Insertion of CO₂ into the carbon–boron bond of a boronic ester ligand

Abstract

New Ru and Zn diazafluorenyl complexes undergo C–H borylation of the diazafluorenyl ligand to form the corresponding diazafluorenylboronic ester complexes, which can insert CO₂ into their C–B bonds to form boryl ester functionalities. The relevance of these new reactivities towards catalytic CO₂ reduction has also been explored.

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Discovery of new fundamental reactivity of CO₂ is of interest because it may lead to new ways to sequester CO₂ and utilize CO₂ as a C₁ feedstock for synthesis.¹ Known reactivity of CO₂ includes coordination to metal centres,² insertion into M–X bonds (where M is a metal center and X is an element, most commonly H or C)^3–5 and adduct formation with a Lewis base (with or without the assistance of a Lewis acid).^6,7 These fundamental reactivities have resulted in the catalytic conversion of CO₂ into a variety of reduced products, which is a topic that has been reviewed^5,7–11 and has seen a flurry of recent progress.^12–31 Our group has demonstrated formal insertion of CO₂ into the C–H bond of an actor diazafluorenyl (L⁻) ligand supported by a spectator metal center.³² We elaborated this work to include metal-free insertions,³³ and catalytic hydroboration²⁸ of CO₂. Although we have not yet studied the catalytic mechanism in detail, one plausible pathway involves borylation of the diazafluorenyl moiety and subsequent insertion of CO₂ into the newly formed C–B bond. To our knowledge, the direct insertion of CO₂ into a C–B bond is unknown prior to this work, and represents a new mode of reactivity for the thermodynamically stable CO₂ molecule.

Related yet distinct C–B bond reactivity was reported by Shoji et al. who employed salts containing the electrophilic Mes₂B⁺ in a deoxygenation-arylation of CO₂.³⁴ In this reaction, one mesityl group is transferred to the carbon of CO₂, generating MesC=O⁺. Piers et al. recently activated CO₂ in ring expansion reactions using the reactive C–B bonds of isomeric doubly reduced diborole and bis-cycloborabutylidene derivatives.‡³⁵ Two examples of related insertions of CO₂ have also recently emerged: Waterman et al. observed an unexpected insertion of CO₂ into reactive C–Si bonds in Zn complexes chelated by a pair of 2-(phosphinomethyl)pyridine ligands,³⁶ and Knopf and Cummins demonstrated formal insertion of CO₂ into three B–H bonds in borohydride salts to form [HB(OCHO)₃]⁻.³⁷

Herein we report the reactivity of [Ru(CO)(H)L(PPh₃)₂], 1a and [ZnL(Mes₂nacnac)], 1b toward pinacolborane (HBpin) to generate the corresponding complexes 2a and 2b of an actor diazafluorenylboronic ester ligand. This actor ligand inserts CO₂ into its C–B bond, which is an unprecedented mode of reactivity for the boronic ester functional group. We also report the catalytic activities of these complexes towards CO₂ reduction with catecholborane (HBcat) and HBpin.

As shown in Scheme 1, 1a was synthesized in 59% yield from the reaction of NaL and [RuHCl(CO)(PPh₃)₃]. The spectroscopic properties of 1a are similar to those of [Ru(H)L(N₂)(PPh₃)₂] (ν(N₂) = 2092 cm⁻¹),³⁸ except that its characteristic infrared absorption occurs at 1918 cm⁻¹ due to stretching of the CO ligand. 1b was synthesized by protonolysis in two steps from diethylzinc with an overall 87% yield. In the ¹H NMR spectrum of 1b one resonance exists for each pair of pyridyl protons, which are related to each other through a mirror plane of symmetry. Protons on nacnac are similarly related, consistent with C_2v symmetry in solution. The solid state structure of 1b (see ESI†) features a ZnN₄ core with distorted tetrahedral geometry.

Scheme 1 Syntheses of 1a (top) and 1b (bottom).

With 1a and 1b in hand, we investigated the stoichiometric reactivity of the diazafluorenyl complexes towards HBpin: when a toluene solution of complex 1a is heated with HBpin, 2a is formed with concomitant formation of H₂ (Scheme 2). Presumably the formation of 2a starts with the formation of an adduct between the carbanion of 1a and HBpin. This interaction brings B–H and C–H bonds into close proximity, which allows the loss of H₂. In the transformation from 1a to 2a the diazafluorenyl ligand acts and the Ru centre spectates.

Scheme 2 Borylation and carboxylation of diazafluorenyl complexes.

The borylation of 1a brings about a change in the carbonyl stretching frequency from 1918 cm⁻¹ in the starting material to 1936 cm⁻¹ in the product. The higher C–O bond strength in 2a indicates the electron-withdrawing nature of the newly formed diazafluorenylboronic ester ligand relative to the parent L⁻ ligand in 1a. In C₆D₆ at ambient temperature, complex 2a displays a singlet at 48.47 ppm in its ³¹P{¹H} NMR spectrum; the hydride resonates at −11.68 ppm as a triplet (²J_P–H = 20 Hz) and the diazafluorenyl moiety shows one signal for each of its six protons in its ¹H NMR spectrum.

