Synthesis and characterization of rhodium(I) and iridium(I) carbonyl phosphine complexes with bis(N-heterocyclic carbene)borate ligands

Journal of Organometallic Chemistry 710 (2012) 36e43

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Synthesis and characterization of rhodium(I) and iridium(I) carbonyl phosphine complexes with bis(N-heterocyclic carbene)borate ligands Fei Chen a, Gao-Feng Wang a, Yi-Zhi Li a, Xue-Tai Chen a, *, Zi-Ling Xue b a

State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Microstructures, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China b Department of Chemistry, University of Tennessee, Knoxville, TN 37996, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 February 2012 Received in revised form 5 March 2012 Accepted 6 March 2012

A series of iridium(I) and rhodium(I) carbonyl phosphine complexes bearing bis(N-heterocyclic carbene) borate ligands [H2B(ImtBu)2]Rh(CO)(PPh3) (5), [F2B(ImtBu)2]Rh(CO)(PPh3) (6), [F2B(ImtBu)2]Ir(CO)(PPh3) (7), [H2B(ImtBu)2]Rh(CO)(PCy3) (8), [F2B(ImtBu)2]Rh(CO)(PCy3) (9), and [F2B(ImtBu)2]Ir(CO)(PCy3) (10) (H2B(ImtBu)2 ¼ dihydrobis(3-tert-butylimidazol-2-ylidene)borate; F2B(ImtBu)2 ¼ difluorobis(3-tertbutylimidazol-2-ylidene)borate) have been prepared and characterized. IR stretching values of the CO ligands, the values of NMR coupling constants JRheP, and electrochemical data are rationalized by the electronic effects of the bis(3-tert-butylimidazol-2-ylidene)borate and phosphine ligands. The results show that the s donor capacity of bis(3-tert-butylimidazol-2-ylidene)borate is stronger than those of the analogous isoelectronic bis(pyrazolyl)borate (Bp) and acetylacetonato (acac) ligands. The molecular structures of complexes 6e10 have been determined by single-crystal X-ray diffraction, which showed square-plane geometries around the metal centers. Ó 2012 Elsevier B.V. All rights reserved.

Keywords: Rhodium Iridium N-Heterocyclic carbene Ligand effect

1. Introduction N-heterocyclic carbenes have been actively used as s donor ligands in a wide variety of transition metal complexes [1e9] since the first report by Arduengo in 1991 [10]. In the past two decades, mono-dentate NHCs have been extensively studied in organometallic chemistry [11e15]. In addition, many bi- or poly-dentate NHCcontaining ligands have been designed and used in the preparation of new organometallic complexes [16e20]. Bis(NHC) ligands bridged by different spacers have attracted particular attention due to the chelating effect of these ligands [21e26]. Bis(N-heterocyclic carbene)borate ligands bridged by a BH2 moiety, which are NHC analogs of the classic bidentate ligands such as bis(pyrazolyl)borate (Bp) [27e30] and acetylacetonato (acac) [31e35], have been actively studied. Dihydrobis(3-alkylimidazol-2-ylidene)borates [H2B(R-ImH)2] (R ¼ Me, Et, iPr) were first introduced by Fehlhammer and coworkers in 2001 [36]. Since then these ligands have been used to coordinate to different metals such as Li(I), Pd(II), Pt(II), Au(I), Ca(II), Sr(II), and Ni(II) [36e39]. More recently, we have prepared and characterized Rh(I)- and Ir(I)-dicarbonyl complexes with the bis(3-

tert-butylimidazol-2-ylidene)borate ligand [H2B(ImtBu)2]Rh(CO)2 (1) and [H2B(ImtBu)2]Ir(CO)2 (2) and observed the unusual reactivity of the BH2 moiety in the formation of the BeF products [F2B(ImtBu)2]Rh(CO)2 (3) and [F2B(ImtBu)2]Ir(CO)2 (4) [40]. The NHC ligand is considered to be a strong s-donor that is able to stabilize the metals in complexes at different oxidation states. Unlike the well established steric and electronic effects of phosphines, the ligand effect of NHC is less understood [11e20]. We have studied the reactions of 1, 3 and 4 with PR3 (R ¼ Ph, Cy). Six new Rh(I) and Ir(I) carbonyl phosphine complexes 5e10 have been isolated and characterized. Here we report their synthesis and characterization. The n(CO) frequencies in the IR spectra, values of the NMR coupling constants JRheP, and electrochemical data of the new complexes have been used to evaluate the electronic effects of bis(3-tert-butylimidazol-2-ylidene)borate and its fluorinated derivative. These properties are compared with those of the Bp and acac ligands.

2. Experimental 2.1. General procedures

* Corresponding author. Fax: þ86 25 83314502. E-mail address: [email protected] (X.-T. Chen). 0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2012.03.005

Unless otherwise noted, all reactions and manipulations were performed under a dry nitrogen atmosphere using the standard

F. Chen et al. / Journal of Organometallic Chemistry 710 (2012) 36e43

37

Scheme 1.

