Comprehensive Organometallic Chemistry III
Elsevier, 2007
Volume 3: Compounds of Groups 13 to 15

3.01 Boron-containing Rings Ligated to Metals, Pages 1-48, R.N. Grimes
3.02 Polyhedral Carboranes, Pages 49-112, M.A. Fox
3.03 s- and p-Block Heteroboranes and Carboranes, Pages 113-131, L.
Wesemann
3.04 d- and f-Block Metallaboranes, Pages 133-174, A.S. Weller
3.05 Metallacarboranes of d- and f-Block Metals, Pages 175-264, N.S. Hosmane
and J.A. Maguire
3.06 Aluminum Organometallics, Pages 265-285, A. Mitra and D.A. Atwood
3.07 Gallium, Indium, and Thallium, Excluding Transition Metal Derivatives,
Pages 287-342, S. Schulz
3.08 d-Block Complexes of Aluminum, Gallium, Indium, and Thallium, Pages
343-407, K.H. Whitmire
3.09 Oligosilanes, Pages 409-512, J. Beckmann
3.10 Compounds with Bonds between Silicon and d-Block Metal Atoms, Pages
513-547, Catherine E. Housecroft
3.11 Organopolysilanes, Pages 549-649, J.R. Koe
3.12 Silicones, Pages 651-697, M.H. Mazurek
3.13 Germanium Organometallics, Pages 699-808, C.S. Weinert
3.14 Tin Organometallics, Pages 809-883, A.G. Davies
3.15 Lead Organometallics, Pages 885-903, M. Weidenbruch
3.16 Arsenic, Antimony, and Bismuth Organometallics, Pages 905-929, H.J.
Breunig and R. Wagner


www.pdfgrip.com

3.01

Boron-containing Rings Ligated to Metals
R N Grimes, University of Virginia, Charlottesville, VA, USA
ª 2007 Elsevier Ltd. All rights reserved.
3.01.1

Background

2

3.01.2

Complexes of C4B Rings

3

3.01.2.1 Mononuclear and Dinuclear Complexes of C4B Rings (Double-Decker
and Triple-Decker Sandwiches)

3

3.01.2.2 Multinuclear C4B Ring Complexes, Clusters, and Extended Systems

8

3.01.3

9

Complexes of C3B2 (Diborolyl) Rings


3.01.3.1 C3B2 Ring Double-decker Sandwiches

9

3.01.3.2 C3B2 Ring Triple-decker Sandwiches and Dimers

13

3.01.3.3 C3B2 Ring Tetradecker and Pentadecker Sandwiches

14

3.01.3.4 Polydecker C3B2 Ring Sandwiches

15

3.01.4

15

Complexes of C2B3 (Triboracyclopentadienyl [Carborane]) Rings

3.01.4.1 C2B3 Ring Double-decker Sandwiches

19

3.01.4.2 C2B3 Ring Triple-decker Sandwiches

24


3.01.4.3 C2B3 Ring Tetradecker Sandwiches

26

3.01.4.4 C2B3 Ring Pentadecker and Hexadecker Sandwiches

27

3.01.4.5 C2B3 Ring Multisandwich Assemblies

28

3.01.5

29

Complexes of C5B (Borabenzene and Boratabenzene) Rings

3.01.5.1 C5B Ring Double-decker Sandwiches

33

3.01.5.2 C5B Ring Dinuclear Complexes (Triple-decker Sandwiches and Dimers)

36

3.01.6

Complexes of C4B2 Rings


37

3.01.6.1 Diboracyclohexadiene Complexes

39

3.01.6.2 Diboranaphthalene and Related Complexes

39

3.01.7

40

Complexes of C3B3 and C2B4 Rings

3.01.7.1 Triboratabenzene Complexes

40

3.01.7.2 Tetraboratabenzene Complexes

40

3.01.8

41

Complexes of C6B, C5B2, and C7B Rings


3.01.8.1 Borepine Complexes

41

3.01.8.2 4-Borataborepine Complexes

41

3.01.8.3 Boratacyclooctatetraene Complexes

41

3.01.9

Complexes of Heterocyclic Ligands

42

3.01.9.1 Complexes of Nitrogen-containing Rings

43

3.01.9.2 Complexes of Phosphorus-containing Rings

43

3.01.9.3 Complexes of Oxygen-containing Rings

44


3.01.9.4 Complexes of Sulfur-containing Rings

44

References

45

1


www.pdfgrip.com
2

Boron-containing Rings Ligated to Metals

3.01.1 Background
Metal complexes of boron ring systems form an extensive area of research that overlaps with and connects three large
fields: organometallic chemistry, organic heterocycles, and the metallacarboranes. This area of chemistry was already
well developed prior to 1993, as summarized by Herberich in COMC (1995);1 since then it has been extensively
investigated and expanded with the discovery of new ring ligands and synthetic routes, novel molecular architectures,
and applications in synthesis, especially catalysis.2 In general, the most important advances during this period fall into
one or more of these categories, and encompass a variety of five- to eight-membered ring systems that are coordinated
to a range of main group and transition metals. Figure 1 presents a selection of the more important ring ligands,
represented as neutral and/or anionic species. This chapter is concerned with metal complexes, and will not, in

Five-membered ring ligands

R
B


B

2–

B

R
1H-Borole
4π

Borollide
6π

R

B

B

R

B

R

B

4–


R

R

1,3-Diborolenyl
3π

1,2,3-Triboracyclopentadienide
6π

4–

R

–

B

B
B

B

R

R
N

B
R


N

R
1,2-Azaborolinyl
5π

1,2,4-Triboracyclopentadienide
6π

3a,7a-Azaborindenyl
6π
–

–

–
B

R

B

R

B

O

R


S

S
1,2-Thiaborollide
6π

1,2-Oxaborollide
6π

1,2-Benzothiaborollide
6π

Six-membered ring ligands
R
–
B

B

R

B

Boratabenzene
6π

Borabenzene
5π


B

B

1,2-Diborabenzene
4π

B

B

N

1,2-Dihydroazaborine
6π

1,4-Diborabenzene
4π

3–

R

–
4–

B
B

R


R

B

B
R
1,3,5-Triboratabenzene
6π

B
B

R

R

B

B

R

R

1,2,3,4-Tetraboratabenzene
6π

1-Boratanaphthalene
6π


Figure 1 Ring ligands. Reproduced with permission from the American Chemical Society.

R


www.pdfgrip.com
Boron-containing Rings Ligated to Metals

general, cover the synthesis and nonmetal chemistry of the ligands per se; in most cases, references dealing with those
aspects can be found in the references cited herein.

3.01.2 Complexes of C4B Rings
3.01.2.1 Mononuclear and Dinuclear Complexes of C4B Rings (Double-Decker
and Triple-Decker Sandwiches)
The chemistry of metal–borole and metal–borollide sandwich compounds has been significantly advanced since the
publication of COMC (1995) (see Table 1). Herberich and his group have prepared a variety of novel complexes of
planar C4B ligands including (i) multinuclear heterometallic systems containing Re–Hg, Re–Cu, Re–Ag, Re–Au, and
Fe–Pt bonds;3 (ii) CN-bridged chains;4 (iii) Fe–Au heterometallic complexes;5 (iv) intramolecular Fe–H–B
hydrogen-bonded species that reversibly rearrange to Fe–H–C systems;6 and (v) several types of triple-decker
sandwich complexes involving Co, Rh, Mn, Ru, and Li.7–12 Compounds 1–5 are representative.

