Catalytic dehydrocoupling of amine-boranes and amines into diaminoboranes: isolation of a Pt(II), Shimoi-type, η¹-BH complex

Abstract

The platinum complex [Pt(I^tBu′)(I^tBu)][BAr^F] is a very efficient catalyst in the synthesis of diaminoboranes through dehydrocoupling of amine-boranes and amines. Shimoi-type, η¹-BH complexes are key intermediates in the process.

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In the last few years amine-boranes and related base-stabilised borane adducts have been shown to produce rich chemistry in which metal catalysed dehydrocoupling processes are involved.¹ During some of these reactions the B–H bonds of the amine borane establish an initial interaction with the metal centre to form complexes exhibiting η¹ or η² coordination modes, undergoing thereafter subsequent reactivities.^1a,2

Dehydrogenation of amine-boranes has been mainly focused on the production and release of dihydrogen and on the generation of amino-boranes that can undergo further dimerization, oligomerization or polymerization processes.^1,2j,3 Less attention has been paid to the production of other boranes, through boron–boron coupling⁴ or the formation of diaminoboranes, (NR₂)₂BH.⁵ With regard to the latter process, only a couple of catalytic processes have been reported by Alcaraz and Sabo-Etienne, using ruthenium^5a,c and by Hill,^5b with calcium and magnesium based catalysts to produce the corresponding diaminoboranes. Catalyst loadings of 2.5% (Ru, Ca, Mg), together with long reaction times (Ru), and even heating (Ca, Mg) were required.

In this communication we wish to report a very efficient platinum(II) complex that promotes diaminoborane formation at a catalyst loading of 0.5% in a few minutes for most of the substrates tested. In addition, we have been able to characterise by means of X-ray diffraction studies the first Shimoi-type η¹-BH complex of a Lewis base-borane adduct and platinum, which provides structural insights into a key intermediate in the dehydrogenation process of amine-boranes and amines.

Previously, we had reported that the [Pt(I^tBu′)(I^tBu)][BAr^F] complex (where I^tBu stands for 1,3-di-tert-butylimidazolylidene and I^tBu′ its cyclometalated form), 1,⁶ is able to promote the dehydrocoupling of dimethylamineborane (NMe₂H·BH₃) into cyclic [NMe₂BH₂]₂ (Scheme 1).⁷ During NMR mechanistic studies, we observed that bis(dimethylamino)borane (NMe₂)₂BH is also formed in small amounts and its yield increases as the concentration of free NMe₂H in the reaction media increases. Therefore, we analysed the ability of complex 1 to act as a catalyst for the formation of diaminoboranes.

Scheme 1 Dehydrocoupling of NMe₂H·BH₃ catalysed by complex 1.

We have first studied the benchmark reaction of tert-butylamineborane (^tBuNH₂·BH₃) and 1 equiv. of tert-butylamine (^tBuNH₂) in CD₂Cl₂, using a catalyst loading of 0.5% (Scheme 2). The reaction was monitored both by NMR spectroscopy and by measuring the increase of gas pressure in a closed system due to the generated H₂ (see the ESI†). The ¹¹B NMR spectrum revealed the formation of a single species in nearly quantitative yield after 11.5 min showing a doublet signal at δ 25.8 ppm (¹J_BH = 127 Hz). In addition, the ¹H NMR spectrum exhibits a broad quartet signal centred at 4.12 ppm that sharpens upon ¹¹B decoupling into a triplet (J_HH = 8.2 Hz). These values are in agreement with the formation of diaminoborane (^tBuNH)₂BH,⁸s1 (Scheme 2). The calculated TON and TOF values for this process are 400 and 2087 h⁻¹, the highest reported to the best of our knowledge (Table 1). At the very end of the reaction, the catalyst remained the cyclometalated species 1, but is slowly hydrogenated into complex [PtH(I^tBu)₂][BAr^F], 2. Other amine-boranes were tested under identical reaction conditions to form symmetrical diaminoboranes. Diethylamine-borane was also shown to be efficiently dehydrogenated in ca. 10 min in the presence of 1 equiv. of NEt₂H and catalyst 1 (TON = 400, TOF = 2400 h⁻¹). On the other hand, pyrrolidine-borane (CH₂)₄NH·BH₃ required longer reaction times (2 h) to be converted into [(CH₂)₄N]₂BH (TON = 380, TOF = 193 h⁻¹). In the latter case, 5% of cyclic dimer [(CH₂)₄NBH₂]₂ is observed in the ¹¹B NMR spectra. Bulkier amine-boranes, such as ⁱPr₂NH·BH₃, do not undergo dehydrocoupling under similar reaction conditions, while heating at 60 °C for 12 h results in the formation of amino-borane NⁱPr₂BH₂, with no evidence of the corresponding diaminoborane.