As shown in Fig. 1, the solid state structure of 2a has been confirmed with X-ray crystallography. The structure of 2a features a pseudo-octahedral Ru centre with two mutually trans phosphine ligands and the hydride and carbonyl ligands oriented in a cis fashion. The diazafluorenylboronic ester ligand chelates the Ru centre through its two nitrogen donor atoms. The Ru1–N1 bond length of 2.171(2) is shorter than the Ru1–N2 bond length of 2.266(2) due to the greater trans influence of the hydride ligand relative to CO. The sum of the bond angles around C5 is 359.9(5)° which suggests sp² hybridization. The B atom is three-coordinate and the O2–B1–O3 group is nearly coplanar with diazafluorenyl (a dihedral angle of 6°) such that π-donation of the carbanion into the vacant p orbital on B is possible. The C5–B1 bond length is 1.512(4) Å, which is similar to pinacol esters of other borylated cyclopentadienyl (Cp) compounds (12-crown-4)LiCpBpin (1.488(8) Å) and (Cp₂Ru)Bpin (1.537(12) Å).^39,40 In such compounds the boratafulvene resonance form also contributes to the bonding picture, having C–B bond lengths between a typical C(sp²)–B(sp²) single bond (e.g., 1.58 Å in BMes₃)⁴¹ and a formal C(sp²)–B(sp²) double bond (e.g., 1.444(8) Å in simple borataalkene [Mes₂B=CH₂]⁻).⁴²

Fig. 1 Molecular structures of 2a and 3a. Ellipsoids are shown at 30% probability. Hydrogen atoms except for hydrides omitted for clarity. Only one disordered component is shown. Co-crystallized molecules are removed for clarity.

Similarly, when 1b was heated in THF in the presence of HBpin an analogous borylation of diazafluorenyl occurred and 2b formed. Similar to 1b, the NMR data for 2b reveal a symmetric structure in solution. 2b crystallizes from toluene/pentane with two molecules in the asymmetric unit (see ESI†). The distorted tetrahedral ZnN₄ core persists; the metric parameters around Zn are similar to those in 1b, and the metric parameters of the diazafluorenylboronic ester ligand are similar to those in 2a.

With 2a and 2b in hand we set about investigating their reactivity towards CO₂. When CO₂ is introduced to a solution of 2a in toluene–diethylether (1 : 1 v/v), the colour of the solution changes from pink to orange (Scheme 2). The formation of 3a can be confirmed by NMR experiments, in which all the ¹H signals have shifted slightly compared to those of 2a and an additional ¹³C signal appears at 162.29 ppm corresponding to the newly formed ester group from CO₂. In the infrared spectrum of 3a, the ester carbonyl stretch appears at 1647 cm⁻¹, and the CO ligand stretch is shifted to 1942 cm⁻¹. X-ray crystallography confirmed the structure of 3a as shown in Fig. 1. Diazafluorenyl is nearly coplanar with its appended carboxylate group, and the B atom is canted out of this plane, giving an O2–C51–O3–B1 dihedral angle of ∼37°. When CO₂ is introduced to a C₆D₅Br solution of 2b, a similar change in colour and in spectral data occurs as the boryl ester product 3b forms; we confirmed the structure by X-ray crystallography (see ESI†).

The insertion of CO₂ into the C–B bond of 2a and 2b is intriguing from the standpoint of providing a new type of reactivity for the thermodynamically stable and environmentally deleterious CO₂ molecule. To understand this transformation further, DFT calculations have been used to locate the transition state of the insertion reaction from 2a to 3a and to obtain the thermodynamic data (see ESI† for details). The transition state of the insertion features a C–C–O–B four-membered ring (Fig. 2). The CO₂ moiety is off linear by 46.4° with the endocyclic C–O bond elongated by 0.09 Å, while the Bpin moiety is bent away from its original position in 2a by 32.6° (measured by the change in Cp_centroid–C–B angle) to accommodate the incoming CO₂ with the B–C bond elongated by 0.08 Å. The endocyclic B–O and C–C distances are 2.11 and 1.66 Å, respectively, indicating that the C–C bond is largely formed in the transition state, but the B–O bond formation is far from complete. This result prompted us to examine the possibility of a two-step mechanism for the insertion reaction, i.e., C–C bond formation first, followed by the migration of the Bpin moiety. All attempts to locate the C–C formation intermediate failed. Although it is still possible that such an intermediate may sit in an extremely shallow well, our computation is consistent with the one-step CO₂ insertion mechanism. The ΔH of the overall reaction is −15.9 kcal mol⁻¹, while the ΔH^≠ is 13.1 kcal mol⁻¹, so it is unsurprising that the insertion reaction occurs readily at ambient temperature.