Table 1 Complexes of C4B rings 1993–2005
Compound a
Synthesis and characterization
Mononuclear C4B ring complexes (double-decker sandwiches)
Lithium
[(Et2O)Li]2[H6(C6H12–C6H4)(CMe3)C12B–C6H3(C6H12–C6H4)]
9-borafluorene complex
Tantalum

Cp*R2Ta[H4C4B–N(CHMe2)2] (R ¼ Me, MeCTNAr, H2PMe3, Cl2,
ClMe)
(Me3P)Me3Ta[H4C4B–N(CHMe2)2]
[Z5-H4C4B–N(CHMe2)2]Ta[Z2-MeCN(C6H3Me2)(Z3-C6H3Me2)NCHCH2C(Me)N(C6H3Me2)]
Cp*Me2Ta(H4C4B–Me)
Cp*Me2Ta[H4C4B–N(CHMe2)2]
Cp*(PMe3)2Ta(H4C4B–R) [R ¼ N(CHMe2)2, Me]
Cp*(PMe3)H2Ta(H4C4B–R) [R ¼ N(CHMe2)2, Me]
Cl3Ta[H4C4B–N(CHMe2)2]
[H4C4B–N(CHMe2)2]TaCl2[C(SiMe3)2H]
Cl2(R2C6H3–NT)Ta[H4C4B–N(CHMe2)H]
Cl3(OCMe2CH2CMeTO)Ta[H4C4B–N(CHMe2)H]
Other Ta(H4C4B–R) complexes
Zirconium and Hafnium
Cp*M[Me4C4B–N(CHMe2)2](m-Cl)2Li(OEt2)2 (M ¼ Zr, Hf)
Cp*M[Me4C4B–NH(CHMe2)2](Cl)2 (M ¼ Zr, Hf)
Cp*ClLM[H5C5B–N(CHMe2)2] (M ¼ Zr, Hf; L ¼ RO, RS;
R ¼ Me,C6H2Me3)
Cp*Cl(Me3SiCUC)Hf[H5C5B–N(CHMe2)2]
Cp*(Me3SiCUC)2Hf[H5C5B–N(CHMe2)2]
Cp*ClLM[H5C5B–N(CHMe2)2] (M ¼ Zr, Hf; L ¼ NMe2H, PMe3)
Cp*(Me3P)ClHf[H5C5B–N(CHMe2)2]
Cp*R2Zr[H4C4B–N(CHMe2)2] (R ¼ Me, CUC–C6H4Me, CUC–CMe3,
CH2Ph)
Cp*IZr[H4C4B–N(CHMe2)2]
Cp*(C3H7)M[H4C4B–N(CHMe2)2] (M ¼ Zr, Hf)
Cp*(C3H7)(L)Hf[H4C4B–N(CHMe2)2] (L ¼ CO, PMe3, NC5H5)
Cp*(PMe3)(H)Hf[H4C4B–N(CHMe2)2]
[1,3-C5H3(SiMe3)2](Et2O)(C6F5)Zr(H4C4B–R) (R ¼ H, Me)


Informationb

References

S, X, H, B, C

19

S, X

13

S, H, C
S, X, H, C

21
21

S, X
S
S, X(Me)
S
S, X, H, C, IR
S, X, H, C, IR
S, X, H, C, IR
S, X, H, C, IR

15
15
15

15
16
16
16
16
16

S, X(Zr)
S, X(Zr)
S, H, C, IR

18
18
18a

S, X, H, C, IR
S, X, H, C, IR
S, X(PMe3), H, C, IR
S, X, H, C, IR
S, H, C

18a
18a
18a
18a
17

S,
S,
S,

S,
S,

17
17
17
17
14a

H, C
X(Hf), H, C, UV
X(CO), H, C, I
H
X, H, F, ethylene
polymerization catalysis

(Continued)

3


www.pdfgrip.com
4

Boron-containing Rings Ligated to Metals

Table 1 (Continued)
Compound a
[1,3-C5H3(SiMe3)2](R9CN)(C6F5)Zr(3-RH3C4B–C6F5) (R ¼ H, Me;
R9 ¼ Me, CMe3)

[1,3-C5H3(SiMe3)2](Me3CNC)2(C6F5)Zr(3-RH3C4B–C6F5) (R ¼ H, Me)
[1,3-C5H3(SiMe3)2](Me3CNC)(C6F5–CTN–CMe3)Zr(3-RH3C4B–C6F5)
(R ¼ H, Me)
Rhenium
(CO)3(H4C4B–Ph)ReÀHgCl
(CO)3(H4C4B–Ph)ReÀL [L ¼ Cu(PPh3)2, AgPPh3, AuPPh3]

Informationb

References

S, X(H, CMe3), H, B, C, F

14

S, H, B, C, F
S, X(H), H, B, C, F

14
14

S, H, C, IR, MS
S, H, C, IR, MS

3
3

S, H, B, C, IR, MS
S, E, ESR
S, H, B, C, IR, MS

S, H, B, C, IR, MS
S, H, B, C, IR, MS
S, X(H, Me, Me; Me, Me, H), H,
B, C, IR, E, MS
S, X, H, C, P, Pt, IR, UV
S, X, H, B, C, P, IR
S, X (wide-angle scattering), H,
B, C, P, IR
S, H, P, IR

6
6
6
6
6
6

Cobalt
CpCo(H4C4BR) (R ¼ H, Me)

He photoelectron (Me)

7

Rhodium
CpRh(H4C4BR) (R ẳ H, Me)
(H4C4BR)RhI(H4C4BR) (R ẳ Ph, Me)
py2IRh(H4C4BPh)
(bpy)IRh(H4C4BPh)
(MeCN)2IRh(H4C4BPh)

(CO)2IRh(H4C4BPh)
(Ph3P)2IRh(H4C4BPh)
[Fe(C5H4PPh2)2]IRh(H4C4BPh)
(norbornadiene)IRh(H4C4BPh)
(MeCN)3Rh(H4C4BPh)ỵ BF4
(py)3Rh(H4C4BPh)ỵ BF4
(C6Me3R3)3Rh(H4C4BPh)ỵ BF4 (R ẳ H, Me)

He photoelectron (Me)
S, H, B, C, MS
S, H, B, C, IR
S, H, B, C, IR, MS
S, H, B, C, IR, MS
S, H, B, C, IR, MS
S, H, B, C, IR, MS
S, H, B, C, IR, MS
S, H, B, C, IR, MS
S, H, B, C, MS
S, H, B, C, MS
S, H, B, C, MS

7
24
8
8
8
8
8
8
8

9
9
9

S, X, H, B, C

10

S, X, H, B, C, Li

11

S, H, B, C, F, ethylene
polymerization catalysis
X

14, 22
22

S, X, H, C
S, H, C, IR

21
16

Manganese
(CO)3Mn(H4C4BMe)Mn(CO)3
(CO)3Mn(H4C4BMe)Co(H4C4BMe)

S, He photoelectron

S, He photoelectron

12
12

Ruthenium
Cp*Ru(Me4C4B-6-CHMeCH2Me)RuCp*

S, X, H, B, IR, MS

23

Iron
(CO)3Fe(Me2H2C4B–Ph)
CpFe(H4C4B–Ph)
(Me3CNC)3Fe(H4C4B–Ph)
(Z4-C5H6)(CO)Fe(Me2H2C4B–Ph)
(Z4-C5H6)(Me3CNC)Fe(Me2H2C4B–Ph)
(Z5-C5H4R0)Fe(R2R92C4B–Ph) (R0 ¼ H, Me; R9 ¼ H, Me,
CH2TCH(CH2)4, (CH2)2CHCH2; R ẳ H, Me)
(H4C4BPh)Fe(CO)2CNPt(CN)(PEt3)2 (four isomers)
[(Ph3P)2Au2](CO)2Fe(H4C4BPh)
{[(Ph3P)3Au3](CO)2Fe(H4C4BPh)}ỵ PF6
(Ph3P)H(CO)2Fe(H4C4BPh)

Dinuclear C4B complexes (triple-decker sandwiches and dimers)
Lithium
[(Me2N)2C2H4]Li[(C4H4)H2C4B–NR2]Li[(C4H4)H2C4B–NR2]
(R ¼ Me, Et)
(tmeda)Li(2,5-Ph2H2C4B–NMe2)Li(tmeda)

