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

Article abstract of DOI:10.1039/c6cc01257d

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 relev

Full text of DOI:10.1039/c6cc01257d

ChemComm
View Article Online
View Journal
COMMUNICATION
Insertion of CO2 into the carbon–boron bond of
a boronic ester ligand†
Cite this:DOI: 10.1039/c6cc01257d
Trevor Janes, Kimberly M. Osten, Adam Pantaleo, Ellen Yan, Yanxin Yang and
Datong Song*
Received 9th February 2016,
Accepted 17th February 2016
DOI: 10.1039/c6cc01257d
www.rsc.org/chemcomm
New Ru and Zn diazafluorenyl complexes undergo C–H borylation of MesCQO+. Piers et al. recently activated CO2 in ring expansion
the diazafluorenyl ligand to form the corresponding diazafluorenyl- reactions using the reactive C–B bonds of isomeric doubly reduced
boronic ester complexes, which can insert CO2 into their C–B bonds diborole and bis-cycloborabutylidene derivatives.‡35 Two examples
to form boryl ester functionalities. The relevance of these new of related insertions of CO2 have also recently emerged: Waterman
reactivities towards catalytic CO2 reduction has also been explored. et al. observed an unexpected insertion of CO2 into reactive C–Si
bonds in Zn complexes chelated by a pair of 2-(phosphinomethyl)-
Discovery of new fundamental reactivity of CO2 is of interest pyridine ligands,36 and Knopf and Cummins demonstrated formal
because it may lead to new ways to sequester CO2 and utilize insertion of CO2 into three B–H bonds in borohydride salts to
CO2 as a C1 feedstock for synthesis.1 Known reactivity of CO2 form [HB(OCHO)3]À.37
includes coordination to metal centres,2 insertion into M–X
Herein we report the reactivity of [Ru(CO)(H)L(PPh3)2], 1a
bonds (where M is a metal center and X is an element, most and [ZnL(Mes2nacnac)], 1b toward pinacolborane (HBpin) to
commonly H or C)3–5 and adduct formation with a Lewis base generate the corresponding complexes 2a and 2b of an actor
(with or without the assistance of a Lewis acid).6,7 These funda- diazafluorenylboronic ester ligand. This actor ligand inserts
mental reactivities have resulted in the catalytic conversion of CO2 CO2 into its C–B bond, which is an unprecedented mode of
into a variety of reduced products, which is a topic that has been reactivity for the boronic ester functional group. We also report
reviewed5,7–11 and has seen a flurry of recent progress.12–31 Our the catalytic activities of these complexes towards CO2 reduction
group has demonstrated formal insertion of CO2 into the C–H bond with catecholborane (HBcat) and HBpin.
of an actor diazafluorenyl (LÀ) ligand supported by a spectator metal
As shown in Scheme 1, 1a was synthesized in 59% yield from
center.32 We elaborated this work to include metal-free insertions,33 the reaction of NaL and [RuHCl(CO)(PPh3)3]. The spectroscopic
and catalytic hydroboration28 of CO2. Although we have not yet properties of 1a are similar to those of [Ru(H)L(N2)(PPh3)2]
studied the catalytic mechanism in detail, one plausible pathway (n(N2) = 2092 cmÀ1),38 except that its characteristic infrared
involves borylation of the diazafluorenyl moiety and subsequent absorption occurs at 1918 cmÀ1 due to stretching of the CO ligand.
insertion of CO2 into the newly formed C–B bond. To our knowl-
edge, 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
Davenport Chemical Research Laboratories, Department of Chemistry,
University of Toronto, 80 St. George Street, Toronto, Ontario, M5S 3H6, Canada.
E-mail: dsong@chem.utoronto.ca
† Electronic supplementary information (ESI) available: Experimental proce-
dures, 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/
Scheme 1 Syntheses of 1a (top) and 1b (bottom).
c6cc01257d
This journal is ©The Royal Society of Chemistry 2016
Chem. Commun.

