme of metal-boron donor-acceptor relationships concludes with the 4,5-dihydro-borepines. l-Phenyl-4,5-dihydroborepine is able to substitute pyridine, acetonitrile or CO... 

Quite in contrast to, e.g., [CoCp2]+, borabenzene metal cations show a pronounced affinity toward hard nucleophiles such as amines, OH-, and to some extent even F- and H20. Qualitatively this affinity increases in the order CoCp2]+ 36 < 1 < 61 (69). [CoCp(C5H5BPh)]+ (1) adds tertiary amines at boron. With pyridine, the pyridinioboratacyclohexadienyl complex 70 is formed (K = 174 5 liters mol-1, in MeCN, 20°C), which can be isolated from CH2C12 as PF6- salt (69). The similar rhodium and iridium cations 36 and 37 form the stable cyanide adducts 71 and 72 (69). 

The empty 2/j,-orbital on boron interacting with the ji-electrons of the butadienylidene system in borole (5) is proved by the pronounced bathochromic shift in the visible spectrum compared with that of its pyridine adduct. This Ti-electron delocalization in boroles is destabilizing and produces a 471-electron antiaromatic system. The antiaromaticity is further evidenced by the rapidity of reactions that remove the 2p ,-orbital on boron from conjugation. These reactions include Lewis complexation, transition metal complexation, or borole ring opening reactions such as Diels-Alder addition, oxidation, and protodeboration. This removal of the antiaromaticity from borole results in transforming the compound to a typical, low reacting, sterically hindered arylborane. 

Figure 11.1.25 shows data from a microdroplet liquid-liquid voltammetry experiment in which lactate anions A are transferred from the aqueous phase into the organic microdroplet phase (here composed of the organic solvent 4-(3-phenylpropyl)-pyridine or PPP containing a Mn(III/II) redox system and naphthyl-2-boronic acid B [120]). Schematically, the oxidation of each metal complex Mn(II)TPP (with TPP = tetraphenylporphyrinato) is generating a positive charge within the microdroplet and this is coupled to the reversible transfer of the anion A ... 

Conceptually, this was first demonstrated using a metallated porphyrin that was functionalized with 2 diols (zinc dicatechol porphyrin) (Fig. 5). After addition of 3-pyridylboronic acid, absorption spectroscopy indicated that the pyridyl moiety was coordinated to the Zn-porphyrin. The observed affinity constant for this interaction, however, was more than 30 times what was expected for a simple pyridyl-Zn-porphyrin complex. It was, therefore, reasoned that the diols reacted with the boronic acids to afford the esters that would result in a cyclic structure (Fig. 5). Indeed, vapor phase osmometry (VPO) confirmed the 2 2 dimeric nature of the complex. Here the ester served as the key covalent linkage, though associated coordination was required between the metal and pyridine to create the cyclic structure. 

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