134.4, 134.3 (C2,6, Ph), 129.2 (C4, Ph), 127.9, 127.8 (C3,5, Ph), 125.7, 125.1 (ring-CH]CHN-tBu), 116.6, 116.7 (ring-CH]CHN-tBu), 56.4, 55.8 (CMe3), 32.2, 31.2(CMe3). 11B NMR (160 MHz, CDCl3, 25  C): d 5.90 (BH2). 31P NMR (202 MHz, CDCl3, 25  C): d 35.0 (d, 1 JRheP ¼ 131.1 Hz, PPh3). FT-IR (KBr): v(CO) 1954 cm1. Anal. Calcd for C33H39BN4OPRh: C, 60.76; H, 6.02; N, 8.59; found: C, 60.68; H, 6.13; N, 8.47. 6, 1H NMR (500 MHz, CDCl3, 25  C): d 7.54 (m, 6H, Ph), 7.31 (m, 9H, Ph), 7.30 (s, 1H, ring-CH]CHNtBu), 7.24 (s, 1H, ring-CH] CHNtBu), 7.04 (s, 1H, ring-CH]CHNtBu), 6.63 (s, 1H, ring-CH] CHNtBu), 1.84 (s, 9H, CMe3), 1.23 (s, 9H, CMe3). 13C NMR (125 MHz, CDCl3, 25  C): d 194.7 (dd, 1JRheC ¼ 39.6 Hz, 3JPeC ¼ 18.1 Hz, CO), 182.0 (dd, 1JRheC ¼ 24.9 Hz, 3JPeC ¼ 11.0 Hz, carbene-C), 177.4 (dd, 1 JRheC ¼ 70.9 Hz, 3JPeC ¼ 45.0 Hz, carbene-C), 135.8 (d, 1 JPeC ¼ 38.2 Hz, C1, Ph), 134.4, 134.3 (C2,6, Ph), 129.3 (C4, Ph), 127.9, 127.8 (C3,5, Ph), 121.9, 121.3 (ring-CH]CHN-tBu), 117.2, 116.4 (ring-CH]CHN-tBu), 56.6, 56.2 (CMe3), 32.0, 31.2 (CMe3). 11B NMR (160 MHz, CDCl3, 25  C): d 2.01 (BF2). 31P NMR (202 MHz, CDCl3, 25  C): d 33.8 (d, 1JRheP ¼ 132.7 Hz, PPh3). 19F NMR (470.5 MHz, CDCl3, 25  C): 135.0 (dq, 2JaFbF ¼ 86.6 Hz, 1a JFB ¼ 26.7 Hz, Fa), 162.5 (dq, 2JaFbF ¼ 84.5 Hz, 1JbF B ¼ 25.1 Hz, Fb). FT-IR (KBr): v(CO) 1958 cm1. Anal. Calcd for C33H37BF2N4OPRh: C, 57.58; H, 5.42; N, 8.14; found: C, 57.56; H, 5.45; N, 8.09. 7, 1H NMR (500 MHz, CDCl3, 25  C): d 7.58 (m, 6H, Ph), 7.32 (m, 9H, Ph), 7.31 (s, 1H, ring-CH]CHNtBu), 7.22 (s, 1H, ring-CH] CHNtBu), 7.08 (s, 1H, ring-CH]CHNtBu), 6.68 (s, 1H, ring-CH] CHNtBu), 1.85 (s, 9H, CMe3), 1.25 (s, 9H, CMe3). 13C NMR (125 MHz, CDCl3, 25  C): d 186.9 (d, 3JPeC ¼ 12.4 Hz, CO), 180.8 (d, 3 JPeC ¼ 5.9 Hz, carbene-C), 170.9 (d, 3JPeC ¼ 10.1 Hz, carbene-C), 135.2 (d, 1JPeC ¼ 48.0 Hz, C1, Ph), 134.4, 134.3 (C2,6, Ph), 129.4 (C4, Ph), 127.8, 127.7 (C3,5, Ph), 121.5, 120.7 (ring-CH]CHN-tBu), 117.6, 116.6 (ring-CH]CHN-tBu), 57.1, 56.7 (CMe3), 32.0, 31.6 (CMe3). 11B NMR (160 MHz, CDCl3, 25  C): d 5.09 (BF2). 31P NMR (202 MHz, CDCl3, 25  C): d 20.52. 19F NMR (470.5 MHz, CDCl3, 25  C): 136.4

