Borazines metal complexes

One of the most fascinating developments in borazine chemistry has been the recent isolation of hexaalkylborazine chromiumtricarbonyl complexes. A reaction was observed when hexamethylborazine was reacted Mo(CO)e but no definite products could be isolated 1S8>. However, with extremely pure starting materials using (CO)3Cr(CHaCN)3 or (CO)3(NH3)3Cr as reactants, the preparation of a borazine-metal complex has been effected iss.ieo). Solvents other than dioxane did not lead to the desired product, and it has been suggested that the reaction proceeds via a dioxane-chromium tricarbonyl complex. The reaction appears to proceed by altering the existing equilibria, indicated in the reaction as depicted in Eq. (43). 

Borazine is isoelectronic and isostructural with benzene and may act as a six-electron donor in complex chemistry. In contrast to arene ligands of arene-transi-tion-metal complexes, coordinated borazines lose their planarity and are slightly puckered . Nevertheless, the B atoms show interactions with metal atoms. 

Other unsaturated boron heterocycles, such as borazines and borabenzenes, form transition metal complexes with the expected nido geometry, as exemplified by compounds (Et3N3B3Et3)Cr(CO)3 (123) and (CBH5BPh)Mn(CO)s (110) (Fig. 29). 

The report of the synthesis of the first borazine transition metal complex was important for me in two respects. Already in May 1967, only 4 months after our communication appeared, I was invited by David Brown to give a main lecture at the following annual meeting of the Royal Society of Chemistry to be held in spring 1968 at Dublin. When I received the letter, I could not believe it at first, because I still considered myself as a nobody in the organometallic community. At the conference, I was lucky that my lecture was scheduled for the afternoon session of the day in which we later had dinner in the Guinness brewery. There, I was able not only to relax but also to enjoy the strong dark... 

Catecholborane and its derivatives had a pivotal role in establishing the class of boryl complexes, since they proved to be the most versatile starting materials for the synthesis of such compounds. This was demonstrated in many works especially by Baker, Hartwig, Marder, Norman, and Smith. ia In the course of our investigations we aimed at boryl complexes with different substituents attached to boron, and we could show that boranes with B-N bonds are also very well suited for the synthesis of boryl complexes. Over the last three years a wide variety of products was obtained by salt elimination reactions between anionic transition metal complexes and haloboranes. Examples include aminoboryl- 4, 5, diborane(4)yl- 6, 7, and rj -borazine complexes 8 (Figure 4). 

Borazine decomposes slowly on storage and is hydrolyzed at elevated temperatures to NH3 and B(OH)3. It is of interest that borazine resembles benzene in forming arene-metal complexes (Section 23-3) thus the hexamethyl-borazine complex, B3N3(CH3)6Cr(CO)3 has been reported and closely resembles C6(CH3)6Cr(CO)3 but is thermally less stable.36... 

Reactive monomeric species of RNBR composition can be isolated by trapping at low temperatures. The monomers are prepared by low-pressure thermal degradation of boranamines or azidoboranes L Cyclization reactions can occur on warming to RT. In the absence of steric hindrance, trimeric borazine rings invariably form, but with sufficiently bulky substituents (e.g., R = R = t-Bu), dimeric species result. However, even with nonbulky substituents many monomers can form cyclic dimers at low temperature if subjected to catalysis by certain transition-metal complexes or the isonitrile t-BuNC... 

Rapid hydrogen releases can be achieved under mild conditions by using transition metal complexes as catalysts. Some noble metal complexes, including Pt, Pd, Rh, Ir, and Ru, were proven to be very effective for promoting hydrogen release from AB. The Rh, Pd, and Ru complexes could catalyze AB to dehydrogenate at room temperature with suppressing the release of volatile cyclotriborazane, borazine, and poly(iminoborane) [107]. The iridium pincer... 

Hunt s group (50, 51) have pioneered the application of the Cl source to organometallics such as the iron tricarbonyl complex of heptafulvene, whose electron impact spectrum shows (M—CO)+ as the heaviest ion, in contrast to the methane Cl spectrum with the ion as base peak. Boron hydrides (52) and borazine (53) have also been studied. The methane Cl spectrum of arenechromium and -molybdenum (54) show protonation at the metal giving a protonated parent or molecular ion. Risby et al. have studied the isobutane Cl mass spectra of lanthanide 2,2,6,6-tetramethylheptane-3,5-dionates[Ln(thd)3] (55) and 1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-oetanedione [H(fod)] lanthanide complexes (56). These latter complexes have been suggested as a means of analysis for the lanthanide elements. 

Boron-bonded p -borazine complexes of transition metals have been prepared by two different approaches (a) nucleophilic substitution of B,fi, fi"-trichloro-borazine with an anionic metal carbonyl reagent and (b) oxidative addition of a B-Br bond of 5,5, B"-tribromoborazine to a zerovalent group 10 complex (see examples in Scheme 9.2). 

The heterocyclic compounds occupy an important place amongst boron-containing ligands [8], It is well known that borazines, having a six-member ring, form ri6(7i)-complexes, for example 554 [953], It was recently shown that cr(B)-metal-coordinated compounds of type 555 may also be obtained on the basis of this ligand system [954a] ... 

In retrospect, it is ironic to it that when I met Ernst Schumacher in 1969 (he was then Professor at the University of Bern in Switzerland) we did not talk about the experiments he did at Zurich in the same building where I was at that time. Instead, his interest focussed on our work on borazine transition metal compounds and we discussed in some detail whether it would be possible to incorporate metal atoms like chromium or molybdenum between the layers of hexagonal boron nitride (BN) in a similar way as it can be done with graphite. In the course of these discussions I did not mention that, after I had moved to Zurich, we had begun to investigate the reactivity of nickelocene towards both nucleophilic and electrophilic substrates. The reason was that we were still at the beginning, and while we had been able to prepare a series of monocyclopentadienyl nickel complexes from Ni(C5H5)2 and Lewis bases, our attempts to obtain alkyl- or acyl-substituted nickelocenes by the Friedel-Crafts reaction failed. 

The electronic difference between benzene and borazine is further supported by the properties of compounds of the type (R<,BjN j)Cr(CO)j. Although the.se are formally analogous to (rj< -QRf,)Cr(CO)j. the bonding is not nearly so strong in the borazine complex—its ring-metal dissociation energy appears to be about one-half that of the arene complex. In addition, there is considerable evidence that the borazine molecule is puckered in the.sc complexes. The actual structure appears to be intermediate between a true tt complex and the extreme rr-only model ... 

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