Four coordinated pyridine

The divalent Co(salen) complex (69a) is one of the most versatile and well-studied Co coordination compounds. It has a long and well-documented history and we shall not restate this here. Recent applications of (69a) as both a synthetic oxygen carrier and as a catalyst for organic transformations are described in Sections 6.1.3.1.2 and 6.1.4.1 respectively. Isotropic shifts in the HNMR spectrum of low-spin Co(salphn) (69b) were investigated in deuterated chloroform, DMF, DMSO, and pyridine.319 Solvent-dependent isotropic shifts indicate that the single unpaired electron, delocalized over the tetradentate 7r-electron system in CHCI3, is an intrinsic property of the planar four-coordinate complex. The high-spin/low-spin equilibrium of the... 

This intramolecular interaction of heteroatoms may be distorted by introducing different substituents onto the 4,6-positions of the ring. In the case of acceptor substituents, (CC13) dioxaboraphosphorinane (97) is not alkylated at the phosphorus atom and is inert to sulfur at the same time it forms a stable complex with pyridine [Eq. (96)]. In the case of donor substituents (Aik) (93)—(94), by contrast, the corresponding sulfides have been obtained and the pyridine complex is formed only with four-coordinated phosphorus derivatives. 

The micelle-encapsulated six coordinated bis(pyridinato) iron(II) complexes of protoporphyrin and OEP have been reported by addition of pyridine to the four coordinate ferrous complex in aqueous micellar solution. The optical spectrum of [Fe(II)(PP)(Py)2] in micelle (Fig. 10) is identical to S = 0 six-coordinate bis(pyridinato) iron(II) porphyrin complex [3]. The magnetic moment measurements in solution confirm their diamagnetic nature. The HNMR spectra are also characteristic low-spin iron(II) resonances (S = 0) with shifts lying in the diamagnetic region (Table 2). 

Gibson described the synthesis of four-coordinate cationic aluminium alkyls 1 which were reported to be well-defined aluminium polymerization catalysts [12]. However, the polydispersities of the products obtained were high (2.9-6.3), showing that there is not a single well-defined active species. The experiments were carried out in metal autoclaves, and Fe and Co complexes of pyridine-diimine ligands are extremely active in ethene polymerization [34], so a transition-metal impurity does not seem an unreasonable explanation. 

Bis-pyridine complexes exist in equilibrium with five- and four-coofdinate species in solution.242 The four-coordinate complexes form polymers, which become predominant above a concentration of 5 x 10 3 M in chloroform. The association constants X, and K2 for 3- or 4- substituted pyridine complexes are well correlated to their pK, (BH+) values and Hammett s a values by equation (33) though deviations are rather large for 3-acetyl- and 2-methylpyridines. In general, K2 is smaller than Ki by a factor of 102-103. 7... 

Co111 (Cor) complexes are reduced to paramagnetic Con(HCor) by NaBH4. The Co11 complexes bind only one molecule of pyridine, and both four- and five-coordinate species are in the low spin state.242 The Con(HCor) contain an extra hydrogen, and the deprotonated [Co11 (Cor)] do not coordinate pyridine (equation 34).246... 

Copper and nickel complexes of the tridentate l-(2-carboxyphenyl)-3,5-diphenyl- (169 X = C02 R = R = Ph) and 1-(2-hydroxyphenyl)-3,5-diphenyl-(169 X = 0 R = R = Ph) formazans were prepared118 by the interaction of the formazan and the appropriate metal acetate in alcohol and were assigned the three-coordinate structures (170 X = O, C02 R = R = Ph M = Ni, Cu) since the diamagnetic nickel complexes were found to be unimolecular in benzene solution. Treatment of the nickel complex (170 X = O, R = R = Ph M = Ni) with pyridine gave a violet crystalline adduct which was assigned the four-coordinate structure (171 X = O R = R = Ph M = Ni). A product similar to the latter could not be obtained from the nickel complex of l-(2-carboxyphenyl)-3,5-diphenylformazan but nickel complexes of this type were obtained from both l-(2-hydroxyphenyl)- (169 X = O, R = CN R = Ph) and l-(2-carboxyphenyl)- (169 X = C02 R = CN R = Ph) 3-cyano-5-phenylformazans. In all three cases a considerable shade change occurred on going from the three-coordinate complex to the pyridine adduct. 

---