compound 78c

Arene−Ruthenium Complexes of 1,1′-Bis(ortho-carborane): Synthesis, Characterization, and Catalysis
Rebekah J. Jeans,† Antony P. Y. Chan,† Laura E. Riley,† James Taylor,† Georgina M. Rosair,† Alan J. Welch,*,† and Igor B. Sivaev‡
†Institute of Chemical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, U.K.
‡A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 28 Vavilov Str., 119991 Moscow, Russia
*S Supporting Information

Compound [1-(1′-closo-1′,2′-C B

H )-closo-1,2-C B H ],

a κ2 coligand following double deprotonation of the {CH} units,7 complementing the earlier studies of Hawthorne and

trivial name 1,1′-bis(ortho

2 10 11

2 10 11

co-workers on homoleptic species [M{(C B H

) } ]n−.29

Figure 1. 1,1′-Bis(ortho-carborane).

of two ortho-carborane units linked by a 2c−2e C−C bond.1 Although it was first reported by Dupont and Hawthorne in 1964,2 it remained relatively underdeveloped for over 40 years, in large measure because a reliable, high-yielding synthesis was not available. That problem has now been overcome,3 and the past decade has witnessed an impressive expansion of the
chemistry of 1,1′-bis(ortho-carborane) with many novel species reported involving both retention and derivatization of the bis(carborane) framework.4−27 In the context of the former, we described the first examples of 1,1′-bis(ortho-carborane) as

Used in this way, the bis(carborane) scaffold provides a rare example of a thermally stable, sterically demanding, dianionic closo-closo-X2(C,C′) chelating ligand. Subsequently, we re-
ported the compound [Ru(κ3-2,2′,3′-{1-(1′-closo-1′,2′-
C2B10H10)-closo-1,2-C2B10H10)}(p-cymene)] (I, Figure 2)
which featured an additional interaction between the bis- (carborane) and metal center in the form of a B-agostic B− H⇀Ru bond.12 This made the bis(carborane) now a closo- closo-X2(C,C′)L ligand, a bonding mode that had been reported only once before.30 Further structural variation was
observed on treatment of I with phosphines, resulting in the first examples of species in which the bis(carborane) acts as a closo-closo-X2(C,B′)L ligand.12 In turn, the treatment of these species with small molecules (CO, MeCN) cleaved the B- agostic B−H⇀Ru bond and afforded the first examples of

Received: June 18, 2019

© XXXX American Chemical Society A DOI: 10.1021/acs.inorgchem.9b01774

Figure 2. [Ru(κ3-2,2′,3′-{1-(1′-closo-1′,2′-C B H


(0.77 mL of a 2.5 M solution, 1.92 mmol), and the reaction miXture was allowed to warm to room temperature and stir for 2 h. The pale- yellow solution was frozen at −196 °C, solid [RuCl2(p-cymene)]2 (0.267 g, 0.437 mmol) was added, and the miXture allowed to thaw and stir overnight to afford a dark-green solution. THF was removed in vacuo, and the crude miXture was dissolved in DCM and filtered through silica. Preparative TLC using an eluent system of DCM and
petrol in a ratio of 1:1 afforded an orange band (Rf = 0.61), the known12 compound [Ru(κ3-2,2′,3′-{1-(1′-closo-1′,2′-C2B10H10)-closo- 1,2-C2B10H10})(p-cymene)] (I, 0.231 g, 51%), and an orange-brown
band (Rf = 0.20) that was subsequently identified as [Ru(κ3-2,2′,11′-



10)}(p-cymene)], compound I.

2 10 10

{1-(7′-nido-7′,8′-C2B9H11)-closo-1,2-C2B10H10}(p-cymene)] (1a,
0.004 g, 1%). C14H35B19Ru requires C 33.0, H 6.92. Found for 1a:

closo-closo-X (C,B′) coordination of bis(carborane),12 a bond-

C 33.2, H 6.86%. 11B{1H} NMR (CDCl3): δ 1.6 (1B), −3.8 (4B),

2 17

−6.5 to −13.6, overlapping resonances with prominent maxima at

ing mode subsequently reported by others.
Compound I was found to be an efficient Lewis acid catalyst for Diels−Alder cycloaddition, presumably operating through initial displacement of the B-agostic B−H⇀Ru interaction by the substrate. In this contribution, we describe the results of experiments designed to tune the Lewis acidity of the metal
center by varying the η-arene ligand. In some cases, partial degradation of one cage of bis(ortho-carborane) is observed, leading to both known and novel metal complexes. Ultimately, however, the targeted analogs of compound I were successfully prepared and used to catalyze Diels−Alder cycloaddition reactions.
Synthesis. EXperiments were performed under dry, oXygen-free N2 using standard Schlenk techniques, although subsequent manipu- lations were sometimes performed in the open laboratory. Solvents were freshly distilled under nitrogen from the appropriate drying agent [THF and 40−60 petroleum ether (petrol); sodium wire: CH2Cl2 (DCM), CH3CN (MeCN), and dimethoXyethane (DME); calcium hydride] and were degassed (three freeze−pump−thaw cycles) before use. Deuterated solvents for NMR spectroscopy
[CDCl , CD Cl , (CD ) CO, and CD CN] were stored over 4 Å

−9.1, −12.2 (total integral 8B), −17.3 (1B), −18.9 (1B), −20.4 (1B),
−23.4 (1B), −26.7 (1B), −36.4 (1B). 1H NMR (CDCl3): δ 5.76 [d, J
= 6.0 Hz, 1H, CH3C6H4CH(CH3)2], 5.74 [d, J = 6.0 Hz, 1H, CH3C6H4CH(CH3)2], 5.55 [d, J = 6.0 Hz, 1H, CH3C6H4CH- (CH3)2], 5.50 [d, J = 6.0 Hz, 1H, CH3C6H4CH(CH3)2], 2.92 (br s, 1H, CcageH), 2.81[app sept, J = 6.9 Hz, 1H, CH3C6H4CH(CH3)2],
2.48 [s, 3H, CH3C6H4CH(CH3)2], 1.41 [app d, J = 6.9 Hz, 6H, CH3C6H4CH(CH3)2]. 1H{11B} NMR (CDCl3): δ as for 1H NMR
plus −2.93 (br s, 1H, BHbridging), −6.48 (s, 1H, BHagostic), −7.81 (s, 1H, BHagostic). EIMS: envelopes centered on m/z 510.3 (M+).
(C6H6)] (1b). Similarly, 1,1′-bis(o-carborane) (0.250 g, 0.873 mmol) in THF (20 mL) at 0 °C was treated with nBuLi (0.77 mL of a 2.5 M
solution, 1.92 mmol) and then [RuCl2(C6H6)]2 (0.218 g, 0.437 mmol) to give a very dark blue solution. Workup by filtration of a DCM solution through silica and then preparative TLC (DCM/petrol 1:1) afforded multiple overlapping bands, the most abundant of which was an orange-brown band (Rf = 0.26) which proved impossible to obtain in a spectroscopically pure form. Nevertheless, prominent in
the 1H{11B} NMR spectrum (CDCl3) were resonances at δ −2.90 (br s, 1H, BHbridging), −6.34 (s, 1H, BHagostic) and −7.83 (s, 1H, BHagostic) tentatively identifying the product as [Ru(κ3-2,2′,11′-{1-(7′-nido- 7′,8′-C2B9H11)-closo-1,2-C2B10H10})(C6H6)] (1b, 0.013 g, 3%).
EIMS: envelopes centered on m/z 454.3 (M+).