Schlenk techniques. Solvents were dried using conventional methods and freshly distilled before use. Complexes 1e4 were prepared according to the methods reported in our previous work [40]. NMR spectra were obtained in CDCl3 on a Bruker AM-500 spectrometer. Chemical shifts were referenced to CHCl3/CDCl3 (d 1 H ¼ 7.28 ppm, d 13C ¼ 77.0 ppm), Et2OeBF3 (d 11B ¼ 0 ppm), 85% phosphoric acid (d 31P ¼ 0 ppm), and CFCl3 (d 19F ¼ 0 ppm). Infrared spectra were obtained on a Nicolet NEXUS870 FT-IR (KBr) spectrometer. Cyclic voltammetry measurements were conducted on a model CHI 660D voltammetric analyzer with a glassy carbon electrode as the working electrode, a polished platinum wire as the counter electrode, Ag/AgNO3 as the reference electrode, and 0.1 M NBu4PF6/CH3CN as the supporting electrolyte at a scan rate of 100 mV/s. Ferrocene was added at the end of each experiment to serve as the internal reference. 2.2. Synthesis A solution of [X2B(ImtBu)2]M(CO)2 (0.5 mmol, X ¼ H, M ¼ Rh, 1; X ¼ F, M ¼ Rh, 3; X ¼ F, M ¼ Ir, 4) and an equimolar amount of PR3 (R ¼ Ph, Cy) in diethyl ether (20 mL) was stirred at room temperature for 0.5 h. The solution was removed and washed with hexane to afford the yellow solid products [X2B(ImtBu)2]M(CO)(PR3) (X ¼ H, M ¼ Rh, R ¼ Ph, 5; X ¼ F, M ¼ Rh, R ¼ Ph, 6; X ¼ F, M ¼ Ir, M ¼ Ir, R ¼ Ph, 7; X ¼ H, M ¼ Rh, R ¼ Cy, 8; X ¼ F, M ¼ Rh, R ¼ Cy, 9; X ¼ F, M ¼ Ir, R ¼ Cy, 10). Recrystallization from a mixture of diethyl ether and hexane yielded crystals of these products. 5, 1H NMR (500 MHz, CDCl3, 25  C): d 7.50 (m, 6H, Ph), 7.32 (m, 9H, Ph), 7.11 (s, 1H, ring-CH]CHNtBu), 7.02 (s, 1H, ring-CH] CHNtBu), 6.98 (s, 1H, ring-CH]CHNtBu), 6.58 (s, 1H, ring-CH] CHNtBu), 4.09 (br, 1H, BH), 3.36(br, 1H, BH), 1.83 (s, 9H, CMe3), 1.20 (s, 9H, CMe3). 13C NMR (125 MHz, CDCl3, 25  C): d 195.4 (dd, 1 JRheC ¼ 39.0 Hz, 3JPeC ¼ 16.8 Hz, CO), 181.5 (dd, 1JRheC ¼ 35.5 Hz, 3 JPeC ¼ 12.8 Hz, carbene-C), 177.3 (dd, 1JRheC ¼ 74.2 Hz, 3 JPeC ¼ 48.8 Hz, carbene-C), 136.2 (d, 1JPeC ¼ 37.8 Hz, C1, Ph),

Table 1 Selected spectral and electrochemical data for carbene complexes 5e10. Complex 5 6

11

B NMR

5.90 2.01

7

5.09

8

5.91

9

1.92

10

1.81

13

13

C NMR C(NHC)eM/ppm

181.5 177.3 182.0 177.4 180.8 170.9 184.6 178.2 185.7 178.7 182.3 174.5

1

3

(dd, JRheC ¼ 35.5, JPeC (dd, 1JRheC ¼ 74.2, 3JPeC (dd, 1JRheC ¼ 24.9, 3JPeC (dd, 1JRheC ¼ 70.9, 3JPeC (d, 3JPeC ¼ 5.9) (d, 3JPeC ¼ 10.1) (dd, 1JRheC ¼ 42.8, 3JPeC (dd, 1JRheC ¼ 94.2, 3JPeC (dd, 1JRheC ¼ 38.8) (dd, 1JRheC ¼ 79.1, 3JPeC (d, 3JPeC ¼ 6.0) (d, 3JPeC ¼ 99.0)

¼ ¼ ¼ ¼

12.8) 48.8) 11.0) 45.0)

19

C NMR C(CO)eM/ppm 1

F NMR

3

195.4 (dd, JRheC ¼ 39.0, JPeC ¼ 16.8) 1

3

d 194.7 (dd, JRheC ¼ 39.6, JPeC ¼ 18.1) 3

186.9 (d, JPeC ¼ 12.4) ¼ 10.3) ¼ 45.0) ¼ 41.0)

196.1 (dd, 1JRheC ¼ 50.0, 3JPeC ¼ 17.1) 195.4 (dd 1JRheC ¼ 62.5, 3JPeC ¼ 17.8) 3

d 187.6 (d, JPeC ¼ 12.9)

e 135.0 162.5 136.4 162.1 e

(dq, (dq, (dq, (dq,

2 ab JFF 2 ab JFF 2 ab JFF 2 ab JFF

¼ ¼ ¼ ¼

86.6, 84.5, 85.8, 84.5,

1 a JFB 1 b JF B 1 a JFB 1 b JF B

¼ 26.7) ¼ 25.1) ¼ 26.1) ¼ 25.9)

136.2 160.6 136.8 161.4

(dq, (dq, (dq, (dq,

2 ab JFF 2 ab JFF 2 ab JFF 2 ab JFF

¼ ¼ ¼ ¼

85.2, 87.0, 87.0, 88.9,

1 a JFB 1 b JF B 1 a JFB 1 b JF B

¼ 30.1) ¼ 30.1) ¼ 28.2) ¼ 28.7)

38

F. Chen et al. / Journal of Organometallic Chemistry 710 (2012) 36e43

Table 2 IR, 31P NMR and cyclic voltammetric data for [M(L)(CO)P] compounds. Entry

Compound

E 0 /V

References

35.0(131.1)

0.14a

This work

1958

33.8(132.7)

0.29a

This work

1953

20.5

0.30a

This work

1946

36.3(124.2)

0.12a

This work

1952

34.8(125.4)

0.23a

This work

1948

19.4

0.29a

This work

v(CO) (cm1)

