Although known for over 90 years, only in the past two decades has the chemistry of diboron(4) compounds been extensively explored. Many interesting structural features and reaction patterns have emerged, and more importantly, these compounds now feature prominently in both metal-catalyzed and metal-free methodologies for the formation of B–C bonds and other processes.

Diboron(4) Compounds: From Structural Curiosity to Synthetic Workhorse

Linus Pauling had a superb intuitive understanding of chemistry, backed by deep intelligence and a prodigious memory. He seldom made mistakes. The best known is perhaps his ill-fated three-stranded DNA structure, but one of the few other examples concerns hydrogen bonds involving fluorine. This is evident from a comparison between the various editions of his classic TMThe Nature of the Chemical Bond∫. In the early editions he wrote: TMonly the most electronegative atoms should form hydrogen bonds, and the strength of the bond should increase with increase in the electronegativity of the two bonded atomso It is found empirically that fluorine forms very strong hydrogen bonds, oxygen weaker ones, and nitrogen still weaker ones.∫ [1] Pauling went on to discuss the strong hydrogen bond in hydrofluoric acid (HF)n and the very strong one in the hydrogen fluoride ion [HF2] and correctly concluded that the proton in the latter should lie in a single minimum potential well or in a double minimum potential with a very small barrier. It was only in the third edition, published in 1960, some twenty years after the first, that Pauling conceded: TMIt is interesting that in general fluorine atoms attached to carbon do not have significant power to act as proton acceptors in the formation of hydrogen bonds in the way that would be anticipated from the large difference in electronegativity of fluorine and carbon.∫ [2] Over the years, many chemists have followed Pauling's first line of thought, and more or less taken it for granted that organic fluorine acts as a powerful acceptor in the formation of hydrogen bonds. Others have looked at the available structural evidence as collected in the Cambridge Crystallographic Structural Database (CSD) and concluded that organic fluorine is at best only a weak hydrogen-bond acceptor. 4] A further intensive search of the CSD, including detailed inspection of individual crystal structures and backed by ab initio calculations on model systems, confirmed that organic fluorine hardly ever accepts hydrogen bonds, that is, it does so only in the absence of a better acceptor. Even in such compounds like crystalline ammonium trifluoroacetate, in which there are four hydrogen donors per anion, there is no hint of N H¥¥¥F hydrogen bonding; the four N H bonds all point towards oxygen atoms of the trifluoroacetate anions. In the ammonium monofluoro structure there is just a hint of a bifurcated hydrogen bond involving carboxylate O and the syn-planar F atom, but the latter is 0.26 ä more distant from the H atom (Figure 1). Likewise, the evidence for hydrogen bonding to organic fluorine in protein±ligand complexes was examined and found to be unconvincing. Hydrogen bonding involving B F bonds should be stronger than that involving C F bonds. Yet even in crystalline ammonium tetrafluoroborate NH4 BF4 , surely the exemplar of such an expected interaction, no short N H¥¥¥F lengths are observed. The authors commented TMit is believed that hydrogen bonding contributes negligibly to the lattice energy of this crystal∫. A search of the CSD reveals only very few crystal structures showing short intermolecular X H¥¥¥F B lengths; one example is 4,4,8,8-tetrafluoropyrazabole, another is 2,2-difluoro-4,6-dimethyl-3-phenyl1,3,2-difluorodiazaborine (Figure 2). One can hardly deny that these should be classified as genuine hydrogen bonds, but there are few such specimens. It seems clear that with its low polarizability and tightly contracted lone pairs, fluorine is unable to compete with stronger hydrogen-bond acceptors such as oxygen or nitrogen. The few authentic examples of O H¥¥¥F or N H¥¥¥F hydrogen bonding involve systems in which approach of the hydrogen atom to other better acceptor atoms is sterically hindered. Indeed, nowadays the occurrence of a genuine hydrogen bond involving organic fluorine seems to be regarded as sufficiently noteworthy that it deserves special mention in the title of the publication, as, for example, ref. [10] .

Organic Fluorine: Odd Man Out

Transmetalation of Unsaturated Carbon Nucleophiles from Boron-Containing Species to the Mid to Late d-Block Metals of Relevance to Catalytic C−X Coupling Reactions (X = C, F, N, O, Pb, S, Se, Te)

The tetrahedral character of the boron atom newly defined—a useful tool to evaluate the N→B bond

This perspective summarizes the chemistry of boroxines and outlines progress towards incorporating these ring structures into functional materials and macromolecular architectures. Special attention is paid to the dynamic covalent chemistry of boroxine ring construction and how these processes lead to novel molecular architectures and functional materials. Also highlighted in this perspective is the rich chemistry surrounding boroxine–ligand interactions and how these interactions flavor many areas of boroxine chemistry.