3 2 2 3 2 3

[1-(1′-closo-1′,2′-C B H )-3-(C H Me )-closo-3,1,2-RuC B H ]

molecular sieves and degassed before use. Preparative thin-layer

3 2 10 11

6 3 3

2 9 10

chromatography (TLC) employed 20 × 20 cm2 Kieselgel F254 glass plates, and column chromatography used 60 Å silica as the stationary phase. Elemental analyses were conducted using an EXeter CE-440 elemental analyzer. NMR spectra were recorded on a Bruker AVIII- 400 [400.1 MHz (1H) and 128.4 MHz (11B)] or an AVHDIII-400
[400.0 MHz (1H) and 128.3 MHz (11B)] spectrometer at room temperature. Electron ionization mass spectrometry (EIMS) and electrospray ionization mass spectrometry (ESIMS) were carried out using a Bruker MicroTOF Focus II mass spectrometer and a Finnigan MAT900XP-Trap mass spectrometer, respectively, at the University of Edinburgh. Starting materials [RuCl2(benzene)]2,31a [Ru C l 2 (mesitylene)] 2 , 31 a [Ru C l 2 ( p -c yme n e) ] 2 , 31b
[RuCl2(hexamethylbenzene)]2,32 1,1′-bis(ortho-carborane),3 [Ru(κ3-

(2c) and [Ru(κ -2,2′,11′-{1-(7′-nido-7′,8′-C2B9H11)-closo-1,2- C2B10H10})(C6H3Me3)] (1c). In an analogous way, 1,1′-bis(o-carborane) (0.250 g, 0.873 mmol) in THF (20 mL) at 0 °C was treated with
nBuLi (0.77 mL of 2.5 M solution, 1.92 mmol) and then [RuCl2(C6H3Me3)]2 (0.255 g, 0.437 mmol), resulting in a dark red- brown solution. Following the filtration of a DCM solution through silica, preparative TLC (DCM/petrol 3:2) afforded a yellow band (Rf
= 0.76) subsequently identified as [1-(1′-closo-1′,2′-C2B10H11)-3- (C6H3Me3)-closo-3,1,2-RuC2B9H10] (2c, 0.013 g, 4%) and an orange band (Rf = 0.60) subsequently identified as [Ru(κ3-2,2′,11′-{1-(7′- nido-7′,8′-C2B9H11)-closo-1,2-C2B10H10})(C6H3Me3)] (1c, 0.026 g, 7%).
2c. C13H33B19Ru requires C 31.5, H 6.71. Found for 2c: C 31.7, H

2,2′,3′-{1-(1′-closo-1′,2′-C B H )-closo-1,2-C B H })(p-cymene)]

6.75%. 11B{1H} NMR (CDCl3): δ 3.3 (1B), 0.4 (1B) −3.3 (3B), −6.3

(I),12 and [Mg(κ2-2,2 2 10


2 10 10

to −14.6, overlapping resonances with prominent maxima at −7.3,

′-{1-(1′ ,2′-C2B10H10)-closo-1,2-

C B H )}(DME) ]23 were prepared by literature methods or slight

−10.6, −12.7 (total integral 12B), −17.4 (2B). 1H NMR (CDCl3): δ

2 10 10 2

variations thereof. All other reagents were supplied commercially.
Deprotonation by nBuLi Followed by Metalation. Note that, with the exception of compound I, these reactions do not afford the target products and all involve the formation in low yields of species resulting from partial cage degradation. Significant amounts of 1,1′- bis(o-carborane) are recovered on workup. Cage degradation is
ascribed to the product(s) of attack on THF by nBuLi (see later). Low yields and the recovery of 1,1′-bis(o-carborane) are believed to result from the general insolubility of [RuCl2(arene)]2 species (the exception being [RuCl2(p-cymene)]2), resulting in inefficient metal- ation and hence reprotonation on workup.
cymene)] (1a). To a cooled (0 °C) solution of 1,1′-bis(o-carborane) (0.250 g, 0.873 mmol) in THF (20 mL) was added dropwise nBuLi

5.84 [s, 3H, C6H3(CH3)3], 3.99 (br s, 1H, CcageH), 3.82 (br s, 1H,
CcageH), 2.41 [s, 9H, C6H3(CH3)3]. EIMS: envelope centered on m/z
493.4 (M+).
1c. C13H33B19Ru requires C 31.5, H 6.71. Found for 1c: C 32.4, H 6.97%. 11B{1H} NMR (CD2Cl2): δ 2.3 (1B), −1.5 to −6.1,
overlapping resonances with prominent maxima at −3.7, −4.9 (total integral 4B), −7.5 to −14.2, overlapping resonances with prominent maxima at −9.1, −9.9, −12.3 (total integral 8B), −17.2 (1B), −18.8
(1B), −20.6 (1B), −23.8 (1B), −27.0 (1B), −36.6 (1B). 1H NMR
(CD2Cl2): δ 5.34 [s, 3H, C6H3(CH3)3, overlapped with CD2Cl2], 2.92 (br s, 1H, CcageH), 2.42 [s, 9H, C6H3(CH3)3]. 1H{11B} NMR
(CD2Cl2): δ as for 1H NMR plus −2.89 (br s, 1H, BHbridging), −6.60 (s, 1H, BHagostic), −7.82 (s, 1H, BHagostic). EIMS: envelope centered on m/z 495.3 (M+).

B DOI: 10.1021/acs.inorgchem.9b01774

(2d) and [Ru(κ3-2,2′,11′-{1-(7′-nido-7′,8′-C2B9H11)-closo-1,2-

[RuCl2(C6H3Me3)]2 (0.255 g, 0.436 mmol) in THF (20 mL), and
following a similar workup (Rf = 0.63), was isolated a yellow solid

C2B10H10})(C6Me6)] (1d). Similarly, the deprotonation of 1,1′-bis(o- carborane) (0.250 g, 0.873 mmol) in THF (20 mL) at 0 °C with

subsequently identified as [{Ru(C6H3Me3)}2(μ-Cl)(μ-κ4-2,2′,3,3′-{1-
(1′-closo-1′,2′-C2B10H9)-closo-1,2-C2B10H9})] (3c, 0.005 g, 1%).