Zirconium
Cp9(C6F5)2Zr[H4C4B–CH2(CH)3B(C6F5)3]ZrCp9 (Zr–F)
[Cp9 ¼ C5H4SiMe3, C5H4Me, C5H5, 1,3-C5H3(SiMe3)2]
[1,3-C5H3(SiMe3)2](C6F5)2Zr[H4C4B–CH2(CH)3B(C6F5)3]
Zr[1,3-C5H3(SiMe3)2] (Zr–F)
Tantalum
Me4Ta[H4C4B–N(CHMe2)]TaMe2[H4C4B–N(CHMe2)]

4
5
5
5

(Continued)


www.pdfgrip.com
Boron-containing Rings Ligated to Metals

Table 1 (Continued)
Compound a

Informationb

References

Cobalt
(H4C4BMe)Co(H4C4BMe)Co(H4C4BMe)
(CO)3Mn(H4C4BMe)Co(H4C4BMe)


He photoelectron (Me)
S, He photoelectron

7
12

Rhodium
(H4C4BMe)Rh(H4C4BMe)Rh(H4C4BMe)
(H4C4BR)Rh(H4C4BR)Rh(H4C4BR) (R ¼ Ph, Me)
(m-Ph2PCH2PPh2)I2Rh(H4C4B–Ph)Rh(H4C4B–Ph)
Cp*Rh(m-I)3Rh(H4C4B–Ph)

He photoelectron (Me)
reaction with I2 ! cubane clusters
S, H, B, C, IR, MS
S, X, H, B, C, IR, MS

7
24
8
8

Multinuclear complexes and extended systems
[(H4C4B–Ph)(CO)3Re]2Pd2
[(CO)3(H4C4B–Ph)Re]2Hg
trans-(NC5H4Me)2Pt-[Fe(CO)2H(H4C4B–Ph)]2
Rh4(H4C4BR)4(m-I)4 (R ¼ Ph, Me)
[pyIRh(H4C4BPh)]2 (cis ỵ trans isomers)
[(CO)IRh(H4C4BPh)]2 (cis ỵ trans isomers)
[Rh(H4C4BPh)ỵ BF4À]x


S,
S,
S,
S,
S,
S,
S,

X, H, IR, MS
X, H, C, IR, MS
X, H, IR, MS
H, B, C, MS
H, B, C, IR, MS
H, B, C, IR, MS
H, B, C, wide-angle X-ray
scattering
S, X, H, B, C

25
3
3
24
8
8
9

INDO (Me); ab initio
INDO (Me); ab initio
INDO (Me); ab initio (H)

INDO (Me); ab initio (H)
INDO (Me); ab initio
INDO (Me); ab initio

12
12
7
7
7
7

(H4C4B–Ph)Rh(m-I)3Rh(H4C4B–Ph)Rh(H4C4B–Ph)
Theoretical studies
Molecular and electronic structure calculations
(CO)3Mn(H4C4BMe)Mn(CO)3
(CO)3Mn(H4C4BMe)Co(H4C4BMe)
CpCo(H4C4BR) (R ¼ H, Me)
CpRh(H4C4BR) (R ¼ H, Me)
(H4C4BMe)Co(H4C4BMe)Co(H4C4BMe)
(H4C4BMe)Rh(H4C4BMe)Rh(H4C4BMe)

9

a

Metals coordinated to C4B rings are shown in boldface.
Legend: S ¼ synthesis, X ¼ X-ray diffraction, H ¼ 1H NMR, B ¼ 11B NMR, C ¼ 13C NMR, F ¼ 19F NMR, P ¼ 31P NMR, Li ¼ 7Li
NMR, Pt ¼ 195Pt NMR, IR ¼ infrared data, MS ¼ mass spectroscopic data, UV ¼ UV–visible data, E ¼ electrochemical data,
ESR ¼ electron spin resonance data.
b


O
C

Ph

B

Re
OC

Hg

Re

Fe

N

O
C

PEt3

Pt

C

Fe


N

C

OC

C
O

C
O

Ph

B

O
C

OC

C
O

B

Ph

PEt3


13

24

+

R1

Ph

B

Au

OC
C
O

R2

PPh3

B
R2

Fe H

Au

Au


PPh3

PPh3

R1, R2 = H, Me

35

46

B

R

B

R

B

R

Rh

R1

Fe

B


Ph

Ph

Rh

R = H, Me

57

O
C

5


www.pdfgrip.com
6

Boron-containing Rings Ligated to Metals

The use of metal–borollide complexes as homogeneous Ziegler–Natta catalysts for ethylene polymerization has
been explored by Bazan and co-workers13 and by Bochmann et al.14,14a Thus, Cp*Me2Ta[H4C4B–N(CHMe2)2] 6
mimics zirconocene in promoting the polymerization of C2H4 at 1 atm and 25  C in the presence of methylaluminoxane (MAO).13 The mononuclear and dinuclear dizirconium complexes 7 and 8 similarly polymerize ethylene when
used with AlMe3 or MAO.14,14a
F

C6F5


F

B

C6F5

F
F
B

TaV Me

N

CHMe2

R

B

R, R′ = H, Me, SiMe3

Zr

C6F5

B

SiMe3


CHMe2
C6F5

Me

R

Zr
C6F5

R = H, Me

Et2O

Zr
C6F5

SiMe3

6

R′

7

8

Aspects of borole complex reactivity have been studied in detail, including the behavior of tantalum sandwiches
bearing alkyl ligands on the metal.15–17 Complexes such as 6 are best regarded as resonance hybrids where strong
B–N -overlap lowers the formal oxidation state of the metal:15,16


N

B

CHMe2
B

+
N

CHMe2

CHMe2

TaV

Me

CHMe2

TaIII Me
Me

Me
Cp*

Cp*

6

Aminoborollide complexes of zirconium and hafnium that incorporate both Lewis acid and Lewis base sites have
been explored.18,18a In molecules such as 9, the amine nitrogen carries an electron lone pair while the d 0 metal center
is acidic:
R
B

M

N:
R
Cl
Cl

OEt2
Li
OEt2

M = Zr, Hf; R = CHMe2

9
Amphoteric molecules of this type, where the acidic and basic sites are relatively close to each other but cannot
interact directly, can heterolytically cleave H–X and C–X bonds where X is a halide, alkoxide, amide, alcohol, thiol,
trimethylsilyl, or alkyl group.18,18a The ability to effect changes in the reactivity of borollide complexes by adjusting
metal oxidation states and ligands allows fine-tuning of catalytic and other properties, which in turn advances the
application of these compounds in synthesis.


www.pdfgrip.com
Boron-containing Rings Ligated to Metals


A tricyclic aromatic system closely related to borole, 9-boratafluorene, might be expected to form Z5-coordinated transition metal sandwich complexes, but so far only the lithium complex has been characterized.19
Monohapto aluminum adducts of neutral 9-borafluorene, in which Al is bound only to the boron atom, have
been prepared.20
B

2–

9-boratafluoreneide

Triple-decker complexes incorporating borole or borollide ring ligands, a well-developed area prior to 1993,
have been explored further in recent years (e.g., 5 and 10–12);10–12,14,21 in most cases, the structures shown have
been established crystallographically. Benzoborollide [H4C6(CH)2BR]2 rings coordinate (tmeda)Liỵ units to
both faces of the five-membered C4B ring to form triple-decker structures.10,11 Treatment of Li2[C4H4B–
N(CHMe2)2] with Me3TaCl2 afforded the extremely electron-deficient (24 electron (24e)) triple-decker 10,21
while the curious dizirconium complexes 8 were generated from (C5H3RR9)Zr(Z3-C4H7)(Z4-C4H6) and B(C6F5)3, a
process in which all three C6F5 ligands were transferred to the metal reagent.14,22 In combination with MAO,
compounds of type 8 (R ¼ H, R9 ¼ H, Me, or SiMe3; R, R9 ¼ SiMe3) catalyze the polymerization of ethylene at
60  C and 1 atm.
CHMe2