Scheme 2 Catalytic synthesis of symmetric diaminoboranes.

Table 1 Catalytic dehydrocoupling of amine-boranes and amines

Entry^a	Amineborane	Amine	% Yield^b (isolated)	TON	TOF (h^−1)
a Reaction conditions: CH2Cl2, rt. b Yields determined by ^11B NMR spectroscopy with respect to all diaminoboranes formed. c Cyclic dimers [NR2BH2]2 and other unidentified species constitute the remaining reaction products (see the ESI). d Isolated yield corresponds to all the possible products formed.
1	^tBuNH2·BH3	^tBuNH2	>99 (99)	400	2087
2	NEt2H·BH3	NEt2H	>99 (88)	400	2400
3	(CH2)4NH·BH3	(CH2)4NH	95^c (81)	380	193
4	^tBuNH2·BH3	NEt2H	>99 (99)	400	3692
5	^tBuNH2·BH3	(CH2)4NH	94 (68)	376	376
6	NEt2H·BH3	(CH2)4NH	93	372	248
7	NMe2H·BH3	^tBuNH2	78^c (71)^d	312	2023
8	NMe2H·BH3	NEt2H	76^c	316	2257

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Asymmetrical diaminoboranes are challenging substrates to be produced catalytically in a selective manner. Hill et al. have recently reported a method for the preparation of this type of substance using group 2 metal catalysts.^5b Although in most of the cases the reaction proceeds with excellent selectivities, long reaction times (24–92 h) and mild heating are usually necessary. Complex 1 has shown very good catalytic behaviour in the formation of unsymmetrical diaminoboranes (Scheme 3). The reaction of ^tBuNH₂·BH₃ with NEt₂H takes place very fast (6.5 min, TON = 400, TOF = 3692 h⁻¹). According to NMR spectroscopy, borane (^tBuNH)(NEt₂)BH is formed almost exclusively (ca. 4% of (^tBuNH)₂BH is observed by ¹H NMR). If the reaction is carried out in the inverse way, starting from NEt₂H·BH₃ and ^tBuNH₂, the process is slower (ca. 20 min), but selectivities are comparable. The same behaviour is observed in the catalytic dehydrocoupling of (CH₂)₄NH·BH₃ and ^tBuNH₂. In this case, the reaction exhibits good selectivities towards the asymmetric diaminoborane (^tBuNH)[(CH₂)₄N]BH (92%), whereas the only symmetric borane detected is [(CH₂)₄N]₂BH (8%), with complete conversion after 2 h at rt. When the reaction is carried out the other way around, that is, starting from ^tBuNH₂·BH₃ and (CH₂)₄NH, the selectivities are only slightly different (85% of (^tBuNH)[(CH₂)₄N]BH, 15% of [(CH₂)₄N]₂BH), although the reaction proceeds in 1 h. The combination of pyrrolidine and diethylamine substrates indicates that the reaction is not selective yielding all possible reaction products (NEt₂)₂BH, [(CH₂)₄N]₂BH and (NEt₂)[(CH₂)₄N]BH in a 1 : 5.4 : 3.1 ratio, respectively, independent of the two possible combinations utilised. A similar behaviour was observed in the reaction of NMe₂H·BH₃ with NEt₂H, although we could not establish the exact ratio due to signal overlapping in the ¹H NMR (see the ESI†). Nevertheless, good selectivities are produced in the reaction of NMe₂H·BH₃ and ^tBuNH₂ (Scheme 3). Very interestingly, the reaction of ^tBuNH₂·BH₃ with ⁱPr₂NH generates diaminoborane (^tBuNH)₂BH (s1) and amino-borane ⁱPr₂N–BH₂ in ca. 5 min at rt. Since 1 does not catalyse the dehydrocoupling of ⁱPr₂NH·BH₃ at rt, ⁱPr₂N–BH₂ is probably formed through the reorganization of amino-borane ^tBuNH–BH₂ and ⁱPr₂NH.⁹

Scheme 3 Catalytic synthesis of asymmetric diaminoboranes. ^a Signal overlapping in the ¹H and ¹¹B{¹H} NMR spectra precluded establishing the exact amount of each diaminoborane (see the ESI†).