nBuLi (0.77 mL of 2.5 M solution, 1.92 mmol) followed by reaction




requires C 34.7, H 5.82. Found for 3c: C 34.5, H

with [RuCl2(C6Me6)]2 (0.293 g, 0.437 mmol) afforded a dark-red solution, the workup of which (initial filtration of a DCM solution through silica and then preparative TLC [DCM/petrol 2:3]) gave a yellow band (Rf = 0.45) subsequently identified as [1-(1′-closo-1′,2′- C2B10H11)-3-(C6Me6)-closo-3,1,2-RuC2B9H10] (2d, 0.025 g, 5%) and
an orange band (Rf = 0.36) subsequently identified as [Ru(κ3-2,2′,11′-
{1-(7′-nido-7′,8′-C2B9H11)-closo-1,2-C2B10H10})(C6Me6)] (1d, 0.065
g, 14%).
2d. C16H39B19Ru requires C 35.7, H 7.31. Found for 2d: C 35.3, H 7.25%. 11B{1H} NMR (CDCl3): δ 5.6 (1B), 0 to −20, overlapping

6.03%. EIMS: envelope centered on m/z 761.3 (M+), 638.2 (M+ −
(C6Me6)] (4d). Similarly, [Mg(κ2-2,2′-{1-(1′-closo-1′,2′-C2B10H10)- closo-1,2-C2B10H10)}(DME)2], prepared from 1,1′-bis(o-carborane) (0.250 g, 0.873 mmol), was allowed to react with [RuCl2(C6Me6)]2
(0.292 g, 0.436 mmol) in THF (20 mL). Solvent was exchanged for DCM, and the dark-brown solution was filtered through silica, following which preparative TLC (DCM/petrol 1:1) afforded a red
band (Rf = 0.57), subsequently identified as [Ru(κ3-2,2′,3′-{1-(1′-

resonances with prominent maxima at −0.9, −2.1, −4.0, −5.3, −7.3,

closo-1′,2′-C2B10H10)-closo-1,2-C2B10H10})(C6Me6)] (4d, 0.067 g,

−8.1, −10.5, −12.8, −15.0, −17.6 (total integral 18B). 1H NMR

14%). C



20Ru requires C 35.1, H 6.99. Found for 4d: C 35.5,

(CDCl3): δ 4.00 (br s, 1H, CcageH), 3.59 (br s, 1H, CcageH), 2.23 [s, 18H, C6(CH3)6]. EIMS: envelope centered on m/z 538.4 (M+).
1d. C16H39B19Ru requires C 35.7, H 7.31. Found for 1d: C 35.7, H
7.27%. 11B{1H} NMR (CDCl3): δ −0.3 (1B), −2.5 to −5.5,

H 7.09%. 11B{1H} NMR (CD2Cl2): δ −1.9 (2B), −3.8 (2B), −7.5 to
−11.0, overlapping resonances with prominent maxima at −8.2, −9.1,
−10.7 (total integral 16B). 1H NMR (CD2Cl2): δ 2.14 [s, C6(CH3)6].
1H{11B} NMR (CD Cl ): δ 2.14 [s, 18H, C (CH ) ], 0.16 (br s, 4H,

overlapping resonances with prominent maxima at −3.3, −4.8 (total

2 2

6 3 6

integral 4B), −8.0 to −13.0, overlapping resonances with prominent maxima at −9.1, −10.7, −12.0 (total integral 7B), −16.8 (2B), −19.1
(1B), −21.1 (1B), −24.9 (1B), −27.1 (1B), −36.6 (1B). 1H NMR
(CDCl3): δ 2.79 (br s, 1H, CcageH), 2.27 [s, 18H, C6(CH3)6].
1H{11B} NMR (CDCl3): δ as for 1H NMR plus −2.97 (br s, 1H, BHbridging), −6.74 (s, 1H, BHagostic), −7.88 (s, 1H, BHagostic). EIMS: envelope centered on m/z 538.4 (M+).

Reactions of 4a, 4b, and 4d with Acetonitrile. [Ru(κ2-2,2′-{1-(1′- closo-1′,2′-C2B10H10)-closo-1,2-C2B10H10})(p-cymene)(NCMe)] (5a). EXcess MeCN (∼1 mL) was added to compound I (= 4a) (0.045
g, 0.0866 mmol) in DCM (1 mL). An immediate color change was observed from orange to yellow. The addition of excess petrol afforded a yellow precipitate which was isolated by filtration, washed
with petrol, and subsequently identified as [Ru(κ2-2,2′-{1-(1′-closo-

Use of [Mg(κ2-2,2′-{1-(1′-closo-1′,2′-C2B10H10)-closo-1,2-
C2B10H10)}(DME)2] as a Bis(o-carborane) Transfer Agent. [Ru(κ3-





})(p-cymene)(NCMe)] (5a, 0.047


g, 97%). C16H37B20NRu requires C 34.3, H 6.65, N 2.50%. Found for

(I). To a solution of [Mg(κ2-2,2′-{1-(1′-closo-1′,2′-C2B10



5a: C 33.3, H 6.53, N 2.49%. 11B{1H} NMR (CD2Cl2): δ −3.3 (4B),
−5.2 to −12.4, overlapping resonances with prominent maxima at

1,2-C2B10H10)}(DME)2] in THF (20 mL), prepared from 1,1′-bis(o- carborane) (0.250 g, 0.873 mmol) according to the literature23 and
used in situ at room temperature, was added [RuCl2(p-cymene)]2 (0.267 g, 0.437 mmol). The reaction miXture was stirred overnight at room temperature to give a near-black solution. THF was removed in vacuo, and the crude miXture was dissolved in DCM and filtered through silica. Purification by preparative TLC (DCM/petrol 1:1)
afforded an orange band (Rf = 0.60) subsequently identified as the known compound12 [Ru(κ3-2,2′,3′-{1-(1′-closo-1′,2′-C2B10H10)-closo- 1,2-C2B10H10})(p-cymene)] (I, 0.186 g, 41%).

−7.0, −8.4, −10.7 (total integral 16B). 1H NMR [(CD3)2CO]: δ 5.62 [d, J = 6.2 Hz, 2H, CH3C6H4CH(CH3)2], 5.43 [d, J = 6.2 Hz, 2H, CH 3 C 6 H 4 CH(CH 3 ) 2 ], 2.80 [sept, J = 7 .0 Hz, 1 H, CH3C6H4CH(CH3)2], 2.76 (s, 3H, NCCH3), 2.04 [s, 3H,
CH3C6H4CH(CH3)2, overlapping with protio-(CD3)2CO resonan- ces], 1.31 [d, J = 7.0 Hz, 6H, CH3C6H4CH(CH3)2]. 1H NMR
(CD2Cl2): δ 5.4 to 5.3 [4H, CH3C6H4CH(CH3)2, overlapping with protio-CD2Cl2 resonances], 2.49 (s, 3H, NCCH3) overlapping with 2.43 [1H, CH3C6H4CH(CH3)2], 2.02 [s, 3H, CH3C6H4CH(CH3)2],
1.27 [d, J = 7.0 Hz, 6H, CH3C6H4CH(CH3)2]. EIMS: envelope

1,2-C B H })] (3b) and [Ru(κ3-2,2′,3′-{1-(1′-closo-1′,2′-C B H )-

centered on m/z 520.3 (M+ − MeCN).

