Azaborolyl Ligands

Oxidation of the borazole complexes [( -C3H3B(Me)NR)2Co] (R = Me, t-Bu, SiMe3) by iodine leads to [(/7 -C3H3B(Me)NR)2Co] (fz = 3-5). The hexafluorophosphate salts are obtained by oxidation using ferricinium hexafluorophosphate (83JOM(256)225, 86JOM(305)1). 

An interesting case of the benzannulated B, N complex is species 58 (X = C1) (00AGE948), prepared from [(OC)Os(PPh3)2Cl(BCl2)j and 8-aminoquinoline. It reacts with tetra- -butyl ammonium iodide and forms 58 (X = I). 

A series of 1,3,2-diazaboroles in a thermal substitution reaction with [Cr(CO)3(AN)3] forms the -coordinated complexes 59 (R = R = Me, Et, /-Pr R = Me, R = Et) (90IC4421). The corresponding dimeric ligand in this reaction yields complex 60 where only one heteroring is -coordinated. 

Interaction of iron(II) chloride with the lithium salt of R4B2N (R = Me, Et) gives sandwiches 61 (R = Me, Et) (67ZAAC1, 96MI4), resembling in electronic properties those of ferrocene (99ICA(288)17). The tt-complex stems from the further complex-formation of 61 (R = Me, Et) with mercury(II) salts via the unsubstituted nitrogen atom. 

5-Dimethyl-l,2,2,3-tetramethyl-A -l,2,5-azasilaboroline with [Fe2(CO)9] gives sandwich 62 and sandwich 63 (82AGE207, 82CB738) with [(r] -Cp)Co(C2H4)2]. With [Ni(CDT)] or in a vapor phase with metallic nickel, sandwich 64 (M = Ni) is formed. The vapor-phase synthesis with iron gives 64 (M = Fe). In all these sandwiches, 62-64, the -coordination of the heterocyclic ligand is realized. 

Another aspect of this problem is the existence of heterocyclic carbenes containing a low-valent gallium center, 49 (95JA5421,98EJIC305,99JA9758, 01 JCS(D)3459). 1,4-Bis(2,6-di-Ao-propylphenyl)-l, 4-diazabuta-l, 3-diene 

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The extensive pioneering work of Schmid and co-workers on transition metal complexes of 1,2-azaborolyl ligands was thoroughly covered in GHEC-II(1996) <1996GHEG-II(3)755>. More recent work has reported additional complexes... 

From the numerous complexes of the 1,2-azaborolyl ligand type, only representatives of those compounds which have been investigated by x-ray structure analysis are mentioned. 

Two main-group sandwich complexes deserve attention. Beryllium forms a complex with one tf-and one i -coordinated 1,2-azaborolyl ligand (Figure 11) <870M435>. It was shown that in solution the ionically bonded f/ -ring can dissociate partially. Thus, starting with a pure enantiomer, a racemate is formed after some time as both ring sites coordinate statistically in the ratio 1 1 (see Section 3.17.4). 

One 1,2-azaborolyl ligand has been prepared by dehydrohalogenation of the P-chloroboracycle that is available from the transmetallation of the stannacycle in Scheme 3.4. Another has been prepared by ring-closing metathesis from a vinyl aminobo-rane (Scheme 3.5). 

The di- and triborolyl ligands tend to i -coordination in sandwichforming reactions. There is a clear-cut tendency for stacking processes followed by the formation of multidecker species and often stabilization of the unusual oxidation states of the transition metals. The route to the linked sandwich and multidecker complexes is attractive for materials chemistry. Thia- and azaborolyl organome-tallic chemistry follows the same trends, although in the azaborolyl complexes the i -rather than i -coordination is sometimes realized. Moreover, coordination via the boron atom is known. In the B, N, Si-heterocycles, the heteroring is j " -coordinated. 

Some half-sandwich complexes with dihydro-l,2-azaborolyl ring ligands have been described, too ... 

A hybrid DFT method has been used to calculate the basicities of 1,2-azaborolyl 9 and 1,2-thiaborolyl 22, which were found to be more basic than cyclopentadienyl. The catalytic performance of early transition metal polymerization catalysts with these heterocyclic ligands has been evaluated by this MO approach <2003MI417-02>. 

This heteroaromatic character of 1,2-azaborolyl rings qualifies them to form numerous transition metal complexes. In contrast to the cyclopentadienyl ligand there is still a remarkable disturbance caused by the boron and nitrogen atoms. This becomes visible if those transition metals are used for complexation that cannot reach the 18-electron configuration in normal sandwich complexes. As a consequence, electron-rich metals tend to increase the distance to the electron-rich nitrogen atoms, whereas electron-poor metals tend to increase the distance to the boron atom. Three examples illustrate these structural features of 1,2-azaborolyl rings. 

In contrast to main-group elements, transition metals usually try to reach high coordination numbers. Therefore, the 1,2-azaborolyl rings are normally t -coordinated, offering 6jt-electrons to the metal, if it is considered formally as an anion, except to the late and electron-rich representatives. Titanium and vanadium each form bent-type complexes as a result of the presence of two chloro ligands and one chloro ligand, respectively. Figures 13 and 14 show the structures. 

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