Boroxine chemistry and applications: A perspective

: The present study was based on the assumption that a neutral Boron Pyrazole species would likely exist in a polymeric structure. Therefore, it should contain the bridging Boron Pyrazole units typical for pyrazaboles but also terminal B bonded Pyrazole groups and thus would be related to B-poly(1-pyrazolyl)pyrazaboles. Furthermore, a (pz)3B-B(pz)3 ion should be similar to a RB(pz)3 ion but for the fact that the former has two polydentate sites. These possibilities should provide for interesting variations as compared to the chemistry of B-(1-pyrazolyl)pyrazaboles and poly(1-pyrazolyl)borates, respectively. Keywords: tetrakis(1-pyrazolyl)dibor ane(4), (monomer, oligomers), bis(pyrazole)-tetrakis(1-pyrazolyl)diborane(4), hexakis(1-pyrazolyl)diborane(4) ate complexes.

Pyrazole derivatives of diborane(4)

: B-Triorganylboroxins, (-BRO-)3 (R = C2H5, C6H5), form 1:1 molar complexes with hydrazine, N,N' -dimethylhydrazine, and N,N'-dimethylhydrazine. At room temperature, the complexes exhibit only one 11B NMR signal suggesting that the species are fluxional with nitrogen coordinating to all three boron atoms of the boroxin ring. The signals broaden on lowering of the temperature and, at -50 C, the signal of (BC2H5O-)3(CH3)2NNH2 appears as two separate peaks in 2:1 ratio indicating that fluxion has been arrested. The complexes (-BRO-)3L with R = C2H5 or C6H5 and L = N2H4 and with R = C6H5 and L = (CH3)2NNH2 can form solvates with excess of the hydrazine and solid 1:2 molar complexes were isolated. However, the second hydrazine is readily lost under reduced pressure or at evaluated temperatures. The complexes of B-triphenylboroxin are thermally much less than those of B-triethylboroxin.

Hydrazine complexes of B-triorganylboroxins

Boron-nitrogen compounds. 113. Boron-halogenated pyrazaboles

Boron-bonded hydrogen or organylthio groups of pyrazaboles, R/sub 2/B(/mu/-pz/sup */)/sub 2/BR/sub 2/ (HPz/sup */ = pyrazole or C-substituted derivatives thereof), can be replaced by reaction with either (C/sub 2/H/sub 5/)/sub 2/O/center dot/BF/sub 3/ or CH/sub 3/OH/center dot/BF/sub 3/ to give the corresponding F/sub 2/B(/mu/-pz/sup */)/sub 2/BF/sub 2/ species. Attempts to replace boron-bonded hydrocarbon groups require much more forcing conditions and usually result in a complete breakdown of the molecule. However, under mild conditions the B-C bond is not attacked and (C/sub 6/H/sub 5/)FB(/mu/-pz)/sub 2/B(C/sub 6/H/sub 5/)F is readily obtained from the reaction of (C/sub 6/H/sub 5/)(C/sub 2/H/sub 5/S)B(/mu/-pz)/sub 2/B(C/sub 6/H/sub 5/)(SC/sub 2/H/sub 5/) with BF/sub 3/. 4,8-Dihalopyrazaboles, RXB(/mu/-ps)/sub 2/BRX, are also obtained from the reaction of dihaloboranes, RBX/sub 2/, with N-(trimethylsilyl)pyrazole, (CH/sub 3/)/sub 3/Si(pz); the halogen of the pyrazabole can be displaced readily be reaction with a Grignard reagent or an alcohol. The compound ((C/sub 2/H/sub 5/)/sub 2/N)(pz)B(/mu/-pz)/sub 2/B(N(C/sub 2/H/sub 5/)/sub 2/)(pz) was also prepared by the cited procedure. The diethylamino group of the latter species is easily displaced by reaction with pyrazole. Several additional pyrazaboles of the type RR'B(/mu/-pz)/sub 2/BBR' have been prepared. (C/sub 6/H/sub 5/)(pz)B(/mu/-pz)/sub 2/B(C/sub 6/H/sub 5/)(pz), which is accessible by various preparative routes, was separated intomore » isomers that exhibit distinctly different /sup 1/H NMR spectra. The mass spectral fragmentations of such (cis and trans) isomers are significantly different with respect to their loss of the first fragments from the parent ions. 18 references, 2 tables.« less

Boron-nitrogen compounds. Part 117. Pyrazaboles of the type RR'B(.mu.-pz)2BRR' and related studies

Preparation d'un derive de B,B'-epidithio dipyrazolo tetrazadiborine a partir de pyrazole et de dimethyl-3,5 phenyl-4 dithiazadiborolidine-1,2,4,3,5

M. K. Das

Papers

Pyrazole derivatives of diborane(4)

Hydrazine complexes of B-triorganylboroxins

Boron-nitrogen compounds. 113. Boron-halogenated pyrazaboles

Boron-nitrogen compounds. Part 117. Pyrazaboles of the type RR'B(.mu.-pz)2BRR' and related studies

Boron-nitrogen compounds. 116. Preparation and crystal structure of a new type of triply bridged pyrazabole