2 10 9

2 10 10 2

closo-1,2-C2B10H10})(C6H6)] (4b). In an analogous manner, to [Mg(κ2- 2,2′-{1-(1′-closo-1′,2′-C2B10H10)-closo-1,2-C2B10H10)}(DME)2] in
THF (20 mL) prepared from 1,1′-bis(o-carborane) (0.250 g, 0.873
mmol), was added [RuCl2(C6H6)]2 (0.218 g, 0.436 mmol). From the
resulting near-black solution, workup by preparative TLC (DCM/ petrol 3:2) afforded a violet band (Rf = 0.47) subsequently identified (via elemental analysis, mass spectrometry, and single-crystal X-ray diffraction) as the paramagnetic compound [{Ru(C H )} (μ-Cl)(μ-

[Ru(κ -2,2′-{1-(1′-closo-1′,2′-C2B10H10)-closo-1,2-C2B10H10})-
(C6H6)(NCMe)] (5b). Similarly, compound 4b (0.025 g, 0.052 mmol)
in DCM (1 mL) was treated with MeCN (∼1 mL), and the yellow product was precipitated by the addition of petrol and washed with petrol and dried. It was subsequently identified as [Ru(κ2-2,2′-{1-(1′- closo-1′,2′-C2B10H10)-closo-1,2-C2B10H10})(C6H6)(NCMe)] (5b,
0.019 g, 70%). C16H37B20NRu requires C 28.6, H 5.79, N 2.78%.
Found for 5b: C 27.7, H 5.86, N 2.71%. 11B{1H} NMR (CD2Cl2): δ

6 6 2

κ4-2,2′,3,3′-{1-(1′-closo-1′,2′-C2B10H9)-closo-1,2-C2B10H9})] (3b,
0.060 g, 10%) and an orange band (Rf = 0.10) subsequently identified as [Ru(κ3-2,2′,3′-{1-(1′-closo-1′,2′-C2B10H10)-closo-1,2-C2B10H10})- (C6H6)] (4b, 0.230 g, 57%).

−3.3 (4B), −4.4 to −11.5, overlapping resonances with prominent
maxima at −5.3, −6.8, −8.4, −10.7 (total integral 16B). 1H NMR (CD2Cl2): δ 5.46 (s, 6H, C6H6), 2.46 (s, 3H, NCCH3). EIMS:
envelopes centered on m/z 505.3 (M+), 463.3 (M+ − MeCN).

3b. C




requires C 28.3, H 4.76. Found for 3b: C 27.5,


H 4.60%. EIMS: envelope centered on m/z 677.2 (M+), 597.1 (M+− C6H6).
4b. C10H26B20Ru requires C 25.9, H 5.65. Found for 4b: C 25.7, H 5.67%. 11B{1H} NMR (CD2Cl2): δ −1.3 (2B) −4.9 (2B), −6.9 (4B),
−8.2 to −12.2, overlapping resonances with prominent maxima at
−9.8 and −11.0 (total integral 12B). 1H NMR (CD Cl ): δ 5.71 (s,

(C6Me6)(NCMe)] (5d). EXcess MeCN (∼1 mL) was added to compound 4d (0.025 g, 0.045 mmol) in DCM (1 mL), resulting in an immediate color change from red to yellow. The solution was concentrated to half volume, and upon subsequent slow evaporation, yellow crystals formed. These were isolated by filtration, washed with
petrol, and ultimately identified as [Ru(κ2-2,2′-{1-(1′-closo-1′,2′-

2 2

C6H6). 1H{11B} NMR (CD2Cl2): δ 5.71 (s, 6H, C6H6), 0.00 (br s,
4H, BHagostic). EIMS: envelope centered on m/z 463.2 (M+).
closo-1,2-C2B10H9})] (3c). In an analogous manner, from [Mg(κ2-2,2′-
{1-(1′-closo-1′,2′-C2B10H10)-closo-1,2-C2B10H10)}(DME)2], prepared from 1,1′-bis(o-carborane) (0.250 g, 0.873 mmol), and

C2B10H10)-closo-1,2-C2B10H10})(C6Me6)(NCMe)] (5d, 0.018 g,
67%). Elemental analysis was unreliable because of the facile loss of MeCN. 11B{1H} NMR (CD3CN): δ 0 to −5, overlapping resonances with prominent maxima at −2.4, −3.5 (total integral 4B), −7 to −11, overlapping resonances with maxima at −7.4, −8.6, −9.6, −10.2 (total integral 14B), −12.5 (2B). 1H NMR (CD3CN): δ 1.98 [s, 18H,

C6(CH3)6], 1.96 [s, 3H, NCCH3]. EIMS: envelope centered on m/z
547.4 (M+−MeCN).
Catalysis: General Procedure for Diels−Alder Cycloaddi- tion. An aliquot of freshly cracked CpH (0.46 mL, 10 equiv) was added under N2 to a solution of the catalyst (1 mol %) in CD2Cl2 (0.8
mL) in a J. Young NMR tube. The dienophile (1 equiv) was added at t = 0, and 1H NMR spectra were recorded every hour. Conversion was monitored by the analysis of the ratio of the aldehyde proton resonances in the products against that in the dienophile. Each run was repeated, and the average conversion was recorded. Reactions were run in an air-conditioned room at 20 °C. Further details are available in the Supporting Information.
Crystallography. Crystals of 1a, 1c, 1d·CH2Cl2, 2c, 2d, 3b· CH2Cl2, 3c, 4d, and 5a·CH2Cl2 were grown by the diffusion of a DCM solution of the appropriate compound and petrol at −20 °C. Crystals of 4b were obtained by vapor diffusion of a fluorobenzene solution and petrol at room temperature in a gloveboX. Crystals of 5b· CH2Cl2 were grown by vapor diffusion of a DCM solution and petrol at −20 °C, and those of 5d·CH3CN were afforded by the slow evaporation of a MeCN solution at room temperature. All were obtained as single crystals except 4d, which crystallizes as a two- component twin. Diffraction data from 1c, 1d·CH2Cl2, 2c, 2d, 3b· CH2Cl2, 3c, 5a·CH2Cl2, and 5d·CH3CN were obtained at 100 K on a Bruker X8 APEXII diffractometer operating with Mo Kα radiation. Data from 1a, 4b, and 5b·CH2Cl2 were collected at 120 K on a Rigaku OXford Diffraction SuperNova diffractometer, using Cu Kα radiation for 1a and Mo Kα radiation for 4b and 5b·CH2Cl2. Data from 4d were obtained at 100 K on a Rigaku AFC12 diffractometer with Mo Kα radiation. Using OLEX2,33 structures were solved by direct methods using the SHELXS34 or SHELXT35 program and refined by full-matriX least-squares using SHELXL.36 In 1d·CH2Cl2 and 5b· CH2Cl2, the DCM molecule of solvation is fully ordered while in 3b· CH2Cl2, it is partially disordered. 5a·CH2Cl2 also contains one molecule of DCM of solvation but this was impossible to model satisfactorily, and the intensity contribution of the badly disordered solvent was removed using the BYPASS procedure37 implemented in OLEX2. Finally, 5d·CH3CN contains a partially disordered molecule of MeCN of solvation. In all cases, cage C atoms bearing only H substituents were clearly distinguished from B atoms by application of the vertex−centroid distance (VCD) and boron−hydrogen distance (BHD) methods,38 except for 2d, in which there is C/B disorder in both the metallacarborane and carborane cages, and 3c and 3b· CH2Cl2, in which all cage atoms bonded to Ru are disordered C/B
50:50, a space group requirement for the latter. In 1c, B3′ is partially disordered, the minor component appearing as a ghost atom above
the C2B3 open face of the nido cage. As a consequence, it was not possible to locate the bridging H atom crystallographically. Cage H atoms (both terminal and bridging) were allowed positional refinement except in the case of 1a where the quality of the data did not allow this; here, terminal H atoms were treated as a miXture of refined, restrained [B−H 1.10(2) Å] and riding (B−H 1.12 Å), while
the bridging H atom was restrained to B9−H, B10−H 1.25(2) Å. All other H atoms were treated as riding, with Cprimary−H 0.98 Å, Csecondary−H 0.99 Å, Ctertiary−H 1.00 Å, and Carene−H 1.00 Å. H-atom
displacement parameters were constrained to 1.2 × Ueq (bound B or
C) except for Me H atoms [1.5 × Ueq (Cmethyl)]. The Supporting Information contains unit cell data and further experimental details.
Synthesis and Characterization. Chart 1 summarizes the four classes of species (1−4) isolated from our reactions and a fifth species (5) ultimately prepared from 4. We have
previously reported compound 4a (hereafter compound I) and shown it to be an efficient catalyst for the Diels−Alder cycloaddition of cyclopentadiene and methacrolein.12 Our objectives here are to extend the scope of this reaction to embrace other dieneophiles and analogous catalysts in which the η-arene ligand is varied.