31

1954

P NMR JRheP

1

H

H B N

N

N

N

1

Rh OC

PPh 3

5 F

F B N

N

N

N Rh

2

OC

PPh 3

6 F

F B N

N

N

N

3

Ir OC

PPh 3

7 H

H B

N

N

N

N Rh

4

OC

PCy3

8 F

F B

N

N

N

N Rh

5

OC

PCy3

9 F

F B

N

N

N

N Ir

6

OC

PCy3

10

F. Chen et al. / Journal of Organometallic Chemistry 710 (2012) 36e43

39

Table 2 (continued ) Entry

Compound

v(CO) (cm1)

31

P NMR JRheP

E 0 /V

References

1

H

H B

N N

N N

7

Rh

1987

44(156)

0.80b,c

[11,14]

1966

51(147)

0.63b,c

[11]

1983

46(177.4)

1.29b,c

[15,35]

1945

58(170.0)

1.21b,c

[15,35]

PPh3

OC

11 H

H B

N N

N N

8

Rh PCy3

OC

12 H 3C

CH 3 O

O

Rh

9

PPh 3

OC

13 H 3C

CH 3 O

O

Rh

10

PCy 3

OC

14 Potentials vs. Fcþ/Fc measured at a scan rate of 0.1 V s1, in 0.1 M [NBu4][PF6]/CH3CN. Only one irreversible oxidation potential was observed. The reported potentials vs. NHE measured at a scan rate of 0.2 V s1, in 0.2 M [NBu4][BF4]/CH3CN or CH2Cl2. The E 0 /V is the first irreversible oxidation potential of those complexes. c E 0 /V vs Fcþ/Fc was obtained by the conversion from the reported potentials vs NHC according to reference [45]. a

b

(dq, 2JaFbF ¼ 85.8 Hz, 1JaFB ¼ 26.1 Hz, Fa), 162.1 (dq, 2JaFbF ¼ 84.5 Hz, 1 b JF B ¼ 25.9 Hz, Fb). FT-IR (KBr): v(CO) 1953 cm1. Anal. Calcd for C33H37BF2N4OPIr: C, 50.97; H, 4.80; N, 7.20; found: C, 50.81; H, 4.83; N, 7.16. 8, 1H NMR (500 MHz, CDCl3, 25  C): 7.11 (s, 1H, ring-CH] CHNtBu), 7.05 (s, 1H, ring-CH]CHNtBu), 6.92 (s, 1H, ring-CH] CHNtBu), 6.79 (s, 1H, ring-CH]CHNtBu), 4.09 (br, 1H, BH), 3.36 (br, 1H, BH), 1.78 (s, 9H, CMe3), 1.61 (s, 9H, CMe3), 1.08e2.25 (m, 33H, Cy). 13C NMR (125 MHz, CDCl3, 25  C): d 196.1 (dd, 1JRheC ¼ 50.0 Hz, 3 JPeC ¼ 17.1 Hz, CO), 184.6 (dd, 1JRheC ¼ 42.8 Hz, 3JPeC ¼ 10.3 Hz, carbene-C), 178.2 (dd, 1JRheC ¼ 94.2 Hz, 3JPeC ¼ 45.0 Hz, carbeneC), 126.2, 124.9 (ring-CH]CHN-tBu), 116.3, 115.5 (ring-CH] CHN-tBu), 56.3, 56.0 (CMe3), 31.1, 30.0 (CMe3), 37.0 (d, 1 JPeC ¼ 16.5 Hz, C1, Cy), 32.1, 32.0, (C2,6, Cy), 28.1, 28.0 (C3,5, Cy), 26.7 (C4, Cy). 11B NMR (160 MHz, CDCl3, 25  C): d 5.91 (BH2). 31P NMR (202 MHz, CDCl3, 25  C): d 36.3 (d, 1JRheP ¼ 124.2 Hz, PCy3). FT-IR (KBr): v(CO) 1946 cm1. Anal. Calcd for C33H57BN4OPRh: C, 59.11; H, 8.57; N, 8.36; found: C, 59.34; H, 8.60; N, 8.30.

9, 1H NMR (500 MHz, CDCl3, 25  C): 7.28 (s, 1H, ring-CH] CHNtBu), 7.27 (s, 1H, ring-CH]CHNtBu), 7.00 (s, 1H, ring-CH] CHNtBu), 6.86 (s, 1H, ring-CH]CHNtBu), 1.79 (s, 9H, C(CH3)3), 1.64 (s, 9H, CMe3), 1.05e2.23 (m, 33H, Cy). 13C NMR (125 MHz, CDCl3, 25  C): d 195.4 (dd, 1JRheC ¼ 62.5 Hz, 3JPeC ¼ 17.8 Hz, CO), 185.7 (d, 1 JRheC ¼ 38.8 Hz, carbene-C), 178.7 (dd, 1JRheC ¼ 79.1 Hz, 3 JPeC ¼ 41.0 Hz, carbene-C), 122.3, 121.2 (ring-CH]CHN-tBu), 117.1, 116.3 (ring-CH]CHN-tBu), 56.6, 56.4 (CMe3), 30.8, 30.1 (CMe3), 36.8 (d, 1JPeC ¼ 16.8 Hz, C1, Cy), 32.0, 31.9 (C2,6, Cy), 28.1, 28.0 (C3,5, Cy), 26.7 (C4, Cy). 11B NMR (160 MHz, CDCl3, 25  C): d 1.92 (BF2). 31P NMR (202 MHz, CDCl3, 25  C): d 34.8 (d, 1JRheP ¼ 125.4 Hz, PCy3). 19F NMR (470.5 MHz, CDCl3, 25  C): 136.2 (dq, 2 ab JFF ¼ 85.2 Hz, 1JaFB ¼ 30.1 Hz, Fa), 160.6 (dq, 2JaFbF ¼ 87.0 Hz, 1 b JF B ¼ 30.1 Hz, Fb). FT-IR (KBr): v(CO) 1952 cm1. Anal. Calcd for C33H37BF2N4OPRh: C, 56.10; H, 7.85; N, 7.93; found: C, 56.23; H, 7.79; N, 7.99. 10, 1H NMR (500 MHz, CDCl3, 25  C): 7.27 (s, 1H, ring-CH] CHNtBu), 7.25 (s, 1H, ring-CH]CHNtBu), 7.06 (s, 1H, ring-CH]