With regard to the mechanism by which these transformations occur, it has been previously shown that dehydrogenation of amine-boranes NR₂H·BH₃ leading to the corresponding aminoboranes NR₂BH₂ is a key step.⁵ Previously, we have reported that complex 1 is able to dehydrogenate NMe₂H·BH₃ into NMe₂BH₂ (that then dimerises) through a distinct mechanism that involves a first step in which the amine-borane interacts with the platinum atom through the BH protons (complex 3a in Scheme 4) followed by nucleophilic addition of free amine present in solution to the activated boron atom.⁷ According to DFT calculations the most stable coordination mode of the BH₃ moiety is η¹-BH (Shimoi type complex) but, unfortunately, no experimental evidence for this type of coordination could be provided. Here we further investigate these adducts by carrying out stoichiometric reactions between complex 1 and amine-boranes at 0 °C in CD₂Cl₂. ^tBuNH₂·BH₃ reacts with 1 leading to a new species that has been postulated as the η¹-BH derivative 3b (Scheme 4). The BH₃ group resonates as a broad signal centred at 0.12 ppm that sharpens upon ¹¹B broadband decoupling. This signal is shifted upfield from the free ^tBuNH₂·BH₃ (δ 1.36). In addition, J_PtH of the CH₂–Pt fragment has a value of 90 Hz, that is, 30 Hz smaller than in complex 1 (120 Hz), consistent with the coordination of a ligand trans to the CH₂ moiety.¹⁰ This value is smaller than that observed in dimethylamine-borane 3a (103 Hz) suggesting that the interaction with the platinum atom is stronger. The ¹¹B NMR spectrum shows a signal at −18.2 ppm that is shifted with respect to free amine-borane (−21.1 ppm). All these data agree well with the formulation of 3b as depicted in Scheme 4.¹¹ Unfortunately, the instability of 3a and 3b due to their propensity to undergo dehydrocoupling precluded their isolation as pure compounds and further characterization. To avoid this problem, tertiary amine-borane NMe₃·BH₃ was used. However, no interaction with 1 was observed, probably due to steric constrains, thus allowing the establishment of a correlation between the bulkiness of the amine-borane and its interaction with 1 (stronger interaction: ^tBuNH₂·BH₃ > NMe₂H·BH₃ > NMe₃·BH₃). Consequently, the less hindered Lewis base stabilised borane C₆H₅N·BH₃ was a judicious choice. This borane reacts with 1 to yield 3c (Scheme 5) quantitatively by NMR spectroscopy. The main feature of 3c is the broad NMR signal centred at 1.25 ppm attributed to the BH₃ protons (2.55 ppm in free C₆H₅N·BH₃). The ¹¹B{¹H} spectrum exhibits a signal at −8.9 ppm, nearly 3 ppm down-field shifted with respect to C₆H₅N·BH₃. The apparent coupling constant ¹J_B,H is ca. 85 Hz, that is, 12 Hz smaller than in free C₆H₅N·BH₃, and compares well with previously described Shimoi-type complexes.^11,12 The coupling constant of the CH₂–Pt protons with ¹⁹⁵Pt of 93 Hz is similar to that of complex 3b. A definite proof of the nature of this compound came from the solid-state structure obtained by X-ray diffraction studies. Colourless crystals suitable for this analysis were obtained by slow diffusion of a concentrated solution of 3c in CH₂Cl₂ into pentane at 0 °C. Fig. 1 depicts an ORTEP-type view of the cation of complex 3c. The complex is four-coordinated, with two N-heterocyclic carbene units in the expected trans arrangement (C(1)–Pt(1)–C(12): 168.38(19)°) one of which is cyclometalated. The fourth coordination site is occupied by the C₆H₅N·BH₃ ligand in which one of the hydride atoms of the BH₃ unit bridges the platinum nucleus. The Pt(1)–H(1B) and H(1B)–B(1) bond distances are 1.96(5) and 1.03(2) Å, respectively. The Pt(1)–H(1B)–B(1) angle of 147.48(485)° together with the long Pt(1)⋯B(1) bond distance (2.8436(5) Å) indicates a negligible interaction between platinum and boron, and therefore this complex is best described as a η¹-BH or Shimoi-type complex.^12,13

Scheme 4 Formation of complexes 3a,b.

Scheme 5 Synthesis of complex 3c.