Fig. 2 Computed transition state structure of the CO₂ insertion reaction from 2a to 3a Colour key: O, red; N, blue; P, orange; B, green; Ru, pink; C, gray; H, white.

This unique reactivity led us to investigate whether borylester 3a is sufficiently reactive towards further reduction of the CO₂-derived moiety. Heating a C₆D₆ solution of 3a to 110 °C in the presence of 20 equivalents of HBpin led to a 0.9 : 1 ratio of 2a : 3a in 8 hours as detected by ¹H NMR spectroscopy, accompanied by the formation of CH₃OBpin and pinBOBpin. This reactivity represents the closing of a synthetic loop: the reactive diazafluorenylboronic ester 2a inserts CO₂ to make boryl ester 3a, which upon reaction with HBpin regenerates 2a and liberates the product of CO₂ reduction.

With the synthetic loop established, we tested the performance of complexes 1 and 2 in catalytic hydroboration of CO₂ with HBcat and HBpin (Table 1). 1a and 1b perform similarly as catalysts: their average turnover frequencies (avg. TOF) are on the order of one per hour, and the methoxyborane derivative CH₃OBR₂ was the major CO₂ reduction product in all cases. In the hydroboration of CO₂ with HBcat, 1a and 1b are not as active as (N-methyl)diazafluorenide (avg. TOF = 16 h⁻¹ at 25 °C). However, when HBpin is used as the reductant, 1a and 1b are capable of more turnovers compared to (N-methyl)diazafluorenide (avg. TOF = 0.28 at 100 °C in CDCl₃).²⁸ Interestingly, 2a and 2b showed comparable catalytic performance for CO₂ hydroboration with HBpin under the same respective conditions compared to the parent compounds 1a and 1b (Table 1). In the Ru case the borylated species led to a slight increase in total TON (from 39 to 60). In the Zn case the borylated version led to a slight decrease in total TON (from 48 to 40). Unfortunately, we could not identify any metal containing species from the catalytic reaction mixtures. Therefore, the relevance of the steps in the synthetic loop to the actual catalytic runs has yet to be determined through further mechanistic studies.

Table 1 Results for the catalytic hydroboration of CO₂ by HBcat and HBpin^a

Entry	Cat.	Borane	T (°C)	Time (h)	TON from formation of each product^b			Total TON^b
					HCO2BR2	CH2(OBR2)2	CH3OBR2
a Reactions were carried out in Schlenk bombs charged with catalyst (0.01 M), borane (1 M), hexamethylbenzene (2–10 mg as an internal standard), C6D5Br (0.6 mL) and CO2 (∼1.5 atm). b TON is based on the number of C–H bonds formed in the reduced product per molecule of catalyst, determined by integration of the ^1H NMR signals against the internal standard. R2BOBR2 is formed in all cases in addition to the carbon-containing CO2-derived products.
1	1a	HBcat	90	45			29	29
2	1a	HBpin	100	45	2		37	39
3	2a	HBpin	100	45	5	2	54	60
4	1b	HBcat	60	20			16	16
5	1b	HBpin	90	20	3	0.1	45	48
6	2b	HBpin	90	20	5	4	31	40

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In summary, new compounds 1a and 1b undergo C–H bond borylation to yield diazafluorenylboronic ester complexes 2a and 2b, which feature the unquenched π-basicity of a carbanionic group directly bound to a boron centre. It is this structural feature that facilitates the unprecedented insertion of CO₂ into their C–B bonds to yield boryl ester products 3a and 3b. DFT calculations suggest the insertion occurs in a concerted fashion. Compound 3a can be converted to 2a when reacted with HBpin, releasing the CO₂ reduction product CH₃OBpin and closing a synthetic loop of CO₂ reduction. Furthermore, compounds 1 and 2 all displayed catalytic activity toward CO₂ hydroboration with HBpin and HBcat. Further mechanistic studies of the catalytic CO₂ reduction, the insertion of other unsaturated substrates into the C–B bond in our diazafluorenylboronic ester ligand, the related reactivities of other C–E bonds, and the corresponding catalytic reactions are under investigation in our laboratory.

We thank NSERC of Canada for funding. T. J. and Y. Y. thank the government of Ontario for OGS and Trillium scholarships, respectively. A. P. and E. Y. thank NSERC for respective CGS-M and USRA awards. We also acknowledge the CFI Project #19119, and the Ontario Research Fund for funding the CSICOMP NMR lab at the University of Toronto enabling the purchase of several new spectrometers.

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Footnotes

† Electronic supplementary information (ESI) available: Experimental procedures, Cartesian coordinates and energies of DFT optimized structures, and IR and NMR spectra. CCDC 1061567, 1061568, 1437476, 1437477 and 1437503. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6cc01257d

‡ The authors posit that CO₂ catalyzes the rearrangement of the bis-cycloborabutylidene into the diborole, which reacts with CO₂ in a pair of ring expansions from five to six members, distinct from from simple insertion into the reactive C–B bonds.

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