View Article Online
Communication
ChemComm
1b was synthesized by protonolysis in two steps from diethyl-
1
zinc with an overall 87% yield. In the H 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 C2v
symmetry in solution. The solid state structure of 1b (see ESI†)
features a ZnN4 core with distorted tetrahedral geometry.
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.
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
The borylation of 1a brings about a change in the carbonyl HBpin an analogous borylation of diazafluorenyl occurred and
stretching frequency from 1918 cmÀ1 in the starting material to 2b formed. Similar to 1b, the NMR data for 2b reveal a symmetric
1936 cmÀ1 in the product. The higher C–O bond strength in 2a structure in solution. 2b crystallizes from toluene/pentane with
indicates the electron-withdrawing nature of the newly formed two molecules in the asymmetric unit (see ESI†). The distorted
diazafluorenylboronic ester ligand relative to the parent LÀ ligand tetrahedral ZnN4 core persists; the metric parameters around Zn
in 1a. In C6D6 at ambient temperature, complex 2a displays a are similar to those in 1b, and the metric parameters of the
singlet at 48.47 ppm in its 31P{1H} NMR spectrum; the hydride diazafluorenylboronic ester ligand are similar to those in 2a.
resonates at À11.68 ppm as a triplet (2JP–H = 20 Hz) and the
With 2a and 2b in hand we set about investigating their
diazafluorenyl moiety shows one signal for each of its six protons reactivity towards CO2. When CO2 is introduced to a solution of
1
in its H NMR spectrum.
2a in toluene–diethylether (1 : 1 v/v), the colour of the solution
As shown in Fig. 1, the solid state structure of 2a has been changes from pink to orange (Scheme 2). The formation of 3a can
1
confirmed with X-ray crystallography. The structure of 2a be confirmed by NMR experiments, in which all the H signals
features a pseudo-octahedral Ru centre with two mutually trans have shifted slightly compared to those of 2a and an additional
phosphine ligands and the hydride and carbonyl ligands 13C signal appears at 162.29 ppm corresponding to the newly
oriented in a cis fashion. The diazafluorenylboronic ester ligand formed ester group from CO2. In the infrared spectrum of 3a, the
chelates the Ru centre through its two nitrogen donor atoms. ester carbonyl stretch appears at 1647 cmÀ1, and the CO ligand
The Ru1–N1 bond length of 2.171(2) is shorter than the Ru1–N2 stretch is shifted to 1942 cmÀ1. X-ray crystallography confirmed
bond length of 2.266(2) due to the greater trans influence of the the structure of 3a as shown in Fig. 1. Diazafluorenyl is nearly
hydride ligand relative to CO. The sum of the bond angles coplanar with its appended carboxylate group, and the B atom is
around C5 is 359.9(5)1 which suggests sp2 hybridization. The canted out of this plane, giving an O2–C51–O3–B1 dihedral angle
B atom is three-coordinate and the O2–B1–O3 group is nearly of B371. When CO2 is introduced to a C6D5Br solution of 2b, a
coplanar with diazafluorenyl (a dihedral angle of 61) such that similar change in colour and in spectral data occurs as the boryl
p-donation of the carbanion into the vacant p orbital on B is ester product 3b forms; we confirmed the structure by X-ray
possible. The C5–B1 bond length is 1.512(4) Å, which is similar crystallography (see ESI†).
to pinacol esters of other borylated cyclopentadienyl (Cp) com-
The insertion of CO2 into the C–B bond of 2a and 2b is
pounds (12-crown-4)LiCpBpin (1.488(8) Å) and (Cp2Ru)Bpin intriguing from the standpoint of providing a new type of
(1.537(12) Å).39,40 In such compounds the boratafulvene reso- reactivity for the thermodynamically stable and environmentally
nance form also contributes to the bonding picture, having C–B deleterious CO2 molecule. To understand this transformation
bond lengths between a typical C(sp2)–B(sp2) single bond (e.g., further, DFT calculations have been used to locate the transition
1.58 Å in BMes3)41 and a formal C(sp2)–B(sp2) double bond (e.g., state of the insertion reaction from 2a to 3a and to obtain the
1.444(8) Å in simple borataalkene [Mes2BQCH2]À).42
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 46.41 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 32.61 (measured by the change
in Cpcentroid–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
Scheme 2 Borylation and carboxylation of diazafluorenyl complexes.
Chem. Commun.
This journal is ©The Royal Society of Chemistry 2016