Me2HC
N

B

B

Me

OC


Me
B

N

B

CHMe2
CHMe2

Me

Ta
Me

Me

B

Me

Mn
C
O

Me

C
O


1016, 21

O
C

Mn

Co
Ta

Me

O
C

Me

Mn
C
O

1112

C
O

C
O


C
O

1212

The electronic structures of the hetero- and homobimetallic triple-deckers 11 and 12 were investigated by
photoelectron spectroscopy and intermediate neglect of differential overlap (INDO) molecular orbital (MO) calculations, which revealed that the metal–borole interaction is stronger in the dimanganese system 12 than in 11, owing to
better Mn–borole versus Co–borole orbital overlap.12
While metal–C4B ring complexes are usually synthesized from existing borole or borollide ligands, the diruthenium
triple-decker 13 (which can also be described as an Ru2C4B seven-vertex closo-metallacarborane) was serendipitously
obtained as a minor product (6% yield) of the reaction of the ruthenaborane nido-1,2-Cp*2H2Ru2B3H7 with 2-butyne
at 85  C;23 the remaining products were open-cage nido-Ru2C2B2 clusters.

Ru
B
Ru

13

7


www.pdfgrip.com
8

Boron-containing Rings Ligated to Metals

3.01.2.2 Multinuclear C4B Ring Complexes, Clusters, and Extended Systems
Dirhodium triple-decker sandwiches such as 5 react with I2 to generate Rh4I4 heterocubane clusters, for example, 14,
together with bis(borole)iodorhodium complexes, 15;8,24 the latter species undergo rapid ring rotation.24 As shown by

Herberich, the heterocubane clusters exhibit interesting chemical reactivity, for example, exchanging RhI(C4H4B–R)
fragments rapidly on the NMR timescale. Treatment of 14 (R ¼ Ph) with Lewis bases L (L ¼ pyridine, CO,
phosphines, etc.) afforded mononuclear or dinuclear products, for example, 16 and 17;8 the dinuclear species
undergoes cis–trans interconversion at a rate dependent on the Lewis base.

I

B
R

B

Rh

Rh

R

I

R
Rh

R = Me, Ph

I

I

Rh

I

B

R

B
R

R

14

R

B

Rh

B

15

B
B

B
R

Rh

I

I

L

I

R

Rh

Rh

L

L

B

R

Rh

I
L

L

L


16

I

Rh
B

R

17

Reaction of the heterocubane cluster 14 (R ¼ Ph) with (Cp*RhI2)2, a reagent that functions both as a Lewis acid
(the metal center) and a Lewis base (the iodide ligands), generates the dirhodium complex 18.8 Complex 14 also adds
to 1,1-bis(diphenylphosphinoferrocene) to form the cyclic species 19.8

Ph

B

Rh

Rh

I
I

Ph2P

PPh2


I

Rh

Fe
B

18

19

In yet another facet of heterocubane chemistry, 14 (R ¼ Ph) has been found to react with Agỵ in acetonitrile to form
the salt (MeCN)3Rh(C4H4B-Ph)ỵ BF4 20, which in turn combines with arenes to generate (arene)Rh(C4H4B-Ph)ỵ
BF4 products.9 Under vacuum, 20 loses MeCN to form the polymer 21, the phenyl group migrating to the Rh center of
a different fragment.


www.pdfgrip.com
Boron-containing Rings Ligated to Metals

+
Ph

B

+
B

Rh

C
Me

N

Rh

N
C
Me

N
C
Me

BF4–

B

BF4–
20

x

21

The first mixed metal borole cluster 22, which contains a planar Re2Pd2 array, was prepared via reaction of the
[(Z5-H4C4B–Ph)Re(CO)3]À anion with PdCl2(NCPh)2.25 In this electron-deficient system, the Pd atoms are formally
14e centers, stabilized by metal–metal binding with each other and with the Re atoms.
B


Re

OC

OC

Pd
CO

Pd

C
O

CO

Re

CO

B

22

3.01.3 Complexes of C3B2 (Diborolyl) Rings
3.01.3.1 C3B2 Ring Double-decker Sandwiches
The pioneering work of Siebert and co-workers on metal–C3B2 sandwich complexes, dating back to the 1960s, has
continued through the 1990s and has opened up significant new aspects of this chemistry (see Table 2).26,27 One
contribution is the finding that the shape of the diborolyl ligand, normally planar, can be altered by manipulation of

Table 2 Complexes of C3B2 (1,3-diborolenyl) rings 1993–2005
Compound a
Synthesis and characterization
Mononuclear C3B2 complexes (double-decker sandwiches)
Iron
Cp*Fe[(CHMe2)2MeC3B2Et2]
CpFe(R2MeC3B2Et2) (R ¼ Et, CHMe2)
CpFe[(CHMe2)2MeC3B2Et2]À
Cp*Ru(R2MeC3B2Et2)
[Cp*Ru(R2MeC3B2Et2)]À (R ¼ Et, CHMe2)
Cp*Ru[Et2MeC3B2(CMe3)2]
Cp*(CO)Ru(Et2MeC3B2EtR) (R ¼ Et, CMe3)
Ruthenium
Cp*(Me3CNC)Ru(R2R9C3B2R02) (R, R9, R0 ¼ Me, Et, Bu,
CH2SiMe3)
Cp*H2Ru(R2R9C3B2R02) (R, R9, R0 ¼ Me, Et, Bu, CH2SiMe3)
Cp*Ru(R2R9C3B2R02) (R, R9, R0 ¼ Me, Et, Bu, CH2SiMe3)
Cp*Ru(TS)(Me3C3B2Me2)

Informationb

References

S, X, H, B, C, MS
S, X(CHMe2), H, B(CHMe2), C(CHMe2),
E(CHMe2), MS
ESR
S, H, B, C, MS
ESR
S, H, B, C, E, MS

S, X(CMe3), H, B, C, MS

28
29
29
29
29
29
29

S, X(Et, Et, Me; Et, Me, Me), H, B, C, MS

30

S, H, B, C, MS
S, H, B, C, MS
S, H, B, C, MS

30
30
36
(Continued)

9


www.pdfgrip.com
10

Boron-containing Rings Ligated to Metals


Table 2 (Continued)
Compound a

Informationb

References

S, H, B, C, MS
S, H, B, C, MS

36
36

S, H, B, C, MS
S, X, H, B, C, MS
S, X, H, B, C, MS

37
31
31

S, X(Me, Me, Me, mesCH2; Me, Et, Me,
Me; Me, Me, CMe3, H; H, Et, Me, Me),
H, B, C, MS

32

Nickel
(C8H12)Ni[cyclo-(CH2)6MeHC3B2Me2]

CpNi[cyclo-(CH2)6MeHC3B2Me2]
(C8H12)Ni[Et2(Me2CH)C3B2Me2]
(C8H12)Ni[(H4C4)Ni(C8H12)(R2CH)C3B2Me2] (R ¼ Me, (CH2)2)

S, X, H, B, C, MS
S, X, H, B, C, MS
S, H, B, C, MS
S, X[CH2)2], H, B, C, MS

31
31
33
33

Platinum
(C8H12)Pt[(H4C4)Pt(C8H12)(R2CH)C3B2Me2] (R ¼ Me, (CH2)2)

S, X(Me), H, B, C, MS

33

Dinuclear C3B2 complexes (triple-decker sandwiches and related clusters)
Iron
CpCo(MeEt2C3B2Et2)Fe(5-Me-2,3,5-C3B7H9)

S, X, H, B, MS

41

Ruthenium

Cp*2Ru2H(R2MeC3B2Me2) (R ¼ Me, Et)
Cp*Ru(Me3C3B2Me2)Rh(Et2C2B4H4)
Cp*Ru(Me3C3B2Me2)Rh(C2B9H11)
Cp*Ru(Me3C3B2Me2)Rh(MeC3B7H9) (Two isomers)
Cp*2Ru2(Me3C3B2Me2)H (adjacent Ru centers)

