Gas-Phase Reaction of Tetraborane(10) and Ethyne:  Molecular Structure of nido-1,2-C₂B₃H₇ in the Gas Phase

Abstract

The molecular structure of nido-1,2-C(2)B(3)H(7), 1, the principal volatile carborane generated in the quenched gas-phase reaction of B(4)H(10) and ethyne at 70 degrees C, has been determined by a combined analysis of gas-phase electron-diffraction data and rotation constants restrained by ab initio computations at the CCSD(T)/TZP' level. The structure is consistent with a geometry having C(s)() symmetry, similar to that of pentaborane(9). The apical position is occupied by a carbon atom, displaced toward B(4) from a position directly above the B(5).B(3) vector, and hydrogen atoms asymmetrically bridge the B-B bonds. The basal atoms are almost coplanar, C(2) lying ca. 2 degrees below the B(3)-B(4)-B(5) plane. Important experimental structural parameters (r(alpha) degrees /pm, angle(alpha)/ degrees ) are r[C(1)-C(2)] = 162.6(6); r[C(1)-B(3)] = 161.4(3); r[C(2)-B(3)] = 154.3(2); r[C(1)-B(4)] = 157.4(5); r[B(3)-B(4)] = 185.7(3); <B(3)-B(4)-B(5) = 80.9(1). In addition to this and the other previously reported carboranes, 2,3-C(2)B(4)H(8), 2-Me-2,3,4-C(3)B(3)H(6), and 4-Me-2-CB(5)H(8), several new derivatives have been identified among the volatile products. These include the dicarbahexaboranes 2,4-Me(2)-2,3-C(2)B(4)H(6) and 5-Et-2,3-C(2)B(4)H(7) (the major volatile products obtained when the reaction is allowed to go to completion, previously reported as tricarbahexaboranes) and the derivatives 2,5-Me(2)-2,3-C(2)B(4)H(6), 4-Et-2,3-C(2)B(4)H(7), 1-Me-2,3,4-C(3)B(3)H(6), 2-Me-2-CB(5)H(8), 3-Me-2-CB(5)H(8), and 2,3,4,5-C(4)B(2)H(6). The complex mechanism of the reaction is discussed in light of these new results.

nido‐Five‐Vertex Clusters: In and Out of Boron Chemistry

“…The 1,2‐dicarba‐ nido ‐pentaborane structure of 1 b was originally assigned from IR, 11 B and 1 H NMR spectroscopy, and mass spectrometry data. The results of a microwave study of 1 b , although announced as a private communication in 1972,5 were finally published in 19886a and in 1998 a combined analysis of gas‐phase electron‐diffraction data and rotation constants restrained by ab initio calculations was reported 6b…”

Quenched gas-phase reactions of tetraborane(10), B4H10, with substituted alkynes: new nido-dicarbapentaboranes and arachno-monocarbapentaboranes

“…This is assigned to the C(1)H peak of 2-Me-1,2-C 2 B 3 H 6 , the sextet pattern arising from coupling with three terminal and two bridging hydrogens at the boron atoms ( Figure 1). 19 The boron-11 NMR peaks with larger intensities at -13.5 and 10 -19.4 ppm are thus assigned to 2-Me-1,2-C 2 B 3 H 6 2a. The substituent at C(2) causes a 'trans effect' on the B(4) boron-11 peak shift compared to the parent compound 1,2-C 2 B 3 H 7 which has peaks at -13.4 and -15.8 for B (3,5) and B(4) atoms respectively.…”

“…Quenched reaction 10,13,19 25-70 o C, 20-60 min B 4 H 10 + HC≡CH → nido-1,2-C 2 B 3 H 7 (25%) 50 Grimes, Bramlett and Vance 7,9 and the Leeds group 16,17,18 have also investigated the flash and completed gas-phase reactions of B 4 H 10 with propyne and 2-butyne under similar reaction conditions (see reactions 4 and 5 below), and Grimes and Ledoux have explored the completed gas-phase reaction of B 4 H 10 55 with trimethylsilylethyne -no volatile carbaborane product was obtained from this latter reaction. 14 …”

“…These reactions have also been explored more recently in Leeds. [16][17][18][19] The reactions were of three 35 types, and the major carbaborane product in each case is shown below: 1) High-energy ('Flash') reaction 6,7,16,17 100 o C, 'Flash' 40 B 4 H 10 + HC≡CH → closo-2,4-C 2 B 5 H 7 (<1% yield) 2) Completed reaction 6,[8][9][10][11][12][13][14][15][16][17][18][19] 25-70 o C, 3 days B 4 H 10 + HC≡CH → nido-2,4-Me 2 -2,3-C 2 B 4 H 6 (7%) 45 …”

nido‐Cluster mit fünf Ecken: ein allgemeines Konzept aus der Borchemie

“…Mikrowellenuntersuchungen von 1 b wurden bereits 1972 erwähnt,5 jedoch erst 1988 ausführlich beschrieben 6a. 1998 wurde eine kombinierte Analyse von Gasphasen‐Elektronenbeugungsdaten und durch Ab‐initio‐Rechnungen verfeinerten Mikrowellendaten veröffentlicht 6b…”