Chart 1. Line Diagrams of the Productsa

aIn zwitterion 1, the negative charge is associated with the open five- atom face of the right carborane cage. Paramagnetic species 3 is a miXture of two isomers with “cis” and “trans” arrangements of the disordered cage C atoms bonded to Ru.

Our initial target was 4b, the simple analog of I with benzene replacing p-cymene. Although I is produced in good yield by the deprotonation of 1,1′-bis(ortho-carborane) in THF with nBuLi followed by reaction with [RuCl2(p- cymene)]2, the analogous reaction using [RuCl2(C6H6)]2 was by no means clean, with TLC revealing a large number
of overlapping bands, each of which was afforded in low yield but none of which was spectroscopically consistent with 4b. The most prominent band was an orange-brown species, but this proved impossible to isolate in pure form by repeated chromatography. Nevertheless, the 1H{11B} NMR spectrum of this band featured tell-tale low-frequency resonances consistent with a B−H−B bridging proton (δ −2.90) and two different
B−H⇀Ru agostic protons (δ −6.34, −7.83), while EIMS
indicated a molecular ion peak centered on m/z 454.3. These data are consistent with the zwitterionic species [Ru(κ3- 2,2′,11′-{1-(7′-nido-7′,8′-C2B9H11)-closo-1,2-C2B10H10})- (C6H6)] (1b).
Switching the arene to mesitylene (1,3,5-trimethybenzene) afforded a somewhat cleaner reaction in that TLC allowed the isolation of the pure form of two products, albeit in low yield. Yellow compound 2c was identified by elemental analysis, mass spectrometry, 11B and 1H NMR spectroscopies, and
ultimately single-crystal X-ray diffraction as [1-(1′-closo-1′,2′- C2B10H11)-3-(C6H3Me3)-closo-3,1,2-RuC2B9H10], while orange
coproduct 1c was characterized in the same way as the zwitterion [Ru(κ3-2,2′,11′-{1-(7′-nido-7′,8′-C2B9H11)-closo- 1,2-C2B10H10})(C6H3Me3)], fully analogous to 1b.

Figure 3. Perspective views of compound 1d. Selected interatomic distances (Å): Ru−C2, 2.1500(14); Ru−H2′, 1.842(19); H2′−B2′, 1.18(2); Ru···B2′, 2.4028(17); Ru−H11′, 1.87(2); H11′−B11′, 1.23(2); Ru···B11′, 2.4364(17); C2−C1, 1.6670(19); C1−C7′, 1.507(2); C7′−C8′,
1.533(2); Ru−C(arene), 2.1861(14)−2.2890(14).
A very similar outcome resulted from the use of [RuCl2(C6Me6)]2, with yellow [1-(1′-closo-1′,2′-C2B10H11)-3- (C6Me6)-closo-3,1,2-RuC2B9H10] (2d) and orange [Ru(κ3- 2,2′,11′-{1-(7′-nido-7′,8′-C2B9H11)-closo-1,2-C2B10H10})-
(C6Me6)] (1d) isolated in yields of 5 and 14%, respectively,
and characterized by the same array of analytical techniques.
Compounds 1b, 1c, and 1d are zwitterionic species in which the primed carborane cage has been partially degraded by the loss of a {B} vertex and carries a negative charge. The open face thus formed has an H atom bridging the B9′−B10′ connectivity, appearing in the 1H{11B} NMR spectrum at ca. δ
−2.9 to −3.0. This cage is not bound to Ru+ by its nonlinking

C atom (which remains protonated, C8′H δ ca. 2.8) but via two B−H⇀Ru agostic interactions at B2′ and B11′, character- istic resonances appearing in the 1H{11B} NMR spectrum at
ca. δ −6.5 to −6.8 and ca. −7.8 to −7.9. The partially degraded bis(carborane) is further bonded to Ru by a direct σ-bond from C2 of the unprimed cage. Overall, the bis(carborane) unit is now a closo-nido-X(C)L2 ligand to the metal center, a previously unreported bonding mode for this ligand. To illustrate these interactions, two views of compound 1d (as an example) are presented in Figure 3.
Compounds 2c and 2d are isomers of 1c and 1d and again result from the deboronation of one cage, but now the interaction between the metal center and the bis(carborane) is quite different, with the unprimed cage being η5-coordinated to Ru and there being no interaction between the metal atom and the primed cage. Thus, compounds 2 are metallacarborane− carborane compounds, fully analogous to those we have
previously reported following deliberate monodeboronation and then metalation of 1,1′-bis(ortho-carborane).10 11B{1H} NMR spectra of compounds 2c and 2d are largely uninformative because of multiple overlapping resonances, a consequence of the asymmetry of the molecules (19 unique B
environments) and a relatively small 11B chemical shift range, although the absence of resonances at low frequency does imply the closo nature of both cages. 1H NMR spectra reveal, in addition to resonances due to the arene ligand, two broad CcageH resonances, one for the metallacarborane cage and the other for the carborane cage.
Both 2c and 2d were also characterized crystallographically, and a perspective view of 2c is shown in Figure 4. In the structure of 2d, there is disorder between C2 and B4 of the ruthenacarborane cage (vertex 2, C/B 53:47; vertex 4, the