40

F. Chen et al. / Journal of Organometallic Chemistry 710 (2012) 36e43

CHNtBu), 7.00 (s, 1H, ring-CH]CHNtBu), 1.80 (s, 9H, CMe3), 1.69 (s, 9H, CMe3), 1.07e2.64 (m, 33H, Cy). 13C NMR (125 MHz, CDCl3, 25  C): d 187.6 (d, 3JPeC ¼ 12.9 Hz, CO), 182.3 (d, 3JPeC ¼ 6.0 Hz, carbene-C), 174.5 (d, 3JPeC ¼ 99.0 Hz, carbene-C), 121.6, 120.7 (ring-CH]CHN-tBu), 117.4, 116.7 (ring-CH]CHN-tBu), 57.1, 56.9 (C(CH3)3), 32.0, 31.9 (C(CH3)3), 37.5 (d, 1JPeC ¼ 24.0 Hz, C1, Cy), 30.0, 29.7 (C2,6, Cy), 28.2, 28.1 (C3,5, Cy), 26.8 (C4, Cy). 11B NMR (160 MHz, CDCl3, 25  C): d 1.81 (BF2). 31P NMR (202 MHz, CDCl3, 25  C): d 19.40. 19F NMR (470.5 MHz, CDCl3, 25  C): 136.8 (dq, 2 ab JFF ¼ 87.0 Hz, 1JaFB ¼ 28.2 Hz, Fa), 161.4 (dq, 2JaFbF ¼ 88.9 Hz, 1 b JF B ¼ 28.7 Hz, Fb). FT-IR (KBr): v(CO) 1948 cm1. Anal. Calcd for C33H37BF2N4OPIr: C, 49.80; H, 6.97; N, 7.04; found: C, 49.83; H, 6.90; N, 7.10. 2.3. Crystal structure determinations Crystallographic data were collected at 291 K. Diffraction data were collected on a Bruker SMART Apex II CCD diffractometer using graphite-monochromated Mo Ka (l ¼ 0.71073 Å) radiation and collected for absorption using SADABS program [41]. The structures were solved by direct methods and refined on F2 against all reflections by full-matrix least-squares methods with SHELXTL (version 6.10) program [42]. 3. Results and discussion 3.1. Synthesis and characterization Complexes 1, 3 and 4 reacted with PR3 (R ¼ Ph, Cy), and one carbonyl ligand in the complexes was replaced in the reactions to give products [H2B(ImtBu)2]Rh(CO)(PPh3) (5), [F2B(ImtBu)2] Rh(CO)(PPh3) (6), [F2B(ImtBu)2]Ir(CO)(PPh3) (7), [H2B(ImtBu)2] Rh(CO)(PCy3) (8), [F2B(ImtBu)2]Rh(CO)(PCy3) (9), and [F2B(ImtBu)2] Ir(CO)(PCy3) (10) in good yields (>95%) (Scheme 1). However, the reactions of [H2B(ImtBu)2]Ir(CO)2 (2) with the phosphine ligands did not yield the phosphine-substituted product. The TLC test showed that the reaction mixture contained several bands, from which no desired product was isolated.

