Crystals of diphenylboron N-methylacethydroxamate are orthorhombic, a = 12.5478(8), b = 7.8735(3), c = 13.6809(5) Å, Z = 4, space group Pnam. The structure was solved by direct methods and was refined by full-matrix least-squares procedures to R = 0.037 and Rw = 0.054 for 1265 reflections with I ≥ 3σ(I). The molecule features a five-membered BO2CN ring which lies in the crystallographic mirror plane. The carbon and nitrogen atoms of the heterocyclic ring are positionally disordered. Mean bond lengths (corrected for libration) are: O—B, 1.550(2), B—C, 1.609(2), O—C/N, 1.340(3), C—N, 1.300(2), C/N—CH, 1.470(2), and C—C(phenyl), 1.394(8) Å.
Es werden Borchelate 2 -6 aus N-Alkyl-bzw. N-Arylhydroxamsauren 1 und verschiedenen Borverbindungen, die fur eine Chelatbildung mit zweizahnigen Liganden geeignet sind, dargestellt. Die spektroskopischen Daten stehen im Einklang mit der cyclischen B,N-Betainstruktur von 2 -6 und reflektieren sowohl die Lewis-Aciditat der chelatisierten Borenium-Ionen als auch das Ausman der Elektronendelokalisierung in den Hydroxamat-Liganden. Boron Chelates of N-Substituted Hydroxamic AcidsBoron chelates 2 -6 are synthesized from N-alkyl-or N-arylhydroxamic acids 1 and various boron compounds that are capable of being chelated by bidentate ligands. The spectroscopic data correspond to the cyclic B,N-betaine structure of 2 -6 and reflect the Lewis acidity of the chelated borenium ions as well as the extent of electron delocalization in the hydroxamate ligands.Im Verlauf unserer Untersuchungen der B,N-Betainstruktur von Hydroxamsaure-Borchelaten -3, haben wir verschiedene Borchelate von N-alkylierten bzw. N-arylierten Hydroxamsauren dargestellt. Die ersten Verbindungen dieses Typs waren von Urnland und Mitarbb. mit dem Diphenylb~rchelat~) und dem Difluorborchelat5) der N-Phenylbenzohydroxamsaure vorgestellt worden. Eine massenspektrometrische Charakterisierung beider Verbindungen folgte spater6). In neueren Arbeiten wurden dann ein Difluorborchelat 2, und ein Diphenylborchelat 3, der N-Methylacetohydroxamsaure jeweils mit Rantgenstrukturanalyse beschrieben und weiterhin einige Difluorborchelate als Kontrollsynthone fur Photocy~lisierungen~) und fur die Gewinnung spektroskopischer Vergleichsdaten beim Studium protonierter Hydroxamsauren * -lo) eingesetzt.Die Borchelate 2-6 , in denen das Proton der N-substituierten Hydroxamsaure 1 durch ein Borenium-Ion (X,B') ersetzt ist, lassen sich am zutreffendsten mit der Grenzformulierung D charakterisieren, da nach den rontgenkristallographischen Befunden2s3) die B,N-Betainform D die Bindungszustande des Resonanzhybrids (C ++ D) am besten wiedergibt. Monzyk und Crumbliss haben an Eisen(II1)-hydroxamaten gezeigt, daR die Komplexstabilitat wesentlich durch die Elektronendonorfahigkeit des N-Substituenten (R') beeinflufit wird, wobei sowohl induktive als auch Resonanz-Effekte die Delokalisie-0 Verlag Chemie GmbH, D
(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
. Can. J. Chem. 61. 2329 (1983). Details of the synthesis, physical properties, and n~olecular structure of 4-isopropylidcne-2,5,5-triphenyl-I ,3-dioxa-4-azonia-2-bora-5-boratacyclopentanc are reported. Crystals of this compound are nlonoclinic, tr = 14.126(3), b = 16.008(1), c = 8.642(2) A, P = 99.052(8)", Z = 4, space group P 2 , / I T . Thc structure was solved by dircct methods and was refinetl by full-matrix least-squares procedures to R = 0.032 and R,,. = 0.033 for I 1 17 reflections with! 2 3 4 1 ) . The nlolecule contains a nearly planar five-membered B-0-B-0-N ring which incorporates both trigonal planar boron (B-0 = 1.333(4) anti 10.418(4), B-C = 1.548(5) A) and tetrahedral boron atoms (B-0 = 1.495(4). B-N = 1.676(4), B-C = 2 at 1.603(5) A). Other important libration-corrected bond distances include: C=N = 1.284(3) and N-0 On rapporte les details relatif 2 la synthise, aux propriCtts physiques et h la structure molCculaire de I'isopropylid&nc-4 triphCnyl-2.5.5 dioxa-1,3 azonia-4 bora-2 borata-5 cyclopcntane. Lcs cristaux dc cc $ompost sont monocliniques et appar- Compound 4 contains diphenylborinic and phenylboronic acid moieties, both of which are detectable by chemical methods such as the blue color reaction with diphenylcarbazone (2, 3) or chelate formation w~t h 2-hydroxyethyiamine (or bis(2-hydroxyethy1)amine) which indicates the presence of diphenylborinic acid (or phenylboronic acid) (3, 4).The ir spectrum of 4 shows a distinct absorption in the C=N stretching region at 1668 c m ' . This band is shifted to a higher frequency compared with compound 3 (1639 cm-') ( I ) which can be explained by a more pronounced iminium salt character of 4.The 'H-nmr spectrum of 4 features resonance signals for the 15 phenyl protons of the PhB-and Ph2B-moieties as well as two sharp singlets, separated by 0.35 ppm, for the two magnetically non-equivalent methyl groups of the isopropylidene substituent. group is a common feature of diphenylboron chelates (6) and leads mostly to very stable fragments. A peak of tnle 264 was also observed in the mass spectrum of 3 (1) and was interpreted as a (monomer 2 + BO)' ion, like similar particles which were found as prominent peaks in the mass spectra of other (alkylideniminoxy)diorganylboranes and considered as characteristic fragments of this type of compound (7). This demonstrates the high stability of the heterocyclic particle (4 -C,H,)' or analogs of it. In support of these findings the mass spectrum of 5 contains the prominent fragment (5 -C2Hs) (5). The peak with the highest mass ( m l e 312) and a relatively high abundance in the spectrum of 4 suggests the formation of (PhBO) -t l i o . r t r -4 -t r : o 1 t i~r -Z -/ 1 o r~1 -j -~1~~1 r t o 1 1 c~to determine the structure of 4, a fill1 X-ray crystallographic A cryst;" bounded by the six hccs (followed by their tlistanees in ExperimentalMethod a A solution of oxybis(diphenylboranc) (0.70 g, 2 mrnol) and acetone oxirne (0.30 g. 4 mmol) in 50 mL benzene was refluxed for 20 rnin using a Dean-Stark trap for the continuo...
. 61, 2493 (1983). The crystal structures of 2,2-d1phenyl-l,3-dioxa-3a-azonia-2-borataindan, 6a, and 4-cyclohexyl-6-methyl-2,2-d~phenyl-1,3-dioxa-3a-azonia-2-borataindan, 6b, reported here represent the first crystallographic examples of coordination complexes of the anionic ligand derivedo from 2-hydroxypyridine N-oxide. Crystals of 6a are monoclinic, a = 11.5210(13), 6 = 10.2860(8), c = 12.7354(15) A, P = olO1 .878(5)", Z = 4, space group P 2 , l c ; and those of 6b are monoclin~c, a = 8.5047 (8)
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