S, H, B, C, MS
S, H, B, C, MS
S, H, B, MS
S, X, H, B, C, MS
S, X, H, B, C, MS

29
39
39
39
36

S, H, B, MS
S, X(1,10), H, B, C, MS
S, X, H, B, E, MS
S, X, MS
S, H, E, MS

40
40
38
41
37


S, X(MeCTCH2, CHMe2), H, B, C, MS

37

S, X(CHMe2, CMe2C5H5CoCp), H, E, MS

37

S, H, B, C, MS
S, X, H, MS
S, H, B, C, MS
S, H, B, C, MS
S, H, B, MS
S, H, B, MS
S, X, H, B, C, MS
S, X, H, B, MS
S, H, B, MS
S, X, H, B, MS
S, X, H, E, MS

37
37
37
42
42
42
42
41
40
41

38

S, MS

32

*

Cp Ru(PR2R9)(Me3C3B2Me2) (R ¼ H, Me; R9 ¼ H, Ph, Me)
Cp*Ru(Me3C3B2Me2)2(CS2)2
Cobalt
Cp*Co(MeEt2HC3B2Et2)
CpCo[cyclo-(CH2)6MeHC3B2Me2]
(CO)3Co[cyclo-(CH2)6MeC3B2Me2]
Rhodium
(C6H5R)Rh(R92R0C3B2R09) (R ¼ Me, H; R9 ¼ Me, Et; R0 ¼ Me,
CMe3; R09 ¼ mesCH2, Me, H)

Cobalt
CpCo(MeEt2C3B2Et2)Co(C2B5H7)
CpCo(MeEt2C3B2Et2)Co(C2B7H9) (Three isomers)
CpCo(MeEt2C3B2Et2)Co(C2B9H11)
CpCo(MeEt2C3B2Et2)Co(5-Me-2,3,5-C3B7H9)
CpCo[(CH2TCMe)(CHMe2)RC3B2Me2]CoCp (R ẳ Me,
MeCTCH2, CHMe2)
{CpCo[(CH2TCMe)(CHMe2)RC3B2Me2]CoCp}ỵ (R ẳ Me,
MeCTCH2, CHMe2)
CpCo[(H4C4)RC3B2Me2]CoCp (R ẳ Me, MeCTCH2, CHMe2,
CMe2C5H5CoCp)
{CpCo[(H4C4)(CHMe2)C3B2Me2]CoCp}ỵ

CpCo[(H4C4)RC3B2Me2]CoCp (R ẳ cyclo-C5H7, cyclo-C5H9)
{CpCo[(H4C4)RC3B2Me2]CoCp}ỵ (R ẳ cyclo-C5H7, cyclo-C5H9)
CpCo(MeEt2C3B2Et2)Co(B9H13)
CpCo(MeEt2C3B2Et2)Co(SB9H9)
CpCo(MeEt2C3B2Et2)Co(S2B9H9)
CpCo(MeEt2C3B2Et2)Co(S2B6H8)
CpCo(MeEt2C3B2Et2)Fe(5-Me-2,3,5-C3B7H9)
CpCo(MeEt2C3B2Et2)Rh(C2B7H9)
CpCo(MeEt2C3B2Et2)Ni(8-Me-2,3,5-C3B7H9)
CpCo(MeEt2C3B2Et2)Ni(C2B9H11)
Rhodium
(Me2RC3B2Me2)Rh(Me2RC3B2Me2)Rh(C6H5Me) (R ¼ Me,
mesCH2)
CpCo(MeEt2C3B2Et2)Rh(C2B7H9)
Cp*Ru(Me3C3B2Me2)Rh(Et2C2B4H4)

S, H, B, MS
S, H, B, C, MS

40
39
(Continued)


www.pdfgrip.com
Boron-containing Rings Ligated to Metals

Table 2 (Continued)
Compound a


Informationb

References

S, H, B, MS
S, X, H, B, C, MS

39
39

S, H, B, C, MS

44

S, X, H, E, MS
S, X, H, B, MS

38
41

S, H, E, MS

38

S, MS

32

Nickel
[CpCo(MeEt2C3B2Et2)]2Ni

{CpNi[(H4C4)MeC3B2Me2]}2Ni
[CpNi(HEt2HC3B2R2)]2Ni (R ¼ H, Me)

S, H, E, MS
S, X, H
S, X(H), H, B, IR(H), MS(H)

38
43
43

Tetranuclear C3B2 complexes (pentadecker sandwiches)
[Cp*Ru(Me3C3B2Me2)RuCl]2

S, H, B, C, MS

30, 39

S, EXAFS, thermogravimetry, differential
calorimetry, electrical conductivity

44

Theoretical studies
Molecular and electronic structure calculations
Triple-decker sandwiches
CpFe(C3B2H5)
CpNi(H3C3B2H2)NiCp

Extended Huăckel

FenskeHall

29
43

Tetradecker sandwiches
[CpNi(H3C3B2H2)]2Ni

FenskeHall

43

*

Cp Ru(Me3C3B2Me2)Rh(C2B9H11)
Cp*Ru(Me3C3B2Me2)Rh(MeC3B7H9) (Two isomers)
Nickel
(C3H5)Ni(RR9R0C3B2Me2)Ni(C6H10) (C6H10 ¼ 1,5-hexadiene;
R ¼ Me, Bu, Hep; R9 ¼ Me, n-C3H7, n-C4H9; R0 ¼ H, Me,
Hep, iHex)
CpCo(MeEt2C3B2Et2)Ni(C2B9H11)
CpCo(MeEt2C3B2Et2)Ni(8-Me-2,3,5-C3B7H9)
Trinuclear C3B2 complexes (tetradecker sandwiches)
Cobalt
[CpCo(MeEt2C3B2Et2)]2Ni
Rhodium
(Et2MeC3B2Me2)Rh(Et2MeC3B2Me2)Rh(Et2MeC3B2Me2)Rh(C6H5Me)

Polydecker sandwiches
[(RR9R0C3B2Me2)Ni](RR9R0C3B2Me2)nNi(RR9R0C3B2Me2)


a

Metals coordinated to C2B3 rings are shown in boldface.
Legend: S ¼ synthesis, X ¼ X-ray diffraction, H ¼ 1H NMR, B ¼ 11B NMR, C ¼ 13C NMR, F ¼ 19F NMR, P ¼ 31P NMR, Li ¼ 7Li
NMR, Pt ¼ 195Pt NMR, IR ¼ infrared data, MS ¼ mass spectroscopic data, UV ¼ UV–visible data, E ¼ electrochemical data,
ESR ¼ electron spin resonance data.
b

the metal electron count. Thus, in complex 23 which contains a formally 16e iron center, the ring is strongly bent out
of planarity with a dihedral angle of 41 .28 This pronounced folding, and the position of the exo-methyl group on the
unique ring carbon atom, suggest sp3 hybridization on that carbon; ESR studies and EHMO calculations show that the
folding is a consequence of metal interaction with the C–B bonds.29,30 Stabilization of this 16e sandwich is attributed
to release of electron density by the methyl groups on the Cp* ligand. Based on the resemblance of its 11B NMR
spectra to that of 23, the ruthenium analog of 23 was deduced to have a similarly folded C3B2 ligand.29

–

Et
Me2HC

Me
Me2HC

B
Et

Et

Fe


B
Cp*FeCl

Me2HC
Me2HC

B
B
Et

23

Me

11


www.pdfgrip.com
12

Boron-containing Rings Ligated to Metals

Oxidative addition of hydrogen to mononuclear Cp*Ru–diborolyl complexes formed the dihydride species 24,
which could not be isolated, and readily lose H2 in vacuum;30 detailed variable-temperature NMR studies were
unable to distinguish between the possible classical Ru(H)2 and non-classical Ru(H2) structures. Acetonitrile,
CO, and other electron donors add similarly and in some cases reversibly, leading to bent structures such
as 25.29,30