Fig. 1 ORTEP-type view of the cation of complex 3c. Hydrogen atoms, except those of the BH₃ unit, have been omitted for clarity (ellipsoids are drawn at 30% probability). Selected bonds (Å) and angles (°): Pt1–H1b 1.96(5), Pt1–C5 2.045(5), B1–H1b 1.03(2); Pt1–H1b–B1 147.48(485), C1–Pt1–C12 168.38(19).

Variable temperature NMR spectroscopy was used to study the dynamic behaviour due to exchange between bridging and terminal BH protons. The resonance for the BH₃ protons collapses into the baseline at ca. 223 K. Upon cooling to 203 K, a new signal of relative integral 2 appears at 2.47 ppm, in the expected region for terminal BH protons. Nevertheless, at this temperature, the signal corresponding to the Pt–H–B proton is not discernible.¹⁴ Further cooling to 183 K provided evidence for a very broad resonance at ca. −4 ppm (approx. integral value of 1), with the terminal BH protons resonating at 2.44 ppm. With respect to the ¹¹B NMR spectra, a very broad signal centred at δ −12.2 is detected at this temperature.

For comparison purposes, we have analysed the interaction of C₆H₅N·BH₃ with the hydrogenated form of complex 1, the hydride derivative [PtH(I^tBu)₂][BAr^F], 2.¹⁵ No evidence of interaction between 2 and C₆H₅N·BH₃ is observed at 298 K. At this temperature, the BH₃ protons resonate at δ 2.49, whereas the platinum-hydride signal appears at δ −25.47 exhibiting a J_Pt,H of 2550 Hz, only marginally different from that of complex 2 in the absence of C₆H₅N·BH₃ (J_Pt,H = 2564 Hz).¹⁵ However, the ¹H NMR spectra at temperatures below 233 K indicate the presence of a new species, 4 (Scheme 6), coexisting with complex 2, in a 0.3 to 1 ratio (4 : 2). This compound is characterised by a signal in the hydride region at −21.85 ppm showing a reduced coupling constant with ¹⁹⁵Pt (J_Pt,H = 1920 Hz) and another broad signal at 0.28 ppm (3H relative integral) that sharpens upon ¹¹B decoupling. As the temperature decreases, the concentration of 4 increases at the expense of 2, reaching a maximum at 188 K (4 : 2 ratio, ca. 4 : 1). The ¹¹B{¹H} NMR spectra at all temperatures exhibit a single very broad signal at −12.3 ppm that does not show splitting upon coupling to ¹H below 208 K. Interestingly, even at 188 K the BH₃ group still shows fast exchange between terminal and bridging hydrogens. The different behaviour of compounds 1 and 2 toward the interaction with C₆H₅N·BH₃ can be easily rationalised in terms of the different steric protection that the cyclometalated I^tBu ligand exerts on the platinum atom compared to its non-cyclometalated form. When the I^tBu ligand is cyclometalated the ^tBu group is tilted¹⁶ away from the metal centre favouring the interaction with amine-boranes.

Scheme 6 Equilibrium between complexes 2 and 4 in the presence of C₆H₅N·BH₃.

Once the amine-borane binds the metal in a Shimoi-type fashion, dehydrogenation leading to amino-boranes, NR₂–BH₂, takes place through a mechanism that involves the intermediacy of boronium cations (NHR₂)₂BH₂⁺ and the neutral platinum hydride [PtH(I^tBu′)(I^tBu)], 5.⁷ It must be noted that we have not observed an interaction between complex 1 and amino-borane NMe₂BH₂ (either using cyclic dimer [NMe₂BH₂]₂ as a precursor¹⁷ or during monitoring the dehydrogenation reaction of 1 and NMe₂H·BH₃ at 0 °C).⁷ At this point, we do not have further information on the mechanism by which the amino-borane is converted into diaminoboranes and the role of complex 1 in the process.

In summary, the coordinatively unsaturated Pt(II) complex 1 is a very efficient catalyst for the synthesis of diaminoboranes achieving TON and TOF values of 400 and 3692 h⁻¹, respectively, the highest reported to date. The process takes place through the initial coordination of the BH protons of amine-boranes to the platinum centre in an end-on mode (Shimoi type) that was demonstrated crystallographically in the pyridine·BH₃ adduct 3c. Ongoing efforts are geared towards unveiling the mechanism by which amino-boranes are transformed into diaminoboranes.

Financial support (FEDER contribution) from the MINECO (Projects CTQ2013-45011-P and CTQ2014-51912-REDC) and the Junta de Andalucía (Project FQM-2126) is gratefully acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental section, H₂ evolution graphics and X-ray crystallographic data. CCDC 1468913. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6cc02720b

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