Bond to Boron

Bonds to Boron.—CNDO/2 M.O. calculations point to the staggered conformation as being preferred for PH3,BH3, PF3,BH3, PH3,BF3, and PF3,BF3, in agreement with the microwave data for the second compound.217 The calculated barriers to rotation and charge transfers are similar to those obtained by ab initio methods. Vibrational spectra for PX3,BC13,218,219 PX3,-BBr3,218 and PX3,BI3,218 where X = Hor D, have been assigned and normal-co-ordinate analyses carried out. 

A comprehensive n.m.r. investigation of the boron hydride and halide addition compounds with a number of alkyl- and aryl-phosphines is reported by Rapp and Drake.220 The H data point to a relationship between 

Bonds to Boron.—With the exception noted below, the papers in this section are concerned with aspects of donor-acceptor behaviour. The exception deals with the formation of P—B tr-bonds by the stepwise replacement of chlorine in BCl3,OEt2 by LiPEtz at low temperature to give the dimeric products (14), (15), and (16).  

Huttner, H.-D. Muller, A. Frank, and H. Lorenz, Angew. Chem. Internat. Edn., 1975,14, 572. 

The trimethylphosphine adducts with BCI3, BBr, and BI3 are isomorphous, and have a staggered conformation with effective 3 symmetry, according to X-ray diffraction results. The P—B bond distance decreases from 1.957 A in the chloride to 1.918 A in the iodide, as expected from the acid strengths, but the differences are within the uncertainty ranges. 

Bonds to Boron.—Microwave spectroscopy of phosphineborane, H3PBH3, and a number of isotopically labelled species indicates a staggered conformation with the following parameters  

A dipole moment of 4.00(3) D was calculated and the barrier to internal rotation assessed at 2.47(5) kcal mol. The relationship between B—P bond length and the stability of the borane adduct is also discussed. 

and Raman data for H3P,BCl3 and DsP.BClg at —196 were interpreted on the basis of symmetry, and a normal-co-ordinate analysis gave a 

Changes in the B—H stretching frequency of borane-phosphite adducts have been correlated with basicity of the phosphite and stability of the adduct, and in six-membered cyclic phosphites such i.r. data show enhanced basicity for axial lone pairs. (Further information on borane adducts can be found in refs. 353, 354, 386, 452, and 497). 

Complex formation between MegP and the mixed halogenoboranes BH3 5, where X = Cl or Br and n = 0—3, together with BHBrCl, BBrCl2, and BBrgCl, was examined by H, P, and B n.m.r. spectroscopy. The only parameter which appeared to correlate with the dieimodynamic stability of the adduct was the chemical shift of the methyl protons on the phosphine. 

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Step 2 The 7C complex rearranges to an organoborane Hydrogen migrates from boron to carbon carrying with it the two electrons m its bond to boron Development of the transition state for this process is shown m 2(a) and its transformation to the organoborane is shown m 2(b)... 

Examine the electrostatic potential map of H3B THE (borane-tetrahydrofuran complex) on Learning By Modeling How does the electrostatic potential of the hydrogens bonded to boron dif fer from the potential of the hydrogens of the tetrahydrofuran ring" ... 

Step 1 Hydride (hydrogen + two electrons) is transferred from boron to the positively polarized carbon of the carbonyl group The carbonyl oxygen bonds to boron... 

With one mole of phosphorus pentachloride, MeNHa.BFa gives a series of mixed halides (13) of which only one (/i = 1) was isolated. In each halide the fluorine atoms were shown to be bonded to boron by the appearance of iiB-isp and absence of spin-spin coupling. An analogous A-phenyl... 

Hydroboration is a stereospecific syn addition that occurs through a four-center TS with simultaneous bonding to boron and hydrogen. The new C—B and C—H bonds are thus both formed from the same face of the double bond. In molecular orbital terms, the addition is viewed as taking place by interaction of the filled alkene it orbital with the empty p orbital on boron, accompanied by concerted C—H bond formation.158... 

In these molecules, the boron atom has only six electrons surrounding it, so it interacts readily with species that can function as electron pair donors. For example, when l reacts with BF3, the product is BF4-, in which sp3 hybrids are formed, so such species are tetrahedral (7 ( symmetry). In most cases, molecules containing boron exhibit one of these types of bonding to boron. The boron hydrides represent a special situation that is described later. 