Ru


R1
R1

B

H
H

B

CO
R2

R1

B

B

Me

R2
R1,

Ru

R1

R2


R2

R2 = H,

R1,

Me

24

R2 = H, Me

25

Reaction of cyclooctyne with (I2B)2CHMe formed a 4,5-cyclooctadiiododiborole, which was methylated with AlMe3
and the product treated with nickel or cobalt reagents to give complexes of type 26.31

Co
B
B

26
(Arene)metal(diborolyl) sandwiches, for example, 27, have been synthesized from [(C2H4)2RhCl]2 and diborole
derivatives.32 In general, the arene ligands in these complexes are labile on heating, as shown by the displacement of
naphthalene from 28 by benzene-d6 to give 29; this lability leads to stacking with the formation of triple-decker
sandwich complexes.32
R4

D


D

D

D

D
R1

Rh

R2
B

R1

C6D6

Rh
R3

B

B
B

D

Rh


–C10H8

B

B

R2
R1 = Me, Et
R2 = Me, CMe3
R3 = H, Me, MesCH2
R4 = H, Me

27

28

29

Novel double-deckers illustrated by 30 and 31 have been generated from 1,4-diborapentafulvene ligands.33 In 31 a
second metal center is coordinated to the BTC bond of the fulvene moiety.


www.pdfgrip.com
Boron-containing Rings Ligated to Metals

M
R

B


Ni
B

R

B

R = Me, (CH2)2
M = Ni, Pt

R

B

R

M

30

31

3.01.3.2 C3B2 Ring Triple-decker Sandwiches and Dimers
Addition of Cp*RuH to monuclear complexes Cp*Ru(MeR1C3B2R2) (analogous to 23) afforded the 30e triple-decker
32.29 Complexes such as 32 can be equivalently represented as seven-vertex pentagonal bipyramidal Ru2C3B2
metallacarborane clusters, suggesting that isomers of such clusters should exist in which one or both metal atoms
occupy equatorial vertices (as is found in dicarbon metallacarboranes34,35). Compounds of this type have, in fact, been
observed; thus, addition of sulfur to the mononuclear complex 33 generated 34, which on standing in hexane solution
at À20  C gave the diruthenium cluster 35 having an Ru–H–Ru bridging hydrogen, whose structure was established

by X-ray crystallography.36 One might expect that 35 could undergo polyhedral rearrangement on heating to form 36,
a process well known in M2C2B3 clusters,34,35 but unfortunately 35 decomposes above 80  C.36 Sulfur can also be
introduced into the ring to create heterocyclic metal complexes, as is discussed in Section 3.01.9.4.

Ru
R1
R1

R2
B

B
R2

Ru H

R1, R2 = H, Me

32

Ru
B

Ru
B
B

33

Ru


S8 or
C3H6S

Ru

H

S

Ru

B

34

B

Ru
B

B

?

B

35

36


H

13


www.pdfgrip.com
14

Boron-containing Rings Ligated to Metals

Paramagnetic 31e dicobalt triple-deckers, for example, CpCo(Et2MeC3B2Et2)CoCp*, have been prepared from
bis(1,3-diborole) double-decker sandwiches and Cp*Co(C2H2)2.37 Reaction of the latter reagent with benzo-1,3diborafulvene derivatives such as 37 afforded triple-deckers 38 and 39 whose electronic structures have been
investigated in detail.37

Co
B

Co

Me

CHMe2

Me

Co

B


Co

Co

Me

B

CMe2

B

B

B

Me

37

38

39

Hybrid sandwich complexes containing both C3B2 and C2Bn (carborane) ligands bound to a common metal center
have been known since 1989, as triple-deckers incorporating C2B3 or C2B4 units together with C3B2 rings.35 In the
period since the publication of COMC (1995), hybrids involving larger 11- and 10-vertex carborane ligands have been
prepared, as in 40 and 41.38–41 A variant on this theme is provided by the diborolyl–thiaboranyl cluster 42 and related
complexes.42


Ru

Ru

B

Co
B

B

B

B
B

Rh
B

C

B

C

B

H
H


C

C
B

B

B

B

B
B

40

B = BH

H

B B

B

B

B

B


Co

Rh

H

C

B

Me
B

B

S
S

B

B H

B

B

41

42


H

B = BH

3.01.3.3 C3B2 Ring Tetradecker and Pentadecker Sandwiches
While triple-decker complexes are ubiquitous in chemistry, covalently bound molecular sandwiches having
more than three decks with the metals linearly aligned are, at this writing, restricted to complexes incorporating
C3B2 or C2B3 planar rings. Soluble molecular sandwiches as large as hexadeckers have been characterized (see
Section 3.01.4.4). Since 1993, several new diborolyl Ni–Ni–Ni and Co–Ni–Co tetradeckers 43 have been
reported,38,43 with structural characterization of the latter.43 A Cl2-bridged ‘‘pentadecker’’ complex has also
been prepared.39


www.pdfgrip.com
Boron-containing Rings Ligated to Metals

Co
B

B

Ni
B

B

Co

43


3.01.3.4 Polydecker C3B2 Ring Sandwiches
Semiconducting black polymers characterized as 44 have been prepared via slow heating of triple-decker
(allyl)nickel diborolyl triple-decker complexes.44 EXAFS studies reveal an Ni–Ni distance of 3.35 A˚ and a powder
conductivity of ca. 10À2 S?cmÀ1. When doped with iodine or oxygen, the conductivity of these polymers
decreases.
R1
R2

R1
R2

B

B

R3

Ni
B

B

n

Ni
R1
R2

R1, R2, R3 = H, alkyl


R3

B

B

R3

44

3.01.4 Complexes of C2B3 (Triboracyclopentadienyl [Carborane]) Rings
Cyclic ligands containing more boron than carbon atoms, such as C2B3, bear a close relationship to the C3B2 and C4B
ring derivatives described in the two preceding sections, yet there are major differences in their preparative routes
and reaction chemistry. Borole and diborolyl derivatives in general are stable organoboron compounds, usually
prepared via boronation of organic precursors, and exist both as free species and as ligands in metal complexes. In
contrast, planar carborane (C2B3) rings, are, with very rare exceptions, known only as covalently bound metal
Z5-complexes (MC2B3 or M2C2B3 metallacarborane clusters), and are obtained by the extraction of the apex boron
from seven-vertex MC2B4 cages.35 The metal–C2B3 clusters (Table 3) conceptually link the metal sandwich
complexes of the organoboron heterocycles with the metallacarboranes, and have features common to both. In this
chapter, only compounds containing planar C2B3 rings coordinated to metal centers are covered; metallacarborane
systems involving C2B4 or larger ligands are treated in a separate chapter.
The preparation of small metallacarboranes having MC2B4, MC2B3, or M2C2B3 cores has been extensively
developed by Grimes and co-workers since the early 1970s,35,45–50 and more recently by Hosmane et al. for MC2B4
systems51 (see Chapter 3.05). Metal complexes of planar C2B3 ring ligands are of interest as building-block units for

15


www.pdfgrip.com
16


Boron-containing Rings Ligated to Metals

Table 3 Complexes of C2B3 rings 1993–2005
Compound a

Informationb

References

Synthesis and characterization
Mononuclear 2,3-C2B3 complexes (double-decker sandwiches)
Iron
Cp*Fe(Et2C2B3H5)
(Z6-C6Me6)Fe(Et2C2B3H4)-5-CUCH

S, H, IR, MS
S, H, B, C, IR, MS

52
53

Ruthenium
(Z5-CHMe2C6H4Me)Ru(Et2C2B3H4-5-Cl)