Complexes 5e10 have been characterized by multinuclear NMR (Table 1). The 1H NMR spectra of complexes 5e10 contain four signals due to the inequivalent protons of the imidazolyl rings, and their resonances are located in 6.58e7.31 ppm. The 11B NMR spectra give a peak at 5.91 to 5.09 ppm. The 31P NMR signal is in the range of 19.4e36.3 ppm. In the 13C NMR spectra of the iridium(I) complexes, the CO ligands were observed at 186.9 ppm (3JPeC ¼ 12.4 Hz) for 7 and 187.6 ppm (3JPeC ¼ 12.9 Hz) for 10. The carbene carbon atoms appear at 180.8 ppm (3JPeC ¼ 5.9 Hz) and 170.9 ppm (3JPeC ¼ 10.1 Hz) for 7 and 182.3 ppm (3JPeC ¼ 6.0 Hz) and 174.5 ppm (3JPeC ¼ 5.9 Hz) for 10. In the 13C NMR spectrum of the Rh(I) complex 5, the CO ligand was observed at 195.4 ppm (1JRheC ¼ 39.0 Hz and 3JPC ¼ 16.8 Hz) and the signals of two carbene carbons appeared at 181.5 ppm (1JRheC ¼ 35.5 Hz and 3 JPeC ¼ 12.8 Hz) and 177.3 ppm (1JRheC ¼ 74.2 Hz and 3 JPeC ¼ 48.8 Hz). The same patterns were also observed in the Rh(I) complexes 6, 8 and 9 (Table 1). The 19F NMR spectra showed two characteristic multiplets at 135.0, 162.5 ppm for 6, 136.4, 162.1 ppm for 7, 136.2, 160.6 ppm for 9, and 136.8, 161.4 ppm for 10, respectively, representing the two magnetically inequivalent fluorine atoms. The splitting pattern of the first multiplet at 135.0 ppm for 6 reflects two coupling interactions. The larger interaction is the geminal 19Fe19F interaction of 2JFeF0 ¼ 86.6 Hz. The smaller 19Fe11B interaction of 1 JFeB ¼ 26.7 Hz gives a characteristic splitting pattern of 1:1:1:1. The second multiplet at 162.5 ppm was also the result of two coupling interactions e A geminal 19Fe19F interaction with 2JFeF’ ¼ 84.5 Hz and a smaller 19Fe11B interaction with 1JFeB ¼ 25.1 Hz. Similar patterns were observed in 7, 9, 10 (Table 1) and 3 and 4 reported in our previous work [40] as well as ruthenium complexes with the difluorobis(pyrazolyl)borate ligand [43]. 3.2. IR,

31

P NMR, and electrochemical properties of the complexes

The n(CO) frequencies in the IR spectra of 5e10 are summarized in Table 2. IR data of the other similar complexes [M(L)(CO)P] with monoanionic bidentate ligands Bp [27] (entries 7 and 8) and acac (entries 9 and 10) are also listed for comparison.

Table 3 Summary of crystallographic data for 6e10. Compound

6

7

8

9

10

Formula Formula weight Crystal system Space group a (Å) b (Å) c (Å) b ( ) V (Å3) Dcalc (g/cm3) Z T (K) Radiation (MoKa) F (000) Absorpt. coefficient (mm1) q range for data collection ( ) Data/restr./paras. Reflections collected Reflections unique Completeness to q ( ) Rint Max. and min. transmission GOF R1/wR2 [I > 2s(I)] R1/wR2 (all data) Largest peak and hole (e/Å3)

C33H37BF2RhN4OP 688.36 Monoclinic P21/c 22.5718(15) 17.237(2) 18.7160(14) 114.493(3) 6626.6(10) 1.380 8 291 0.71073 2832 0.606 1.7 to 26.0 12 986/0/787 12 986 7108 26.0 (99.8%) 0.045 0.902 and 0.877 1.052 0.0549/0.1068 0.0915/0.1115 0.59/0.58

C33H37BF2IrN4OP$CH2Cl2 862.57 Orthorhombic Pbca 17.780(3) 21.550(4) 18.823(3) 90 7212(2) 1.589 8 296 0.71073 3424 3.938 1.8 to 26.0 7082/0/421 37 542 7082 26.0 (100%) 0.180 0.554 and 0.474 0.994 0.0578/0.1179 0.1171/0.1251 3.04/1.83

C33H57BN4OPRh 670.52 Monoclinic P21/c 9.502(2) 30.504(6) 12.299(3) 105.805(3) 3430.1(13) 1.298 4 296 0.71073 1424 0.575 1.9 to 26.0 6740/0/376 18 613 6740 26.00 (99.9%) 0.059 0.891 and 0.859 1.001 0.0446/0.0940 0.0760/0.1024 0.90/0.56

C33H55BF2RhN4OP 706.50 Monoclinic P21/c 9.556(7) 30.56(2) 12.303(9) 105.794(11) 3457(4) 1.357 4 291 0.71073 1488 0.583 1.8 to 26.0 6794/0/394 18 840 6794 26.00 (99.9%) 0.058 0.883 and 0.854 1.075 0.0536/0.1180 0.1008/0.1325 0.87/0.76

C33H55BF2IrN4OP 795.81 Monoclinic P21/c 9.729(4) 30.864(11) 12.389(5) 106.143(5) 3573(2) 1.479 4 291 0.71073 1616 3.822 1.8 to 26.0 6987/0/392 19 024 6987 26.00 (99.8%) 0.037 0.515 and 0.437 0.983 0.0374/0.0943 0.0568/0.1019 0.53/0.88

F. Chen et al. / Journal of Organometallic Chemistry 710 (2012) 36e43

41

Table 4 Selected bond distances (Å) and bond angles ( ) for complexes 6e10. Complex

Bond lengths (Å) MeP1 MeC1 MeC8 MeC15 C15eO1 B1eF1 B1eF2 Bond angles ( ) C1eMeC8 C1eMeC15 C8eMeC15 C1eMeP1 C8eMeP1 C15eMeP1 F1eBeF2

7$CH2Cl2

8

2.3084(14) 2.046(4) 2.067(5) 1.760(6) 1.193(8) 1.363(6) 1.385(8)

