Oxygen-Bridged Borate Anions from Tris(pentafluorophenyl)borane:  Synthesis, NMR Characterization, and Reactivity

Abstract

The previously known anion [(C6F5)3B(mu-OH)B(C6F5)3]- (2) has been prepared by a two-step procedure, involving deprotonation of (C6F5)3BOH2 to give [B(C6F5)3OH]- (1), followed by addition of B(C6F5)3. The solution structure and the dynamics of 2 have been investigated by 1H and 19F NMR spectroscopy. The reaction of [NHEt3]2 with NEt3 resulted in the formation of [NHEt3]+ [(C6F5)3BOH]-, [NHEt3]+ [(C6F5)3BH]-, and (C6F5)3B- (CH2CH=N+ Et2). This indicates that in the presence of a nucleophile anion 2 can dissociate to B(C6F5)3 and 1. The reaction of [HDMAN]2 with 1,8-bis(dimethylamino)naphthalene (DMAN) confirmed this trend. In the presence of water, 2 transformed into the adduct [(C6F5)3BO(H)H...O(H)B(C6F5)3]- (3), containing the borate 1 hydrogen-bonded to a water molecule coordinated to B(C6F5)3. The same compound is formed by treating (C6F5)3BOH2 with 0.5 equiv of a base. A competition study established that for 1 the Lewis acid-base interaction with B(C6F5)3 is about 5 times preferred over H-bonding to (C6F5)3BOH2. The X-ray single-crystal analysis of [2-methyl-3H-indolium]3 provided the first experimental observation of an asymmetric H-bond in the [H3O2]- moiety, the measured O-H and H...O bond distances being significantly different [1.14(2) vs 1.26(2) A]. The reaction of NEt3 with an equimolar mixture of B(C6F5)3 and bis(pentafluorophenyl)borinic acid, (C6F5)2BOH, afforded the novel borinatoborate salt [NHEt3]+ [(C6F5)3BOB(C6F5)2]- ([NHEt3]4). X-ray diffraction showed that the B-O bond distances are significantly shorter than in [(C6F5)3B(mu-OH)B(C6F5)3]-. Variable-temperature 19F NMR revealed high mobility of the five aryl rings, at variance with the more crowded anion 2. 2D NMR correlation experiments showed that in CD2Cl2 the two anions [(C6F5)3BOH]- and [(C6F5)3BH]- form tight ion pairs with [NHEt3]+, in which the NH proton establishes a conventional (BO...HN) or an unconventional (BH...HN), respectively, hydrogen bond with the anion. The diborate anions 2-4, on the contrary, gave loose ion pairs with the ammonium cation, due both to the delocalized anionic charge and to the more sterically encumbered position of the oxygen atoms that should act as H-bond acceptors.

Non‐Metal‐Mediated Homogeneous Hydrogenation of CO2 to CH3OH

“…[10] The 1 H NMR (C 7 D 8 ) spectrum of 2 reveals a ca. 2 ppm downfield shift of the NH 2 protons relative to 1 and also displays a septet resonance at 8.24 ppm (J HF = 2 Hz), consistent with a H…F interaction between the formate proton and each of the six ortho-F's of the B(C 6 F 5 ) 3 unit; this assignment was confirmed through selective heteronuclear 19 F decoupling experiments.…”

“…[9] We have found that admission of CO 2 to a toluene solution of 1 at 100°C produces the unique formato-borate complex [TMPH][HCO 2 B(C 6 F 5 ) 3 ] (2) in quantitative yield; the reaction can be conveniently monitored using solution 19 F NMR. [10] The 1 H NMR (C 7 D 8 ) spectrum of 2 reveals a ca.…”

“…The establishment of equilibrium concentrations of CO 2 and 1 from 2 is followed by reversible decomposition of the borohydride salt into free H 2 , TMP and B(C 6 F 5 ) 3 ; evidence for this process was established through 19 F NMR in which a solution of 1, when heated to 160°C, shows the presence of 17% B(C 6 F 5 ) 3 . Attack of this B(C 6 F 5 ) 3 upon the acyl oxygen atom of 2 produces an intermediate (Scheme 3, A) which is thought to be the origin of the broad doublet seen at 174.5 ppm in the 13 C NMR spectrum; the downfield shift and line-shape corroborate an augmented electron deficiency due to the coordination of an additional bulky B(C 6 F 5 ) 3 molecule around a 3 single carbon centre.…”

“…of C 6 F 5 H and CH 3 OB(C 6 F 5 ) 2 , whilst that of the former was slower, proceeding to the boroxin [OB(C 6 F 5 )] 3 [18] (presumably via HOB(C 6 F 5 ) 2 ); all of these are identified as products in the completed reaction using 1 H, 19 Since the only labile source of protons in the decomposition reactions of 3 and 4 is the TMPH cation, this suggests that recombination of the ion pairs to form TMP and ROH·B(C 6 F 5 ) 3 (R = CH 3 , H) must occur (Scheme 4); corroborating this hypothesis is the detection of H 2 O·B(C 6 F 5 ) 3 H-bonded with various anions in the MS(ES). [19] Whilst studies have shown that such adducts can dissociate to give ROH and free B(C 6 F 5 ) 3 , [20] at these temperatures protonation of the ipso-C on the C 6 F 5 rings appears to be faster (yielding ROB(C 6 F 5 ) 2 and C 6 F 5 H), which preclude any catalytic turnover. Finally, addition of one equivalent of CO 2 to a 1:1 mixture of TMP/B(C 6 F 5 ) 3 (4 eq., excess, under H 2 atmosphere) in C 7 D 8 demonstrated quantitative conversion to CH 3 OB(C 6 F 5 ) 2 via 2, after 6 days at 160°C.…”

Borohydrides from Organic Hydrides: Reactions of Hantzsch’s Esters with B(C6F5)3

“…[15][16][17][18][19][20] In scattered examples, bulky amines have been shown to act as hydride donors [21] in reactions with BA C H T U N G T R E N N U N G (C 6 F 5 ) 3 via a C À H activation alpha to nitrogen [Eqs. (1)(2)(3)].…”