Boronic coordination with Lewis bases

The strength or coordinating power of different Lewis acids can vary widely against different Lewis bases. Thus, for example, in the case of boron trihalides, boron trifluoride coordinates best with fluorides, but not with chlorides, bromides, or iodides. In coordination with Lewis bases such as amines and phosphines, BF3 shows preference to the former (as determined by equilibrium constant measurements).66 The same set of bases behaves differently with the Ag+ ion. The Ag+ ion complexes phosphines much more strongly than amines. In the case of halides (F, CP, Br, and P), fluoride is the most effective base in protic acid solution. However, the order... 

The boron atom in boron trifluoride is hybridized to the sp planar configuration and consequently is coordinatively unsaturated, ie, a Lewis acid. Its chemistry centers around satisfying this unsaturation by the formation with Lewis bases of adducts that are nearly tetrahedral sp [ The electrophilic properties (acid strengths) of the trihaloboranes have been found to increase in the order BF < BCl < BBr < BI (3,4). 

Being an electron deficient compound, boron trifluoride forms complexes with Lewis bases and compounds that have unshared pair(s) of electrons. With ammonia, it forms boron trifluoride ammonia. Similar coordination compounds are formed with monoethylamine, BF3-NH2C2H5 diethyl ether, CH3CH20(BF3)CH2CH3 and methanol, BF3—OHCH3. It forms a sohd complex HNO3-2BF3 with concentrated nitric acid. 

Borin (12) with an sp2 hybridized boron atom has six ir-electrons and is expected to be resonance stabilized. The boron atom has an empty sp2 orbital and should therefore be a strong Lewis acid. The ring system (12) is accordingly known only coordinated to Lewis bases, mostly to carbanions, to give a ring system isoelectronic with benzene. Thus systems such as the 1-arylborinate anions (83) and 5-aryldibenzo[6,e]borinate anions (84) are by far the best-known species. 

Coordination interaction is also a factor that needs to be considered. Because a trigonal uncharged boronate contains a boron atom with an empty orbital, this can serve as an electron receptor for a coordination interaction. In addition to the previous report that unprotonated amines and carboxyl groups can serve as electron donors and thus can coordinate with boronic acids, the coordination of Lewis bases [e.g., fluoride ion) with boron can also occur, which can enhance the complexation between cis-diol-containing compounds and boronic acids. However, if there is a hydroxy group adjacent to the amine [e.g., ethanolamine), this hydroxy group can also interact with the boronate, which would block esterification between the boronate ligand... 

In the chemistry of polyhedral boron hydrides, boron-centered cations were postulated to be key intermediates of an electrophile-induced nucleophilic substitution mechanism that is responsible for the formation of a variety of boron-substituted derivatives [14], Such boron-centered cations can be easily generated by abstraction of a hydride by the treatment of polyhedral boron hydrides with Lewis or Bronsted acids [15], Similar to the classical chelate-restrained borinium cations based on 3-coordinate boron, these species, which we called quasi-borinium cations, have an unstabilized p orbital and are strong electrophiles (Scheme 6.1). Such quasi-borinium cations are highly reactive and react with even weak nucleophiles, such as ether or nitrile solvent molecules giving the corresponding oxonium and nitrilium derivatives whose properties are close to those of similar complexes of transition metals [15-17]. 

Boron trihalides are strong Lewis acids that react with a wide collection of Lewis bases. Many adducts form with donor atoms from Group 15 (N, P, As) or Group 16 (O, S). Metal fluorides transfer F ion to BF3 to give tetrafluoroborate salts LiF + BF3 LiBF4 Tetrafluoroborate anion is an important derivative of BF3 because it is nonreactive. With four <7 bonds, [BF4 ] anion has no tendency to coordinate further ligands. Tetrafluoroborate salts are used in synthesis when a bulky inert anion is necessary. 

The more generalised picture provided by Lewis, who defined acids as molecules or ions capable of coordinating with unshared electron pairs, and bases as molecules or ions which have such unshared electron pairs available for coordination, has already been referred to (p. 29). Lewis acids include such species as boron trifluoride (1) which reacts with trimethylamine to form a solid salt (m.p. 128°) ... 