Figure 4. Perspective view of compound 2c. Selected interatomic distances (Å): Ru3−C1, 2.2805(15); Ru3−C2, 2.2201(16); C2−C1, 1.667(2); C1−C1′, 1.544(2); C1′−C2′, 1.658(2); Ru−C(arene),

complement) and between C2′ and B4′ of the carborane cage (both vertices, C/B 50:50) while the structure of 2c is fully ordered. It is clear from Figure 4 that an important feature of
the structure of 2c is a pronounced bend back of the mesitylene ligand away from the bulky carborane substituent on C2. This bend back is consistently observed in [1-C2B10-3- arene/Cp/Cp*-3,1,2-MC2B9] species10,25 and is conveniently quantified by θ, the dihedral angle between the plane of the hydrocarbon ligand and the B5B6B11B12B9 plane of the metallacarborane (the lower pentagonal belt, usually taken as the reference plane in 3,1,2-MC2B9 icosahedra).39 In 2c, θ is 18.58(5)°, and in 2d, it is 19.70(5)°. Both of these are greater than that in previously reported p-cymene analog 2a [hereafter compound II, θ = 16.08(9)°],10 consistent with the effective steric bulk of the arenes increasing in the order p-cymene < mesitylene < hexamethylbenzene.
The identification of products 1b−d and 2c,d involving partial degradation of one of the cages of 1,1′-bis(ortho- carborane) prompted us to reinvestigate the synthesis of
compound I. Originally, having reacted the carborane with nBuLi (slight excess) in THF, we stirred the solution at room temperature for 1 h before the addition of [RuCl2(p- cymene)]2.12 In repeating the reaction, we now allowed the solution to stir for 2 h before metalation. On workup, this afforded not only compound I in somewhat better yield than before (51 vs 37%) but also a trace amount (ca. 1%) of a new compound as an additional mobile band on TLC. This new

E DOI: 10.1021/acs.inorgchem.9b01774
Inorg. Chem. XXXX, XXX, XXX−XXX

Scheme 1. Deprotonation of 1,1′-Bis(ortho-carborane) with nBuLi Followed by Reaction with [RuCl2(arene)]2

compound was quickly identified as zwitterion [Ru(κ3-2,2′,11′-
{1-(7′-nido-7′,8′-C2B9H11)-closo-1,2-C2B10H10}(p-cymene)] (1a) on the basis of the characteristic resonances for B−H−B bridging and B−H⇀Ru agostic H atoms in the 1H{11B} NMR spectrum and ultimately confirmed as such by a crystallo-
graphic study.
We therefore conclude that small amounts of deboronated species 1 and 2 are typically formed in these reactions, possibly aided by extended stir-time prior to metalation. Partial deboronation of 1,1′-bis(ortho-carborane) in the presence of nBuLi in THF has been noted previously.17,24 We believe that
the most likely source of this deboronation is nucleophilic attack on 1,1′-bis(ortho-carborane) by an ω-alkenolate ion formed by the decomposition of THF by nBuLi at room temperature.40 We therefore concur with Spokoyny et al. that nBuLi is not a suitable reagent for the deprotonation of 1,1′- bis(ortho-carborane).24 Deprotonation of the single-cage [closo-1,2-C2B10H12] with nBuLi in THF is a clean reaction and is used extensively, but each cage of 1,1′-bis(ortho- carborane) is electron-withdrawing relative to H, rendering the other more susceptible to nucleophilic attack.10,11 Although this explains the formation of species 1 and 2 in the above reactions, it does not rationalize the fact that only the reaction with [RuCl2(p-cymene)]2 affords target species 4 (4a = I). However, of all of the [RuCl2(arene)]2 dimers, the p-cymene
compound is by far the most soluble, and this is likely to be a major factor here.
Scheme 1 summarizes the reactions described so far. The

closo-1′,2′-C2B10H10)-closo-1,2-C2B10H10})(p-cymene)] (I).10 Reaction between [Mg(κ2-2,2′-{1-(1′-closo-1′,2′-C2B10H10)- closo-1,2-C2B10H10)}(DME)2], prepared according to the
literature23 and used in situ following spectroscopic con- firmation of its purity, and [RuCl2(p-cymene)]2 in THF afforded compound I (identified by 11B{1H} and 1H{11B} NMR spectroscopies) in 41% yield following workup, a yield fully comparable with that previously reported.10
Encouraged by this success, we next used the transfer agent to target an analog of I that we had failed to prepare by the nBuLi/metalation approach. Reaction of the magnesium compound with [RuCl2(C6H6)]2 in THF resulted in an extremely dark, nearly black solution, which on separation by TLC afforded two products: violet paramagnetic species
C2B10H9)-closo-1,2-C2B10H9})] (3b) and, pleasingly, orange compound [Ru(κ3-2,2′,3′-{1-(1′-closo-1′,2′-C2B10H10)-closo- 1,2-C2B10H10})(C6H6)] (4b), the latter in a yield of 57%.
Compound 3b was characterized by elemental analysis, mass spectrometry, and single-crystal X-ray diffraction (Figure 5). It
is without precedent, featuring a formal {C2B10H9−C2B10H9}4− fragment derived by the quadruple deprotonation of 1,1′- bis(ortho-carborane) bridging two {Ru(C6H6)} fragments
which are additionally bridged by a Cl ligand. Evidently the
{Ru2(C6H6)2Cl} unit arises from only partial fragmentation of the [RuCl2(C6H6)]2 dimer on reaction with [Mg(κ2-2,2′-{1- (1′-closo-1′,2′-C2B10H10)-closo-1,2-C2B10H10)}(DME)2]. Thus,
3b is formally a miXed-valence RuII/RuIII species with the

formation of a range of compounds 4 with differing arene ligands via the initial deprotonation of 1,1′-bis(ortho- carborane) with nBuLi in THF was thus shown to be unfeasible. Viñas, TeiXidor, and co-workers have performed extensive studies of alternative ethereal solvents for the
deprotonation of carboranes,41 but seeking a different approach, we turned to the recently reported bis(carborane) transfer reagent [Mg(κ2-2,2′-{1-(1′-closo-1′,2′-C2B10H10)-closo- 1,2-C2B10H10)}(DME)2].23 We began by testing the ability of this species to transfer bis(carborane) to an {Ru(p-cymene)} fragment to afford known compound [Ru(κ3-2,2′,3′-{1-(1′-

bis(carborane) acting as a closo-closo-X4(C,C′,B,B′) ligand to the Ru2 unit. The molecule has effective C2v symmetry, one of
whose mirror planes coincides with a crystallographic mirror which lies perpendicular to the Ru2Cl plane, requiring the cage vertices σ-bonded to the Ru atoms to be C/B 50:50 disordered. The species must therefore exist as an equal miXture of two isomers of the molecule (Figure 6) with cis and trans arrangements of the cage C atoms bonded to Ru. These isomers could not be separated on TLC because the product moves as tight band, and they crystallize together, resulting in the disordered structure.