2.306(3) 2.062(8) 2.107(9) 1.869(12) 1.120(15) 1.344(14) 1.369(14)

2.3333(12) 2.065(4) 2.124(4) 1.830(4) 1.150(5) e e

2.321(2) 2.083(5) 2.102(5) 1.834(5) 1.142(6) 1.374(6) 1.387(6)

2.3230(17) 2.053(6) 2.134(5) 1.931(6) 1.061(7) 1.366(7) 1.408(7)

81.72(17) 92.3(2) 169.6(3) 164.52(13) 96.71(15) 86.8(2) 109.6(5)

81.8(3) 95.0(4) 176.4(4) 162.6(3) 95.4(3) 88.2(4) 106.9(8)

82.52(14) 89.27(15) 168.62(16) 165.25(11) 97.99(10) 87.95(13) e

82.70(19) 88.5(2) 168.8(2) 165.25(16) 97.79(14) 88.97(16) 110.7(4)

83.2(2) 90.6(2) 171.0(2) 165.42(17) 96.61(14) 87.86(16) 110.5(5)

6 Molecule A

Molecule B

2.3093(13) 2.067(5) 2.050(4) 1.831(5) 1.165(6) 1.368(7) 1.401(7) 81.93(16) 93.83(19) 175.5(2) 163.92(10) 94.09(13) 89.59(16) 110.6(4)

The n(CO) frequencies in 5e10 decrease with an increase in the s/pdonor capacity of the phosphine ligands, i.e., 5 (1954 cm1) > 8 (1946 cm1), 6 (1958 cm1) > 9 (1952 cm1), 7 (1953 cm1) > 10 (1948 cm1). Similar changes in n(CO) frequencies were observed in the IR spectra of [RhBp(CO)(PR3)] [27] and [Rh(acac)(CO)(PR3)] [34]: [RhBp(CO)(PPh3)] (11, 1987 cm1) > [RhBp(CO)(PCy3)] (12, 1966 cm1) and [Rh(acac)(CO)(PPh3)] (13, 1983 cm1) > [Rh(acac)(CO)(PCy3)] (14, 1945 cm1). The lower n(CO) frequencies were found for complexes with the [H2B(ImtBu)2] ligand. For rhodium compounds with the same phosphine ligand, the n(CO) frequencies follow the order 5 < 6, 8 < 9. Even though the electron-donating ability of the fluorinated derivative has been diminished by the fluorine substitution, it is still a stronger donor than monoanionic bidentate Bp and acac ligands: [H2B(ImtBu)2Rh(CO)PPh3] (5, 1954 cm1) < [F2B(ImtBu)2Rh(CO)PPh3] (6, 1958 cm1) < [Rh(acac) (CO)PPh3] (13, 1983 cm1) < [RhBp(CO)PPh3] (11, 1987 cm1). An analysis of the coupling constants JRheP in the 31P NMR spectra leads to the same conclusion that the electronic donor capacity of the chelating bis-carbene ligands is stronger than those of Bp and acac ligands. Following Tolman’s rule for the complexes in which the electronic effect dominates, the JRheP values decrease with a decrease in the n(CO) frequencies [44]. JRheP decreases when the ligand s/p-donor character increases in the order [F2B(ImtBu)2]Rh(CO)(PPh3) (6, 132.7 Hz) > [H2B(ImtBu)2] Rh(CO)(PPh3) (5, 131.1 Hz) and [F2B(ImtBu)2]Rh(CO)(PCy3) (9, 125.4 Hz) > [H2B(ImtBu)2]Rh(CO)(PCy3) (8, 124.2 Hz). Furthermore, the JRheP coupling constants of [H2B(ImtBu)]Rh(CO)(PR3) (5, 131.1 Hz; 8, 124.2 Hz) and [F2B(ImtBu)]Rh(CO)(PR3) (6, 132.7 Hz; 9, 125.4 Hz) are lower than those of [RhBp(CO)(PR3)] (11, 156 Hz; 12, 147 Hz) and [Rh(acac)(CO)(PR3)] (13, 177.4 Hz; 14, 170.0 Hz) (Table 2). The order of the JRheP coupling constants are: [F2B(ImtBu)2]Rh(CO)(PPh3) (6, 132.7 Hz) > [F2B(ImtBu)2] Rh(CO)(PCy3) (9, 125.4 Hz), [H2B(ImtBu)2]Rh(CO)(PPh3) (5, 131.1 Hz) > [H2B(ImtBu)2]Rh(CO)(PCy3) (8, 124.2 Hz). These agree well with the electron-donating capacity of the phosphines, i.e., PCy3 > PPh3. The cyclic voltammograms of 5e10 were run in 0.1 M NBu4PF6/ CH3CN at a scan rate of 100 mV/s (Table 2). Unlike two irreversible oxidation waves observed in [RhBp(CO)(PR3)] and [Rh(acac)(CO)(PR3)] [22,30], only one irreversible oxidation wave was observed in the E0 range of 0.12e0.30 V vs. Fcþ/Fc for 5e10. This is probably due to the loss of CO or phosphine ligands following the oxidation of the metal centers in 5e10. It is clear that the oxidation

