MICROWAVE SPECTRA, MOLECULAR STRUCTURE AND AROMATIC CHARACTER OF BN-NAPHTHALENE (4A,8A-AZABORANAPHTHALENE) AARON M. PEJLOVAS, STEPHEN G. KUKOLICH, University of Arizona ARTHUR J. ASHE III, University of Michigan, ADAM M. DALY, University of Arizona . ISMS – 16 – P1531

B-N substitution in AROMATIC Molecules How aromatic is AZABORINE? S. Y. Liu – 2009 > OSU - 2010 PLANAR MOLECULE 0 = 0.02 amuÅ2 BOND LENGTH 1.45Å BETWEEN SINGLE AND DOUBLE BOND LENGTH C) eqQ’s CONSISTENT WITH PARTIAL DELOCALIZATION of p ELECTRONS Parameters for BN-NAPTHALENE synthesis by A. J. Ashe 2014 PLANAR MOLECULE 0 = -0.159 amuÅ2 B-N BOND 1.47(2) Å, BETWEEN SINGLE AND DOUBLE BOND LENGTH C) eqQ’s CONSISTENT WITH PARTIAL DELOCALIZATION of p ELECTRONS 2 2 2

 Synthesis of BN-naphthalene.13 Allyltributylstannane and boron trichloride > diallylboron chloride treat with diallylamine then trimethylamine > 1:1 addition product. Ring closing produces the bicyclic BN-heterocycle. React this with Pd-black > mixture of BN-naphthalene and BN-tetralin, separated by column chromatography on silica gel a) BCl3 b) HN(C3H5)2 c) NEt3 d) cat. (Cy3P)2(PhCH)RuCl2 e) Pd black 13. Rohr, A. D.; Kampf, J. W.; Ashe, A. J., III Organometallics 2014, 33, 1318. 3 3

 Microwave Measurements Flygare-Balle Spectrometer – Homodyne Detection, BN napthalene room temp Isotopologues > normal with 10B, 11B, and all unique single 13C and the 15N (with 11B), in natural abundance Measurements 3-11 GHz > 1982 Spectrometer Measurements 2-3 GHz > BIG Spectrometer – 2011 > 48” dia. Mirrors, 35” diff. pump

Measured lines 2-11 GHz Example transitions > 10B14N (129 pulsed beam cycles), 11B14N 13C4 or 13C12 (2436 pulsed beam cycles), and 11B15N (7803 pulsed beam cycles)

J′ Ka’ Kc’ F’ J″ Ka” Kc” F” obs o-c 1 1 1 3 0 0 0 2 3896.0701 0.0000 Measured rotational transitions for the 11B15N isotopologue.. Values shown are in MHz. Only A, B, and C were fit with these transitions. J′ Ka’ Kc’ F’ J″ Ka” Kc” F” obs o-c 1 1 1 3 0 0 0 2 3896.0701 0.0000 2 1 2 4 1 0 1 3 5619.2651 -0.0002 3 1 3 2 2 0 2 1 7189.5835 0.0003 4 1 4 3 3 0 3 2 8663.3870 0.0025 3 0 3 3 8664.1787 -0.0027 5 1 5 4 4 0 4 4 10115.7393 6

Experimental and calculated parameters for (11B14N-naphthalene). Basis sets - B3LYP/aug-cc-pVTZ, B3LYP/aug-cc-pVQZ, MP2/aug-cc-pVTZ and B97D/aug-cc-pVTZ 7

Experimental and calculated parameters for (11B14N-naphthalene). Basis sets - B3LYP/aug-cc-pVTZ, B3LYP/aug-cc-pVQZ, MP2/aug-cc-pVTZ and B97D/aug-cc-pVTZ Azaborine shown for comparison 8

 Structure Fit Inertial Defect Δ = -0.159 amu Å2 , so planar structure and C2v symmetry assumed 10 variable parameters describing relative positions (cartesian coordinates) of heavy atoms H atom coordinates relative to bonded atom from B3LYP calculation The standard deviation for the structure fit was 0.151 MHz Uncertainties in heavy atom coordinates > ±0.005 Å. 9 9