S, H, C, MS

74

S, H, B, C, IR, MS


52

S, H, B, C, IR, MS
S, H, B, C, MS
E(Cl)
E
S, H, B, IR

52
52
61
54
67

Cytotoxic/antitumor activity
Cytotoxic/antitumor activity
NaH/Me2CBrNO2 ! closoCp*Co(Et2C2B3X2Br) oxidative
cage closure
S, H, B, C, IR, MS
S, H, B, C, IR, MS
S, H, B, C, IR, MS
S, H, B, C, IR, UV, MS

68
68
63

Cobalt
CpCo(Et2C2B3H3-4-X-6-Y) (X ¼ Br, Y ¼ I; X ¼ Br, Y ¼ H; X ¼ I,

Y ¼ H)
CpCo(Et2C2B3H3-4,6-X2) (X ẳ Br, I)
CpCo(Et2C2B3H4-5-X) (X ẳ Cl, Br, I)
Cp*Co(Et2C2B3H5)
Cp2Coỵ [nido, closo-(Et2C2B3H4-5-R)Co(Et2C2B4H4)]À (R ¼ H, Me)
cobaltocenium salts
Cp*Co(Et2C2B3H4)-5-X (X ¼ H, Br)
Cp*Co(Et2C2B3H3)-4,6-I2
Cp*Co(Et2C2B3H3)-4,6-X2 (X ¼ Cl, Br)

Cp*Co(Et2C2B3H4)-5-R (R ¼ NMe2, OCMe3)
R ¼ CUCH, CUCSiMe3
Cp*Co(Et2C2B3H3)-4,6-(CUCSiMe3)2
[Cp*Co(Et2C2B3H4-5-CUC)]2
Other Cp*Co(Et2C2B3H5) B-substituted derivatives
Other (hydrocarbon)Co(Et2C2B3) derivatives
[Cp*Co(Et2C2B3H4)]2
[Cp*Co(Et2C2B3H3)]2 (bent isomer)
[Cp*Co(Et2C2B3H3)]2 (planar isomer)
[Cp*Co(Et2C2B3H4)]2 (B–B connected)
(Z5-C5H4I)Co(Et2C2B3H5)
[Z5-C5H4C(O)Cl]Co(Et2C2B3H5)
(Et2C2B4H4)CoH(Et2C2B3H4Me)
(Et2C2B4H4)CoH(Et2C2B3H4-5-n-C4H9)
Iridium
Cp*Ir(Et2C2B3H4-4-R) (R ¼ H, Cl)
Cp*Ir(Et2C2B3H4-5-Cl)

S,
S,

S,
S,
S,
S,
S,
S,

X, H, B, UV, MS
X, H, B, IR, UV
X, H, B
H, B
H, B, C, IR, MS
H, B, C, IR, MS
H, B, UV, MS
H

56
54, 58
72
54
75
55, 57
59
64
64
64
57
60
75
65


S, H, B, IR, MS
S, H, IR, MS

66a
66a

S, H(correlated)(X ¼ H), E, ESR, IR,
MS, Moăssbauer (X ẳ H)
S, H(correlated), E, ESR, IR, MS

70

Ruthenium
(Z6-MeC6H4CHMe2)Ru(Et2C2B3H2Me)CoH(Et2C2B3H5)
(Z6-MeC6H4CHMe2)Ru(Et2C2B3H2Me)Co(Et2C2B3H5)
Cp*HRu(Et2C2B3H3)CoCp*

S, H, B, UV, MS
S, UV, ESR, MS
S, H, C, IR, UV, MS

75
75
73

Molybdenum
(CO)4Mo(Et2C2B3H3)CoCp*
(CO)4Mo(Et2C2B3H2-5-CH2Ph)CoCp*


S, X, H, B, C, IR, UV, MS
S, X, H, IR, UV, E, MS

76
76

S, X(H,CH2Ph), H, B(R ¼ H),
C(R ¼ H), IR, UV, E(CH2Ph)
Cytotoxic/antitumor activity

76

Dinuclear 2,3-C2B3 complexes (triple-decker sandwiches)
Iron
[Cp*Fe(Et2C2B3H2-5-X)CoCp*]n (n ẳ ỵ1, 0, 1, 2; X ẳ H, Cl, Br)
[Cp*Fe(Et2C2B3H3)CoCp]n (n ¼ 0, À1, À2)

Tungsten
(CO)4W(Et2C2B3H2-5-R)CoCp* (R ¼ H, CH2Ph)
(CO)4W(Et2C2B3H3)CoCp*

70

68
(Continued)


www.pdfgrip.com
Boron-containing Rings Ligated to Metals


Table 3 (Continued)
Compound a

Informationb

References

Tantalum
CpCl2Ta(Et2C2B3H3)CoCp*
CpCl2Ta(Et2C2B3Br3)CoCp*
CpCl2Ta(Et2C2B3HI2)CoCp*
CpLL9Ta(Et2C2B3H3)CoCp* (L ¼ Me, CH2Ph; L9 ¼ Cl, Me,
CH2Ph, CH2CMe3)

S,
S,
S,
S,

79
80
79, 80
79

Niobium
CpCl2Nb(Et2C2B3H3)CoCp*
Cobalt
Cp*Co(Et2C2B3H2Me)CoH(Et2C2B3H5)
Cp*Co(Et2C2B3H2Me)Co(Et2C2B3H5)
Cp*Co(Et2C2B3H2Me)CoHn(Et2C2B3H5) (n ¼ 0,1)

CpCo(Et2C2B3Br3)CoCp
Cp*Co(C2B3H5)CoCp*
Cp*Co(Et2C2B3H3)CoCp*
Cp*Co[Et2C2B3H-4,6-(CUCH)2]CoCp*
Cp*Co[Et2C2B3H2-5-CUCSiMe3]CoCp*
Cp*Co[Et2C2B3H2-5-CUCSiMe3]CoCp*n (n ẳ 0, 1, 1)
[Cp*Co(R2C2B3H3)CoCp*]n (n ẳ ỵ1, 0, À1; R ¼ H, Me, Et)
[CpCo(Et2C2B3H3)CoCp*]n (n ¼ 0, À1)
(Z5-NC4Me4)Co(Et2C2B3H3)Co(Z5-NC4Me4)
Cp*Co(Et2C2B3H2Me)CoHn(Et2C2B4H4) (n ¼ 0,1)
Cp*Co(Et2C2B3H2Me)CoH(Et2C2B4H4)
Cp*Co(Et2C2B3H2Me)Co(Et2C2B4H4) (M ¼ CoH, Co)
Cp*Co(Et2C2B3H3)Mo(CO)4
Cp*Co(Et2C2B3H2-5-CH2Ph)Mo(CO)4
Cp*Co(Et2C2B3H2-5-R)W(CO)4 (R ¼ H, CH2Ph)
Cp*Co(Et2C2B3H3)W(CO)4
Cp*Co(Et2C2B3H3)NbCpCl2
Cp*Co(Et2C2B3H3)TaCl2Cp
Cp*Co(Et2C2B3Br3)TaCl2Cp
Cp*Co(Et2C2B3HI2)TaCl2Cp
Cp*Co(Et2C2B3H3)TaCpLL9 (L ¼ Me, CH2Ph; L9 ¼ Cl, Me,
CH2Ph, CH2CMe3)
[Cp*Co(Et2C2B3H2-5-X)Fe(Cp*)]n (n ẳ ỵ1, 0, 1, 2; X ẳ H, Cl, Br)
[CpCo(Et2C2B3H3)FeCp*]n (n ¼ 0, À1, À2)
Cp*Co(Et2C2B3H3)RuHCp*
(Et2C2B3H5)CoH(Et2C2B3H2Me)Ru(Z6-MeC6H4CHMe2)
(Et2C2B3H5)Co(Et2C2B3H2Me)Ru(Z6-MeC6H4CHMe2)
Cp*Co(Et2C2B3H4-4-Cl)IrCp*
Cp*Co(Et2C2B3Me3)Ni(Et2C2B4H4)
Other Co(C2B3)M triple-decker complexes