9

10

potentials of the F-substituted complexes are higher than the corresponding parent complexes, i.e., 5 (0.14 V) < 6 (0.29 V), 8 (0.12 V) < 9 (0.23 V), indicating that the electron-donating capacity of the fluorinated derivative has been reduced by the fluorine substitution. The order of the measured oxidation potential values of the complexes (5 > 8; 6 > 9; 7 > 10) follows the reverse order of the electron-donor capacity of the phosphine, i.e. PCy3 > PPh3. The oxidation potentials of 5e10 are lower than those of [Rh(acac)(CO)(PR3)] [i.e., 5 (0.14 V) < 6 (0.29 V) < 13 (0.89 V); 8 (0.12 V) < 9 (0.23 V) < 14 (0.81 V)] [35] and [RhBp(CO)(PR3)] [i.e., 5 (0.14 V) < 6 (0.29 V) < 11 (0.40 V)] [27], indicating a much stronger electron-donor character of the carbene ligands than that of acac or Bp. 3.3. Single-crystal X-ray structures of 6e10 Crystals suitable for X-ray diffraction studies were obtained by diffusion of hexane into diethyl ether solutions. The summary of

Fig. 1. Molecular structure of complex 6 showing 30% probability ellipsoids. Hydrogen atoms are omitted for clarity.

42

F. Chen et al. / Journal of Organometallic Chemistry 710 (2012) 36e43

Fig. 2. Molecular structure of complex 7 showing 30% probability ellipsoids. Hydrogen atoms and solvent atoms are omitted for clarity.

crystallographic data and selected bond distances and angles for 6e10 are shown in Tables 3 and 4, respectively. The structures of 6e10 are shown in Figs. 1e5. For 6, the asymmetric unit contains two crystallographically nonequivalent molecules A and B. The conformations of A and B are almost the same, with only small differences in bond distances and bond angles. These complexes show the expected square-planar geometry with slight distortion around the meter center. The metal center is coordinated by the bidentate X2B(ImtBu)2 (X ¼ H, F), one CO, and one phosphine ligand. The values of the P1eMeC1 and C8eMeC15 angles are in the range of 162.6 e175.5 , and deviated from the ideal value of 180 . The macrocyclic six-membered ring adopts a pseudo-twistboat conformation similar to the reported metal complexes of the bis(3-tert-butylimidazol-2-ylidene)borate ligand [36e40].

Fig. 3. Molecular structure of complex 8 showing 30% probability ellipsoids. Hydrogen atoms are omitted for clarity.

Fig. 4. Molecular structure of complex 9 showing 30% probability ellipsoids. Hydrogen atoms are omitted for clarity.

The RheCcarbene bond lengths in 6 [2.050(4), 2.067(5) Å], 8 [2.065(4), 2.124(4) Å], and 9 [2.083(5), 2.102(5) Å] are in the range of the reported Rh(I)eCNHC bonds (1.914e2.102 Å) [36]. The IreCcarbene bond lengths in 7 [2.062(8), 2.107(9) Å] and 10 [2.053(6), 2.134(5) Å] are also typical for IreNHC coordination [36]. The metal-CO distances are 1.800, 1.869, 1.830, 1.834, and 1.937 Å for 6e10, respectively, which are close to those of the dicarbonyl analogs with the same NHC ligands [40]. The metal-P distances of 2.309, 2.306, 2.333, 2.321 and 2.323 Å for 6e10, respectively, are in the range of those reported in [(NHC)Ir(CO)(PPh3)2] (2.321 Å) [46].

Fig. 5. Molecular structure of complex 10 showing 30% probability ellipsoids. Hydrogen atoms are omitted for clarity.

F. Chen et al. / Journal of Organometallic Chemistry 710 (2012) 36e43

The two BeF bond lengths in the four complexes (1.344e1.408 Å) are close to the average BeF bond length (1.38 Å) in similar Fsubstituted borate metal complexes [40,43,47]. The bond angles C1eMeC8 range from 81.80 to 83.20 for 6e10, and they are smaller than those of 1e4 (83.23 e83.79 ). The dihedral angles between two imidazol-2-ylidene rings are in the range of 64.38 e68.80 for 6e10. 4. Conclusions A series of Ir(I) and Rh(I) carbonyl phosphine complexes with bis(N-heterocyclic carbene)borate ligands have been prepared by substituting one carbonyl in the parent dicarbonyl complexes with phosphine ligands (PPh3 or PCy3). The n(CO) frequencies in the IR spectra, values of coupling constant JRheP, and electrochemical data have been rationalized by the electronic effects of the bis(3-tertbutylimidazol-2-ylidene)borate and its fluorinated derivative. In addition, these properties have been compared with those of Bp and acac ligands. The results here furthermore support that the s donor capacity of bis(3-tert-butylimidazol-2-ylidene)borate is stronger than those of isoelectronic bidentate Bp and acac ligands. Acknowledgment We are grateful for the financial support from the Natural Science Grant of China (No. 21071078 to X.-T.C), and the U.S. National Science Foundation (CHE-1012173 to Z.-L.X). Appendix A. Supplementary material CCDC ID: 850122(6), 850121(7), 850123(8), 850124(9), 850125(10) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif or from the Cambridge Crystallographic Data Centre, 12 union road, CambridgeCB2 1EZ, UK; fax: (þ44) 1223336-033; or e-mail: [email protected]. References [1] [2] [3] [4]

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