The chlorine atoms are bonded to boron atoms as expected on the basis of the difference in electronegativity. 

A number of other models were considered and tested (for example, direct B—H bonding). The most significant test was the IR vibrational spectrum, where a sharp absorption band at 1875 cm-1 was found, corresponding to the Si—H stretch mode softened by the proximity of the B-atom. Had the hydrogen been bonded to boron, a sharp absorption band at 2560 cm-1 would have been expected. Also, Johnson (1985) showed that deuteration produced the expected isotopic shift. The most definitive and elegant proof of the correctness of the Si-H-B bonding model was provided by Watkins and coworkers (1990), on the basis of a parametric vibrational interaction between the isotopes D and 10B. 

Halogen bonded to boron or carbon of the CNB grouping of the molecule seems to be of similar activity. Reactions of monomeric iminoboranes with organothiols lead to C-S substituted iminoboranes which will be discussed later (c.f. Sect. VI). 

Iminoboranes may be identified as a class of molecules with an imino group NR and a varying group X (e.g., F, RO, R2N, R3C) bonded to boron. Iminoboranes (XBNR) belong to the family of neutral two-coordinated boron species which may be arranged systematically in the following way ... 

Much is known empirically on B-NMR data (39). Restricting boron compounds to noncyclic molecules with no or one nitrogen atom and two, three, or four carbon atoms bonded to boron and omitting sterically overcrowded alkyl groups R and R, we find four classes with the following typical chemical shift ranges (Et20 BF3 as the external standard) ... 

Trimethylamine replaces PH3 quantitatively in BH3 PH3 to give BH3 N(CH3)3. Liquid or gaseous ammonia also replaces phosphine in the compound BH3 PH3 to the extent of 52-58% or up to 75%, respectively. The hydrogen atoms bonded to boron are partially, or completely, replaced by chlorine on treatment with hydrogen chloride, depending on the reaction conditions. The first stage of the reaction with hydrogen chloride is the formation of the colourless, viscous liquid BHjCl PH3 (see below). 

In systems derived from PH3 BH3, changes in the coupling constant, /pH, are found which are similar to those seen in compounds derived from PH3, i.e. compounds with phosphorus in coordination number 3. The observation that the alteration does not correspond to that seen for derivatives of PHi suggests that the very weak donor-acceptor bond in PH3 BH3 does not affect the hybridisation of phosphorus, as compared with that in free PH3, as much as might at first be expected. A small increase in the electron withdrawing action by halogen substitution of the hydrogen atoms bonded to boron causes an increase in the s character of the P-H bonds. 

Addition Reactions. In general, polar molecules such as hydrogen halides add across the B—N bonds, the more electronegative group bonding to boron (91). The adducts are cydotriborazanes such as the product formed by reaction of B-trichloroborazine and hydrogen chloride (eq. 35). X-ray crystal analysis shows the structure exists in a chair conformation (124). 

Fig. 10-5.—The atomic arrangement in e molecule of decaborane, Bi0Hu. Note that the 10 boron atoms are approximately at corners of a regular icosahedron. Each of the two remaining corners of the icosahedron may be considered to have been replaced by two bridging hydrogen atoms. The other 12 hydrogen atoms are bonded to boron atoms in such a way that the B—H bonds extend out from the center of the icosahedron.

An additional / -donor ligand results in the most stable boron cations because of the filled octet of boron and the complete coordination sphere.583 Boron in cation 240 has a distorted tetrahedral geometry and a N(l)—B—C angle of 99.5°.583 In cation 241 the boron atom is complexed to [Cp Fe(CO)2].595 The Cp group is bonded to boron in an r 5 fashion and the Cp-B-Fe vector is essentially linear (bond angle = 177.86°). Both spectroscopic data and DFT calculations (B3LYP with LANL2DZ and 6-31 +G ... 

Atom bonded to boron is indicated in bold. bReferenced to BF3-OEt2 unless otherwise indicated. 

The alkyl group migrates with the two electrons from its bond to boron and as a result the migration occurs with retention of the stereochemistry of the alkyl group. 

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