DOI: 10.1021/acs.inorgchem.9b01774
Inorg. Chem. XXXX, XXX, XXX−XXX

Figure 5. Perspective view of compound 3b. Atoms shown in red are C/B 50:50 disordered, and the molecule has crystallographically
imposed Cs symmetry about the plane defined by B12B12′Cl. Selected interatomic distances (Å): Ru−BC2, 2.1070(18); Ru−BC2′, 2.1070(18); Ru−Cl, 2.3341(4); C1−BC2, 1.716(3); C1′−BC2′,
1.720(3); C1′−C2′, 1.658(2); Ru−C(arene), 2.2241(18)−2.311(2).

Figure 6. cis and trans isomers of compound 3b.

The major product of the reaction that afforded 3b was target compound [Ru(κ3-2,2′,3′-{1-(1′-closo-1′,2′-C2B10H10)- closo-1,2-C2B10H10})(C6H6)] (4b). This was initially charac- terized by elemental analysis, mass spectrometry, and 11B{1H} and 1H{11B} NMR spectroscopies. Although the 11B{1H} NMR spectrum is relatively uninformative, the absence of any
CcageH resonances in the 1H NMR spectrum and the clear presence of a resonance associated with the B-agostic B−H⇀ Ru interaction being integrated for 4H in the 1H{11B} NMR spectrum identify 4b as a direct analog of compound I.12 However, there is only one B−H⇀Ru interaction in these species in which the interaction is fluXional among B atoms 3, 3′, 6, and 6′ in solution at room temperature. The fluXionality can be partially arrested by cooling in the case of I,12 but for 4b, it persists, even at 198 K in CD2Cl2.
Confirmation of the nature of 4b was provided by the results of a structural determination (Figure 7). The Ru atom is bound to the bis(carborane) by two Ru−C bonds and,
additionally, a B-agostic interaction from B3′H3′, making the bis(carborane) a closo-closo-X2(C,C′)L ligand and affording the
Ru center an 18e configuration.
The success of [Mg(κ2-2,2′-{1-(1′-closo-1′,2′-C2B10H10)-
closo-1,2-C2B10H10)}(DME)2] as a bis(carborane) transfer
agent in these two reactions prompted us to similarly explore its reactivity with [RuCl2(C6H3Me3)]2 and [RuCl2(C6Me6)]2. In the former case, the only product that could be isolated by TLC was a low yield of paramagnetic species [{Ru-
C2B10H9)-closo-1,2-C2B10H9})] (3c), an analog of 3b charac-
terized by microanalysis, mass spectrometry, and single-crystal X-ray diffraction. We attribute the general lack of success of this reaction, at least in part, to the very limited solubility of

Figure 7. Perspective view of compound 4b. Selected interatomic distances (Å): Ru−C2, 2.1086(8); Ru−C2′, 2.1292(8); Ru−H3′, 1.924(15); H2′−B3′, 1.154(15); Ru···B3′, 2.4003(10); C2−Cl,
1.6604(11); C1−C1′, 1.5116(11); C1′−C2′, 1.7046(11); Ru−
C(arene), 2.1777(9)−2.2443(9).

[RuCl2(C6H3Me3)]2. Reaction between the magnesium reagent and [RuCl2(C6Me6)]2 also afforded only one isolatable
product, but this time sought-after species [Ru(κ3-2,2′,3′-{1- (1′-closo-1′,2′-C2B10H10)-closo-1,2-C2B10H10})(C6Me6)] (4d).
Initial characterization by elemental analysis was comple-
mented by that from 11B{1H} and 1H{11B} NMR spectros- copies, although no useful mass spectrometric data could be obtained from 4d. As was the case with I and 4b, the 1H{11B} NMR spectrum of 4d at room temperature shows a broad, low- frequency singlet integrated for 4H and assigned to fluXional B−H⇀Ru atoms, and the presence of one such interaction in the solid state was subsequently confirmed by a crystallo- graphic study. Similarly to 4b, the fluXionality of the B−H⇀Ru interaction in 4d could not be arrested at 198 K. Scheme 2 summarizes the reactions performed with the bis(carborane) transfer regent.
As previously noted,10 we believe that the displacement of the B-agostic B−H⇀Ru interaction in species of structure type 4 is the first step in these compounds acting as Lewis acid catalysts. Therefore, prior to full catalytic studies, we tested the likely potential of I, 4b, and 4d to act as Lewis acid catalysts by investigating their reactivity with weakly binding ligand acetonitrile. If MeCN is able to displace the agostic bond, then the possibility exists that, in a cycloaddition reaction, the incoming dienophile will similarly displace this interaction, be activated by bonding to the metal center, and subsequently interact with the diene. The addition of an excess of MeCN to a solution of I, 4b, and 4d in DCM resulted in an immediate
lightening of the color of the solution, signifying reaction. Products [Ru(κ2-2,2′-{1-(1′-closo-1′,2′-C2B10H10)-closo-1,2- C2B10H10})(p-cymene)(NCMe)] (5a), [Ru(κ2-2,2′-{1-(1′- closo-1′,2′-C2B10H10)-closo-1,2-C2B10H10})(C6H6)(NCMe)]
(5b), and [Ru(κ2-2,2′-{1-(1′-closo-1′,2′-C2B10H10)-closo-1,2-
C2B10H10})(C6Me6)(NCMe)] (5d) were subsequently iso-
lated in high yields (Scheme 3).
These three new compounds were characterized by microanalysis (except for 5d, from which MeCN is easily lost, leading to unreliable results), mass spectrometry (all show M+ − MeCN as the highest-mass peak), 11B{1H}, and 1H and 1H{11B} NMR spectroscopies (in CD3CN for 5d for the
reason stated above). Significantly, the 1H{11B} NMR spectra showed no resonance due to B−H⇀Ru, with this being replaced by a simple singlet for the NCCH3 protons. Note that it was necessary to record the 1H NMR spectra of 5a in two different solvents. In (CD3)2CO, the resonance assigned to CH3C6H4CH(CH3)2 lies on top of the protio solvent signals,

Scheme 2. Reactions Using the 1,1′-Bis(ortho-carborane) Transfer Reagent

Scheme 3. Displacement of the B-Agostic B−H⇀Ru Bond by the Reaction of 4 with MeCN

and although this is resolved in CD2Cl2, there is now overlap between the CH3C6H4CH(CH3)2 resonances and solvent and between NCCH3 and CH3C6H4CH(CH3)2 resonances. The compound is insufficiently soluble in CDCl3 for this solvent to be useful. Finally, all three compounds were characterized crystallographically. Figure 8 shows a perspective view of a molecule of 5a as an example.
We also tested zwitterionic species 1a, 1b, 1c, and 1d in reaction with MeCN because these also have B-agostic B−H⇀ Ru interactions which could in principle also be cleaved by an incoming 2e ligand. However, no reaction was observed in all cases, with the retention of the characteristic BHagostic