View Article Online
ChemComm
Communication
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 hÀ1 at 25 1C).
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 1C 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.
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.
In summary, new compounds 1a and 1b undergo C–H bond
borylation to yield diazafluorenylboronic ester complexes 2a
and 2b, which feature the unquenched p-basicity of a carba-
nionic group directly bound to a boron centre. It is this
examine the possibility of a two-step mechanism for the inser- structural feature that facilitates the unprecedented insertion
tion reaction, i.e., C–C bond formation first, followed by the of CO2 into their C–B bonds to yield boryl ester products 3a and
migration of the Bpin moiety. All attempts to locate the C–C 3b. DFT calculations suggest the insertion occurs in a concerted
formation intermediate failed. Although it is still possible that fashion. Compound 3a can be converted to 2a when reacted
such an intermediate may sit in an extremely shallow well, our with HBpin, releasing the CO2 reduction product CH3OBpin
computation is consistent with the one-step CO2 insertion and closing a synthetic loop of CO2 reduction. Furthermore,
mechanism. The DH of the overall reaction is À15.9 kcal molÀ1
,
compounds 1 and 2 all displayed catalytic activity toward CO2
while the DHa is 13.1 kcal molÀ1, so it is unsurprising that the hydroboration with HBpin and HBcat. Further mechanistic
insertion reaction occurs readily at ambient temperature.
studies of the catalytic CO2 reduction, the insertion of other
This unique reactivity led us to investigate whether borylester 3a unsaturated substrates into the C–B bond in our diazafluor-
is sufficiently reactive towards further reduction of the CO2-derived enylboronic ester ligand, the related reactivities of other C–E
moiety. Heating a C6D6 solution of 3a to 110 1C in the presence of 20 bonds, and the corresponding catalytic reactions are under
equivalents of HBpin led to a 0.9 : 1 ratio of 2a :3a in 8 hours as investigation in our laboratory.
detected by 1H NMR spectroscopy, accompanied by the formation of
We thank NSERC of Canada for funding. T. J. and Y. Y. thank
CH3OBpin and pinBOBpin. This reactivity represents the closing of a the government of Ontario for OGS and Trillium scholarships,
synthetic loop: the reactive diazafluorenylboronic ester 2a inserts respectively. A. P. and E. Y. thank NSERC for respective CGS-M
CO2 to make boryl ester 3a, which upon reaction with HBpin and USRA awards. We also acknowledge the CFI Project #19119,
regenerates 2a and liberates the product of CO2 reduction.
and the Ontario Research Fund for funding the CSICOMP NMR
With the synthetic loop established, we tested the perfor- lab at the University of Toronto enabling the purchase of several
mance of complexes 1 and 2 in catalytic hydroboration of CO2 new spectrometers.
Table 1 Results for the catalytic hydroboration of CO2 by HBcat and HBpina
TON from formation of each productb
Entry
Cat.
Borane
T (1C)
Time (h)
HCO2BR2
CH2(OBR2)2
CH3OBR2
Total TONb
1
2
3
4
5
6
1a
1a
2a
1b
1b
2b
HBcat
HBpin
HBpin
HBcat
HBpin
HBpin
90
100
100
60
90
90
45
45
45
20
20
20
29
37
54
16
45
31
29
39
60
16
48
40
2
5
2
3
5
0.1
4
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),
b
C6D5Br (0.6 mL) and CO2 (B1.5 atm). 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.
This journal is ©The Royal Society of Chemistry 2016
Chem. Commun.