 Structure and Atom Cooridinates a (calc.) b (calc.) a (exp.) b (exp.) Krait-|a| Krait-|b| C1 -2.390 -0.741 -2.400 -0.743 2.397(7) 0.743(2) C2 -1.188 -1.369 -1.203 -1.373 1.193(1) 1.372(1) C3 -1.371 1.461 -1.354 1.466 1.376(1) 1.464(1) C4 -2.490 0.681 -2.499 0.689 2.496(8) 0.684(2) B17 0.000 0.806 0.796 0.279(5) 0.795(2) N18 -0.669 -0.674 0.08*i(2) 0.675(2) 10 10

r(B-N) Interatomic Distances Microwave Fit Value (Å)  Comparison of B-N bond distances for BN-naphthalene and other molecules with the B-N bond.  The B-N bond lengths agree where there is -donation from the N p- orbital to the empty p-orbital of B.  This results in some double bond character which shortens the bond length. r(B-N) Interatomic Distances Microwave Fit Value (Å) Calculated Value (Å) BN-naphthalenea 1.470 1.475 1,2-dihydro-azaborineb 1.45 1.437 BN-cyclohexenec - 1.40 H3NBF3d 1.59 BH3NH3e 1.6576 H2NBH2f 1.391 HCN-BF3g 2.47 11

 Quadrupole Coupling & Townes-Dailey analysis -(Stew Novick slide) Extended Towned-Dailey analysis of Azaborine by Novick (S. E. Novick, J. Mol. Spectrosc. 267, (2011), 13–18) NITROGEN populations of the sp2 and 2pz orbitals in azaborine.   STO-3G npz 1.00 1.20 1.40 1.50 1.60 2.00 1.58 ng1 (nH) 0.91 1.11 1.31 1.41 1.51 1.91 1.28 ng2 + ng3 (nC+nB) 1.76 2.16 2.56 2.76 2.96 3.76 2.52 ntotal about N 4.47 5.27 5.67 6.07 7.67 5.38 Occupations Townes-Dailey NBO N (pz) 1.3 1.4 1.45 NB (sp2) 1.5 1.6 NC+NC (sp2) 2.3 2.5 2.6 Ntotal 5.0 5.4 5.6 Charge 0.0 -0.43 -0.63 Electron orbital occupations for NITROGEN in BN-naphthalene > very similar to occupations for 1,2-dihydro-1,2-azaborine, which behaves very similar to other aromatic N-containing molecules such as pyrrole. SIGNIFICANT AROMATIC CHARACTER for BN napthalene and Azaborine 12 12

 Quadrupole Coupling & Townes-Dailey analysis -(Stew Novick slide) Extended Towned-Dailey analysis of Azaborine by Novick (S. E. Novick, J. Mol. Spectrosc. 267, (2011), 13–18) nitrogen electron populations of the sp2 and 2pz orbitals in azaborine.   STO-3G npz 1.00 1.20 1.40 1.50 1.60 2.00 1.58 ng1 (nH) 0.91 1.11 1.31 1.41 1.51 1.91 1.28 ng2 + ng3 (nC+nB) 1.76 2.16 2.56 2.76 2.96 3.76 2.52 ntotal about N 4.47 5.27 5.67 6.07 7.67 5.38 Occupations Townes-Dailey NBO N pz 1.3 1.4 1.45 NB (sp2) 1.5 1.6 NC+NC (sp2) 2.3 2.5 2.6 Ntotal 5.0 5.4 5.6 Charge 0.0 -0.43 -0.63 Electron orbital occupations for N in BN-naphthalene > very similar to occupations for 1,2-dihydro-1,2-azaborine, which behaves very similar to other aromatic N-containing molecules such as pyrrole. SIGNIFICANT AROMATIC CHARACTER for BN napthalene and Azaborine 13 13

 Arthur Ashe III, UNIVERSITY of MICHIGAN Acknowledgements:  Arthur Ashe III, UNIVERSITY of MICHIGAN  Lu Kang, KENNISAW STATE UNIVERSITY (for help with early measurements on this molecule  N$F – We thank the National Science Foundation for Grants CHE-1057796, CHE-0721505 14 14

ISMS – 16 – P1531