Figure 8. Perspective view of compound 5a. Selected interatomic distances (Å): Ru−C2, 2.139(2); Ru−C2′, 2.131(2); Ru−N, 2.0310(19); C2−Cl, 1.703(3); C1−C1′, 1.530(3); C1′−C2′,
1.681(3); N−C100, 1.139(3); Ru−C(arene), 2.216(2)−2.262(2).

resonances in the 1H{11B} NMR spectrum. Clearly the B- agostic B−H⇀Ru bonds in compounds of type 1 are significantly stronger than those in compounds of type 4, and this is fully consistent with the greater hydridic nature of the former H atoms which resonate between ca. δ −6 and −8 in the 1H{11B} NMR spectrum, compared to ca. δ 0 for the bridging H atoms in compounds of type 4.
In summary, we now have three examples of structure type 4 (I, 4b, and 4d) that have the potential to act as Lewis acid catalysts. These compounds differ only in the nature of the arene η-bonded to the Ru center. Conventional wisdom is that, in terms of their electron-donating abilities, C6Me6 > p-cymene
> C6H6, in which case the Lewis acid strengths of the three potential catalysts would be 4b > I > 4d. Before testing this, we sought evidence for the relative electron-donating strengths of the three arenes from the structural studies we have performed on I,10 4b, 4d, 5a, 5b, and 5d (Table 1).
For the sequence 4b → I → 4d, the Ru···B3′ distance increases from 2.40 to 2.43 to ca. 2.52 Å, indicating a weaker

Table 1. Structure-Based Assessment (Å) of the Relative Electron-Donating Properties of the Arene Ligands in Species 4 and 5


aTwo crystallographically independent molecules.

Scheme 4. Diels−Alder Cycloaddition Reaction Catalyzed by Lewis Acids I, 4b, and 4d

interaction between the B3′−H3′ bond and the metal as the series progresses (the Ru···B length is considered more reliable than either the Ru−H or B−H length because of the relatively large errors in the last two distances). Note that the steady increase in the Ru···B3′ distance is not simply a consequence of the increasing steric bulk of the arene because the Ru−C2 and Ru−C2′ distances remain effectively constant. Thus, the B-agostic interaction progressively weakens along the series 4b
→ I → 4d, reflecting an increasingly electron-rich metal center as the arene changes from C6H6 to p-cymene to C6Me6. Moreover, in acetonitrile adducts 5 there is a small but clear progressive change in the N C distance (N−C100), affording the same conclusion with regard to the relative electron- donating properties of the arenes; the Ru center is more electron-rich in the series 5d > 5a > 5b, resulting in more back- bonding to the nitrile CN−π* orbital and a longer bond.
Catalysis. To compare the Lewis acidities of compounds I, 4b, and 4d, we used each of them as catalysts in the Diels− Alder cycloaddition reactions between cyclopentadiene and three dienophiles (methacrolein, ethylacrolein, and E-croto-
naldehyde), as summarized in Scheme 4. Full details of the catalytic studies and graphs of % conversion of substrates to products versus time by catalyst and by dienophile are available in the Supporting Information.
All three catalysts are active, showing increased conversions relative to control reactions (no catalyst present). For all three catalysts, the reactions with methacrolein and ethylacrolein were much faster than those with E-crotonaldehyde, as illustrated in Figure 9 for the case of catalyst 4d. After 360

Figure 9. Performance of compound 4d in catalyzing the Diels−Alder cycloaddition reactions.

favorable secondary orbital overlap between aldehyde and ene functions in the product, whereas for methacrolein and ethylacrolein, the exo/endo ratio is typically ca. 90:10, the result of steric crowding between R1 and the bridgehead atom. Such reactivity differences between the dienophiles in Diels− Alder reactions are well-established.42
With regard to the relative efficiencies of the three catalysts, for the reactions involving methacrolein and ethylacrolein, benzene compound 4b performs best, achieving >90% conversion within 1 h and effectively quantitative conversion within 2 h. With these substrates there is little difference between p-cymene compound I and hexamethylbenzene compound 4d, with both achieving ca. 50−60% conversion after 4 h, although I performs slightly better than 4d. Figure 10

Figure 10. Performance of compounds 4b, I, and 4d versus the control in catalyzing the Diels−Alder cycloaddition reaction involving ethylacrolein.

demonstrates the trend in the case of the ethylacrolein reaction. We recognize that a contributing factor in the relative catalytic activity of 4b > I > 4d could be the increasing steric bulk of the arene (making it more difficult for the dienophile to interact with the metal center), but for Lewis acid catalysis, the relative electron-donating ability of the three arene ligands (demonstrated by the analysis in Table 1) is also likely to be important. In this particular system, both electronic and steric effects work in harmony. With E-crotonaldehyde, the reaction is much slower irrespective of the catalyst, as previously noted, and although 4d appears to perform worst, there is effectively no difference between the performance of 4b and I.

min, compound 4d achieved ca. 65% conversion for the dienophiles with an α-substituent (methacrolein and ethyl- acrolein) compared to <20% for that without (E-crotonalde- hyde). Another difference is that with E-crotonaldehyde the exo/endo product ratio is typically ca. 45:55, with the preference for the kinetic endo isomer being the result of

Attempts to prepare arene ligand analogs of a previously reported catalytically active arene ruthenium complex of 1,1′- bis(o-carborane), using nBuLi in THF were frustrated by the deboronation of one cage, leading instead to metallacarbor- ane−carborane compounds and/or novel zwitterionic species in which the bis(carborane) unit is a closo-nido-X(C)L2 ligand

to the metal center, a previously unreported bonding mode. Switching to the use of a magnesium bis(carborane) transfer reagent was successful, however, and two new analogs of the catalyst were isolated and characterized, although we also isolated two examples of a novel type of complex in which the
bis(carborane) acts as a closo-closo-X4(C,C′,B,B′) ligand to a
paramagnetic Ru2 unit. The abilities of three catalysts to catalyze Diels−Alder cycloaddition reactions between cyclo- pentadiene and three dienophiles were compared and found to reflect both the electron-donating strength and steric bulk of the arene ligand.
*S Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorg- chem.9b01774.
NMR spectra of all new products and details of the catalytic experiments (PDF)
Accession Codes
CCDC 1921133−1921144 contain the supplementary crys- tallographic data for this paper. These data can be obtained free of charge via, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Corresponding Author
*E-mail: [email protected].
Alan J. Welch: 0000-0003-4236-2475
The authors declare no competing financial interest.
We thank the Engineering and Physical Sciences Research Council and the CRITICAT Centre for Doctoral Training for financial support (Ph.D. studentships to R.J.J. and A.P.Y.C.; grant code EP/L016419/1). We also thank the U.K. National Crystallography Service for data collection for compound 4d and Dr. G. S. Nichol (Univ. of Edinburgh) for data collection for compounds 1a, 4b, and 5b·CH2Cl2.
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