Crystal and molecular structure of difluoroboron N-methylacethydroxamate (2,2-difluoro-4,5-dimethyl-1,3-dioxa-4-azonia-2-boranatacyclopent-4-ene)

Abstract

(1977). Metal hydroxamates are well known (1 1-15), but structural formulations are (as with hydroxamic acid or hydroxamate anions) still subject to some controversy (6,15). Spectroscopic data do not provide sufficient information for a definitive formulation. Evidence for at least partial C-N double bond and C-O single bond character is mainly given by X-ray crystallographic studies of organotin (16-18), iron (19), and zinc (20) complexes of hydroxamic acids and a nickel thiohydroxamate (15,21). Chelated protons are also easiiy displaced by "pseudometal" cations (22) X,Bf which are well established (23-27) as remarkable acceptors in the formation of chelate complexes. The bidentale dianionic ligand in Id should be well suited for the synthesis of boron chelates and in fact hydroxamic acids and their N-substituted derivatives readily form X,B-chelates 2 (8,9,26,(28)(29)(30)(31).' The uv spectrum of the anion 3 suggests the presence of C=N double bonding (8, 9 ) whereas for a cyclic thiohydroxamate the thione form, analogous to 2a, is favored but not unambiguously proven (30). With the assumption that a betainic structure 4a represents the best formal description of the chelates 2a-d, the study of complexes of Nsubstituted2 hydroxamic acids with electronaccepting boron compounds seemed very useful. One of the simplest molecules of this type is the N-methylacethydroxamate of the extremely electron-deficient difluoroborenium ion ( 2 or 4, R = R' = Me, X = F). The ir spectrum of Nmethylacethydroxamic acid (1. R = R' = Me) (32) displays an absorption around 1625 cm-' which can be assigned to C=O of the hydroxamic or to C=N of the hydroximic form (33). In the difluoroboron chelate 2 or 4a this band is shifted to a higher frequency (about 1675 cm-l) into the region of C=Nf vibrations of iminium salts, which is a general effect (34) associated with quaternization of a C=N group. The iminium structure 2b-d or 4a is also supported by the 'H-nmr spectrum, which shows a low field shift of the N-methyl protons at 6 = 3.70 ppm which can be compared with the values for N-methyl protons in nitrones 6 (R = Me) of 3.70 ppm. (6 (ppm) for N-methyl protons in N-methyl-C

Structural studies of organoboron compounds. XII. Crystal and molecular structures of (acetylacetonato)diphenylboron and (tropolonato)diphenylboron

“…These values are both near the short end of the range 1.603(4) A in 4 to 1.639 (8) A in 5 observed for Ar,B chelates (1-3, 5, 6, 9, 11). Differences between chemically equivalent B-C distances have been observed earlier (3,5,6,28).…”

“…Unit-cell parameters were refined by leastsquares on 2 sin Blh values for 25 reflections measured on a diffractometer with Mo K a , radiation (h = 0.70930 A). Crystal data at 22OC are: C I~H I~B O , fw = 264.14 Triclinic, a = 8.3557 (8), b = 9.3519 (8), c = 9.6103 (9) A, a = 96.094(5), P = 94.904 (6), y = 93.489(6)", V = 742.1(1) A3, Z = 2, p, = 1.182 g cm-', F(000) = 280,p(Mo Ka) = 0.70 cm-I. (14) 7421 (20) 8294 (14) 66 ( 5) 56401(15) 65011 (22) 84762 (14) 61 H (4) 7182 (16) 6809 (20) 9290 (16) 79 ( 6) 66497(16) 60914 (26) 90986 (15) 7 1 H (5) 7733 (17) 4797 (19) 9862 (17) 80 ( 6) 69727(17) 48778 (27) 94994 (16) 75 H (6) 6713 (17) 2980 (22) 9786 (18) 92 ( 7) 63461 (17) 37567 (24) 94171 (15) 68 H (7) 4903 (15) 2675 (21) 8959 (16) 80 ( 6) 52479 (14) 35418 ( After data reduction, no absorption correction was made in view of the low value of p. Of the 4297 independent reflections measured, 2465 (57.4%) had intensities greater than 3o(I) above background where cr2(I) = S + 2B + (0.04(S -B))2 with S = scan count and B = background count.…”

“…observed in the structure of (N-methylacethydroxamato)diphenylboron (11) which also contains a planar five-membered chelate ring with two oxygen atoms bonded to boron. In 4,4-dimethyl-2,2-diphenyl-1,3-dioxa-4-azonia-2-boranatacy~lopen-tane, 5 (3), which has a non-planar chelate ring, the EL0 distances are 1.506(7) and 1.556 (8) A. The B-0 distances in 1 and 2 are longer than the true single bonds in the tetrahedrally coordinated ethanolamine esters of diarylborinic acids which have a mean bond length of 1.477(5) A (1,2,6) In all three structures, the two B-C distances are somewhat different, this difference being significant only in the case of 2.…”

Structural studies of organoboron compounds. XVIII. Crystal and molecular structures of 2,2-diphenyl-1,3-dioxa-3a-azonia-2-borataindan and 4-cyclohexyl-6-methyl-2,2-diphenyl-1,3-dioxa-3a-azonia-2-borataindan

“…With the exception of the C(18) methyl substituent in 6b (displacement = 0.008(7) A), all substituents of the pyridine rings are significantly displaced from the mean plane of the ring in the same direction (displacements range from 0.013(1) A for O(2) in 6a to 0.083 (2) A for the same atom in 6b, mean 0.049 A). Unlike the planar five-membered OBONC rings in the closely related molecules 8 (13,14), the analogous rings in 6a and 6b both exhibit non-planar B-envelope conformations, the deviation from planarity being greater in the case of 6b (see Table 5). In view of the chemical similarities between 6 and 8, the non-planarity of the five-membered rings in the present structures is most likely a result of packing forces since intramolecular steric interactions are minimal.…”

“…1983 Me \o /Me Me\ tMe N-C chelate 6 (17). In view of the B,N-betaine formulation 8 of appeared to be the betaine form 8, based on the analysis of the open-chain hydroxamic acid boron complexes, which has been bond distances which indicate an approximate contribution of verified by X-ray crystallographic analyses of the boron 75% by the resonance structure 8 (13,14). chelates 8 (X = F, Ph) of N-methylacethydroxamic acid (13, No X-ray structure analyses have yet been reported for 14), it was of special interest to determine which of the reso-2-hydroxypyridine N-oxide or for complexes of this ligand (6).…”