Pyrazole‐Organoboranes, VI. Monomeric and Dimeric 9‐Pyrazolyl‐9‐borabicyclo[3.3.1]nonanes

Abstract

Bis(9H-9-borabicyclo[3.3.l]nonane) (9H-Q-BBN)z reacts with (tb),mPz, and (tb),ePz, respectively] to give the 9-pyrazolyl-9-pyrazole (Pz) and its 4-bromo, 3-methyl, 3-phenyl, 3,5-dimethyl, borabicyclo[3.3.l]nonanes with the dimeric structures (1)2 -(3)2 3-methyl-5-phenyL 3S-dipheny1, 3S-di-tert-buty1, 3,5-diada-(X-ray, NMR, and MS analysis), and 4 -11 with monomeric mantyl, 3,5-di-terf-butyl-l-methyl, and 3,5-di-tert-butyl-4-ethyl structures (X-ray, NMR, MS In a previous publication*) we have shown that substituents in the 3-and/or 5-position of pyrazoles hinder the coordination of these N bases to the boron atom(s) of bis(l,5-cyclooctanediylboryl) oxide or sulfide. Dialkyl-pyrazolyl-boranes in the monomeric form I with a trigonal boron atom should become accessible by utilizing these steric interactions. This class of boron-nitrogen compounds has been extensively studied, and in all cases its members have been found to exist in the dimeric state U3-5) (X-ray'), NMR'), and MS8) analysis). The only monomeric pyrazolylborane species reported so far are the diamino-pyrazolylboranes 1119', in which the Lewis acidity of the trigonal boron atom can be assumed to be significantly reduced by n orbital interactions with the three neighbouring nitrogen atoms. In a preliminary account of this work") we have shown that pyrazoles with very bulky substituents in their 3-and 5-positions react with bis(9H-9-borabicyclo-[3.3.l]nonane) (9H-9-BBN)2 to form the corresponding monomeric 9-pyrazolyl-9-borabicyclo[3.3.1]nonanes. In this publication we show how the reaction of (9H-9-BBN)z with a number of pyrazoles with progressively increased number and sizes of the substituents gradually decreases the stability of the dimeric form and eventually leads to stable monomeric diorgano-pyrazolyl-boranes. Results and DiscussionThe dimeric 9-pyrazolyl-9-borabicyclo[3.3.l]nonane (1)* has previously been obtained in an indirect way by the reaction of the B20N2 heterocycle 1V with (9H-9-BBN);l).We have now found that (l)z is more readily accessible by

(2,2′-Bipyridyl-N,N′)(bicyclo[3.3.1]nonane-C1,C5)boronium trifluoromethanesulfonate

“…The dimensions of the 9-borabicyclo[3.3.1]nonane group are comparable to those found in related structures (Brock et al, 1992;Fraenk et al, 1999;Yalpani et al, 1990;Wrackmeyer et al, 1995). The B-C bond lengths are 1.6150 16…”

B–N Cleavage in (9‐BBN)bis(pyrazolyl)borate NiII Complexes

“…The B–N cleavage occurs during the dimerization process, with elimination of a PPh 3 and a BBN‐pz. In accordance with this, the eliminated BBN‐pz could be observed by 11 B NMR at 55.6 ppm, which is consistent with the reported chemical shift for a BBN‐pz monomer …”

Pyrazabole formation by thermolysis of the free acid of the hydrotris(pyrazolyl)borate ion

“…This latter distance is longer than in only one other pyrazabole. 15 A survey 16 of relevant precedents 4,8,13,14 reveals that the bonds from B to terminal pyrazolyls are always shorter than those to the bridging ones. The mean B-N (terminal) distances in A and B are 1.517 [2] and 1.496 [3] Å respectively; it appears that the longer B-N (ring) distances in B are offset by shorter B-N (terminal) distances, these being shorter than in any other B-pyrazolylpyrazabole.…”

“…This degree of difference has never before been observed, and it is difficult to rationalise its origins since there appears to be little correlation between the nature of pyrazabole B-borne substituents, the position they occupy and the length of their bonds to the B atom in preceding structures: thiolato, 4 pyrazolyl 8 or F groups 17 might be somewhat more strongly bound at pe than pa sites, Cl groups show no preference 17 and, interestingly, phenyl or large alkyls may show stronger binding at pa sites than pe. 15,18 There are no clear intermolecular interactions such as would be responsible for the difference in lengths in A. All 'pyridinic' N atoms in both A and B structures make intermolecular contacts with H atoms, the closest of which are 2.52(2) Å for N(8) ؒ ؒ ؒ H(4) (1 Ϫ x, Ϫy, Ϫz) in A and 2.41(2) Å for N(8) ؒ ؒ ؒ H(9) (x Ϫ 0.5, y, 0.5 Ϫ z) in B; the former contact probably contributes to the difference in bond lengths in A, but the magnitude of the difference suggests that other factors also play a role.…”