The picture of the nitrogen atoms in diazadiboretidines acting as Lewis base centers is also supported by the formation of a 1 1 coordination compound with TiCl4 [Eq. (58)] (91). The B-NMR signal of 22.7 ppm indicates a highfield shift, which cannot be due to d-electrons from tetravalent d -titanium. X-Ray structural analysis shows that bridging chlorine atoms provide the observed electronic saturation of the boron atoms. 

A boron to which is attached a Lewis base is isoelectronic with carbon and thus the Lewis base-bridge hydrogen feature in VII-N6 is undesirable. After the rearrangement the Lewis base is not on the apex (VI -N6), i.e., since it is isoelectronic with carbon it prefers the four-coordinate base site and thus becomes isoelectronic with IV-N6. 

The first species produced in cationic polymerizations are carbocations, and these were unknown as such prior to World War II. It is now known that pure Lewis acids, such as boron trifluoride and aluminum chloride, are not effective as initiators. A trace of a proton-containing Lewis base, such as water, is also required. The Lewis base coordinates with the electrophilic Lewis acid, and the proton is the actual initiator. Since cations cannot exist alone, they are accompanied by a counterion, also called a gegenion. 

Such a bond, in which the donor molecule (or anion) provides both bonding electrons and the acceptor cation provides the empty orbital, is called a coordinate or dative bond. The resulting aggregation is called a complex. Actually, any molecule with an empty orbital in its valence shell, such as the gas boron trifluoride, can in principle act as an electron pair acceptor, and indeed BF3 reacts with ammonia (which has a lone pair, NH3) to form a complex H3N ->BF3. Our concern here, however, is with metal cations, and these usually form complexes with from 2 to 12 donor molecules at once, depending on the sizes and electronic structures of the cation and donor molecules. The bound donor molecules are called ligands (from the Latin ligare, to bind), and the acceptor and donor species may be regarded as Lewis acids and Lewis bases, respectively. 

With this repertoire of bonding possibilities at our disposal, we car construct the molecular structures of various boron-hydrogen compounds, both neutral species and anions. The simplest is the tetrahydroborate126 or borohydride ion, BH. Although borane is unstable with respect to dimerization, the addition of a Lewis base, H , satisfies the fourth valency of boron and provides a stable entity. Other Lewis bases can coordinate as well. 

The ability of the boron atom of 59 to engage in a donor-acceptor interaction was illustrated with DMAP and DABCO (DABCO = diazabi-cyclo-[2.2.2]-octane) that readily formed the corresponding Lewis adducts. Interestingly, a similar behavior was retained after coordination of the phosphorus atom to palladium. The formation of the Lewis base adducts 66a and 66b of complex 65 (Scheme 38) was supported by solid-state 31P and nB CP/MAS-NMR spectroscopy (<5 1 B = 5-6 ppm), although the occurrence of decomposition and/or dissociation processes impeded spectroscopic characterization in solution and recrystallization to obtain X-ray quality crystals. Compounds 66a and 66b substantiate the ability of ambiphilic compounds to engage concomitantly into the coordination of donor and acceptor moieties. Such a dual behavior opens interesting perspectives for the preparation of metallo-polymers and multimetallic complexes. 

The anion is a regular tetrahedron (isoelectronic with CH4) and might alternatively (and perhaps preferably ) be described as B (H)4 since B and H are of about the same electronegativity, the negative charge should be about equally shared over all five atoms. Many molecules and ions containing tetrahedrally-coordinated boron can be regarded as Lewis acid-Lewis base adducts X3B <— L. 

The starting material bis(pinacolato)diboron is a poor Lewis acid and 1 B-NMR of KOAc and B2bin2 in DMSO-d6 shows no evidence of the coordination of the acetoxy anion to a boron atom leading to a tetrahedral activated species. However, the formation of an (acetato)palladium(II) complex after the oxidative addition of the halide influences the reaction rate of the transmetalation step. The Pd-O bond, which consists of a hard Lewis base with a soft Lewis acid, is more reactive than a Pd-X (X=Br, I) bond. In addition, the high oxophilicity of boron has to be considered as a driving force for the transmetalation step, which involves an acetato ligand. 

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