View Article Online
Communication
ChemComm
18 K. Beydoun, G. Ghattas, K. Thenert, J. Klankermayer and W. Leitner,
Angew. Chem., Int. Ed., 2014, 53, 11010.
19 E. Blondiaux, J. Pouessel and T. Cantat, Angew. Chem., Int. Ed., 2014,
53, 12186.
Notes and References
‡ 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.
´
´
20 M.-A. Courtemanche, M.-A. Legare, E. Rochette and F.-G. Fontaine,
Chem. Commun., 2015, 51, 6858.
´
´
21 R. Declercq, G. Bouhadir, D. Bourissou, M.-A. Legare, M.-A.
Courtemanche, K. S. Nahi, N. Bouchard, F.-G. Fontaine and
L. Maron, ACS Catal., 2015, 5, 2513.
1 M. Aresta, Carbon Dioxide as Chemical Feedstock, Wiley-VCH, Weinheim,
2010.
2 J. Mascetti, Metal Coordination of CO2, Encyclopedia of Inorganic and
Bioinorganic Chemistry, Wiley, Chichester, 2014.
3 X. Yin and J. R. Moss, Coord. Chem. Rev., 1999, 181, 27.
4 S. P. Bew, Compr. Org. Funct. Group Transform. II, 2005, 19–125.
5 M. Cokoja, C. Bruckmeier, B. Rieger, W. A. Herrmann and
F. E. Ku¨hn, Angew. Chem., Int. Ed., 2011, 50, 8510.
6 J. L. Murphy, K. N. Robertson, R. A. Kemp, H. M. Tuononen and
J. A. C. Clyburne, Chem. Commun., 2015, 51, 3942.
7 D. W. Stephan and G. Erker, Chem. Sci., 2014, 5, 2625–2641.
8 E. E. Benson, C. P. Kubiak, A. J. Sathrum and J. M. Smieja, Chem.
Soc. Rev., 2009, 38, 89.
22 C. Liu, J. Xie, G. Tian, W. Li and Q. Zhou, Chem. Sci., 2015, 6, 2928.
23 F. A. LeBlanc, W. E. Piers and M. Parvez, Angew. Chem., Int. Ed.,
2014, 53, 789.
24 T. T. Metsanen and M. Oestreich, Organometallics, 2015, 34, 543.
25 S. Bontemps, L. Vendier and S. Sabo-Etienne, J. Am. Chem. Soc.,
2014, 136, 4419.
26 K. Fujiwara, S. Yasuda and T. Mizuta, Organometallics, 2014, 33, 6692.
27 C. C. Chong and R. Kinjo, Angew. Chem., Int. Ed., 2015, 54, 12116.
28 Y. Yang, M. Xu and D. Song, Chem. Commun., 2015, 51, 11293.
¨
´
29 X. Frogneux, E. Blondiaux, P. Thuery and T. Cantat, ACS Catal.,
2015, 5, 3983.
9 A. M. Appel, J. E. Bercaw, A. B. Bocarsly, H. Dobbek, D. L. DuBois,
M. Dupuis, J. G. Ferry, E. Fujita, R. Hille, P. J. A. Kenis, C. A. Kerfeld,
R. H. Morris, C. H. F. Peden, A. R. Portis, S. W. Ragsdale,
T. B. Rauchfuss, J. N. H. Reek, L. C. Seefeldt, R. K. Thauer and
G. L. Waldrop, Chem. Rev., 2013, 113, 6621.
10 D. Yu, S. P. Teong and Y. Zhang, Coord. Chem. Rev., 2015, 293, 279.
11 Q. Liu, L. Wu, R. Jackstell and M. Beller, Nat. Commun., 2015,
6, 5933.
12 Z. Lu, H. Hausmann, S. Becker and H. A. Wegner, J. Am. Chem. Soc.,
2015, 137, 5332.
13 J. A. B. Abdalla, I. M. Riddlestone, R. Tirfoin and S. Aldridge, Angew.
Chem., Int. Ed., 2015, 54, 5098.
14 M. D. Anker, M. Arrowsmith, P. Bellham, M. S. Hill, G. Kociok-Kohn,
D. J. Liptrot, M. F. Mahon and C. Weetman, Chem. Sci., 2014,
5, 2826.
15 J. R. Khusnutdinova, J. A. Garg and D. Milstein, ACS Catal., 2015,
5, 2416.
16 N. M. Rezayee, C. A. Huff and M. S. Sanford, J. Am. Chem. Soc., 2015,
137, 1028.
17 S. Wesselbaum, V. Moha, M. Meuresch, S. Brosinski, K. M. Thenert,
J. Kothe, T. vom Stein, U. Englert, M. Holscher, J. Klankermayer and
W. Leitner, Chem. Sci., 2015, 6, 693.
´
30 G. Jin, C. G. Werncke, Y. Escudie, S. Sabo-Etienne and S. Bontemps,
J. Am. Chem. Soc., 2015, 137, 9563.
31 S. Bagherzadeh and N. P. Mankad, J. Am. Chem. Soc., 2015,
137, 10898.
32 V. T. Annibale and D. Song, Chem. Commun., 2012, 48, 5416.
33 V. T. Annibale, D. A. Dalessandro and D. Song, J. Am. Chem. Soc.,
2013, 135, 16175.
34 Y. Shoji, N. Tanaka, K. Mikami, M. Uchiyama and T. Fukushima,
Nat. Chem., 2014, 6, 498.
35 J. F. Araneda, W. E. Piers, M. J. Sgro and M. Parvez, Organometallics,
2015, 34, 3408.
36 G. I. McGrew, P. A. Khatri, W. E. Geiger, R. A. Kemp and R. Waterman,
Chem. Commun., 2015, 51, 15804.
37 I. Knopf and C. C. Cummins, Organometallics, 2015, 34, 1601.
38 E. Stepowska, H. Jiang and D. Song, Chem. Commun., 2010, 46, 556.
39 G. E. Herberich and A. Fischer, Organometallics, 1996, 15, 58.
40 M. Sato, G. Maruyama and A. Tanemura, J. Organomet. Chem., 2002,
655, 23.
41 J. F. Blount, P. Finocchiaro, D. Gust and K. Mislow, J. Am. Chem.
Soc., 1973, 95, 7019.
42 M. M. Olmstead, P. P. Power, K. J. Weese and R. J. Doedens, J. Am.
Chem. Soc., 1987, 109, 2541.
Chem. Commun.
This journal is ©The Royal Society of Chemistry 2016