X, H, B, C, IR, E, UV, MS
X, H, B, C, MS
H, B, MS
X(Me2, CH2Ph; Cl), H, B, C, IR,
UV, MS

S, H, B, C, IR, UV, E, MS

79

S, X, H, B, UV, MS
S, UV, ESR, MS
S, UV, MS
S, H, B, C, MS
X, H(correlated)
S, X, H, B, C, E
S, H, B, C, IR, UV, MS
S, H, B, C, IR, UV, E, MS
IR(spectroelectrochem), UV, E
S, X(Et; n ¼ 0, À1), E(Et),
H(correlated), B (Et), C
S, X(n ¼ 0), H(correlated), B
X
S, H, UV, MS
S, H, B, UV, MS
S, ESR, UV, MS
S, X, H, B, C, IR, UV, MS
S, X, H, IR, UV, E, MS
S, X(H,CH2Ph), H, B(R ¼ H),

C(R ¼ H), IR, UV, E(CH2Ph)
Cytotoxic/antitumor activity
S, H, B, C, IR, E, UV, MS
S, X, H, B, C, IR, E, UV, MS
S, X, H, B, C, MS
S, H, B, MS
S, X(Me2, CH2Ph; Cl), H, B, C, IR,
UV, MS
S, H(correlated)(X ẳ H), E, ESR, IR,
MS, Moăssbauer (X ẳ H)
S, H(correlated), E, ESR, IR, MS
S, H, C, IR, UV, MS
S, H, B, UV, MS
S, H, B, UV, MS
S, H, B, UV, MS
S, H, B, UV, MS

75
75
78
52
70
70
72
54
72
70
64, 70
77
78

75
75
76
76
76
68
79
79
80
79, 80
79
70
70
73
75
75
66a
75
72

Iridium
Cp*Ir(Et2C2B3H4-4-Cl)CoCp*
Cp*Ir(Et2C2B3H4-4-Cl)IrCp*

S, H, B, UV, MS
S, X(Ir), H, B, IR, UV, MS

66a
66a


Nickel
(Et2C2B4H4)Ni(Et2C2B3Me3)CoCp*

S, H, B, UV, MS

75

Trinuclear 2,3-C2B3 complexes (tetradecker sandwiches)
Molybdenum
[Cp*Co(Et2C2B3H3)]2Mo(CO)2

S, X, H, B, C, IR, E, UV

76

Ruthenium
Cp*Co(Et2C2B3H2-5-Cl)Ru(Et2C2B3H-4,5-Cl2)CoCp*
[(Z6-MeC6H4CHMe2)Ru(Et2C2B3H2-5-X)]2Co (X ¼ Me, Cl)

E, ESR
S, X(Me), H, IR, UV, MS

84
74
(Continued)

17


www.pdfgrip.com

18

Boron-containing Rings Ligated to Metals

Table 3 (Continued)
Compound a

Informationb

References

S, H, C, IR, UV, MS
S, X, IR, UV, MS

74
74

S, IR, UV, MS

74
74

S, UV, ESR, MS
S, UV, ESR, MS
S, H, B, UV, MS
E

75
75
75

84

S, X, H, B, C, IR, E, UV
S, UV, ESR, MS, oxidative
fusion ! Co2C4B8 clusters
S, H, UV, MS
S, X(Cl), H, B, C, IR, UV, MS
S, H, UV, MS
S, H, UV, MS
S, H, UV, MS
S, H, UV, MS
S, H, B, C, E, Ms
S(electrochem), H, F, E, MS
E, ESR
S, X(Me), H, IR, UV, MS
S, H, C, IR, UV, MS
S, X, IR, UV, MS

76
83

S,
S,
S,
S,

IR, UV, MS
H, UV, MS
H, UV, MS
X(Cl), H, B, C, IR, UV, MS


74
66a
66a
82

Rhodium
Cp*Co(Et2C2B3H4-5-Cl)RhH(Et2C2B3H4-5-Cl)IrCp
Cp*Co(Et2C2B3H4-5-Cl)RhH(Et2C2B3H4-5-Cl)CoCp*
[Cp*Co(Et2C2B3H4-5-R)]2RhH (R ¼ H, Cl)
Cp*Ir(Et2C2B3H4-5-Cl)RhH(Et2C2B3H4-5-Cl)IrCp*
Cp*Ir(Et2C2B3H4-5-Cl)RhH(Et2C2B3H4-5-Cl)CoCp*

S,
S,
S,
S,
S,

H, UV, MS
H, UV, MS
X(Cl), H, B, C, IR, UV, MS
H, UV, MS
H, UV, MS

66a
66a
82
66a
66a


Iridium
Cp*Co(Et2C2B3H4-5-Cl)Ir(Et2C2B3H4-5-Cl)CoCp*
Cp*Ir(Et2C2B3H4-5-Cl)RhH(Et2C2B3H4-5-Cl)IrCp*
Cp*Ir(Et2C2B3H4-5-Cl)RhH(Et2C2B3H4-5-Cl)CoCp*

S, H, UV, MS
S, H, UV, MS
S, H, UV, MS

66a
66a
66a

Nickel
[Cp*Co(Et2C2B3H-4,5-Cl2)]2Ni
[Cp*Co(Et2C2B3FCl2)]2Ni
[(Z6-MeC6H4CHMe2)Ru(Et2C2B3Me3)]2Ni

S, H, B, C, E, Ms
S(electrochem), H, F, E, MS
S, IR, UV, MS

84
84
74

S, UV, ESR, MS
S, H, B, UV, ESR (n ¼ 0), MS


85
85

S, H, UV, ESR, MS

85

S, X(n ¼ 1), H, UV, ESR, MS

78,85

S, X, UV, ESR, MS

85

6

[(Z -MeC6H4CHMe2)Ru(Et2C2B3H2-5-Cl)]2CoH
(Z6-MeC6H4CHMe2)Ru(Et2C2B3H2-5-Me)Co(Et2C2B3H2-5Et)Ru(Z6-MeC6H4CHMe2)
[(Z6-MeC6H4CHMe2)Ru(Et2C2B3Me3)]2Co
Other Ru tetradecker sandwiches
Cobalt
Cp*Co(Et2C2B3H2Me)Co(Et2C2B3H2Me)CoH(Et2C2B3H5)
Cp*Co(Et2C2B3H2Me)Co(Et2C2B3H2Me)CoH(Et2C2B4H4)
Cp*Co(Et2C2B3H2Me)Ni(Et2C2B3H2Me)CoH(Et2C2B4H4)
[Cp*Co(Et2C2B3H2-5-X)]2Co [X ¼ H, Me, Cl, Br, C(O)Me,
CH2CUCMe]
[Cp*Co(Et2C2B3H3)]2Mo(CO)2
[Cp*Co(Et2C2B3H2-5-R)]2FeH (R ¼ Me, Cl)
Cp*Co(Et2C2B3H4-5-Cl)RhH(Et2C2B3H4-5-Cl)CoCp*

[Cp*Co(Et2C2B3H4-5-R)]2RhH (R ¼ H, Cl)
Cp*Co(Et2C2B3H4-5-Cl)Co(Et2C2B3H4-5-Cl)CoCp*
Cp*Ir(Et2C2B3H4-5-Cl)RhH(Et2C2B3H4-5-Cl)IrCp*
Cp*Ir(Et2C2B3H4-5-Cl)RhH(Et2C2B3H4-5-Cl)CoCp*
Cp*Co(Et2C2B3H4-5-Cl)Ir(Et2C2B3H4-5-Cl)CoCp*
[Cp*Co(Et2C2B3H-4,5-Cl2)]2Ni
[Cp*Co(Et2C2B3FCl2)]2Ni
Cp*Co(Et2C2B3H2-5-Cl)Ru(Et2C2B3H-4,5-Cl2)CoCp*
[(Z6-MeC6H4CHMe2)Ru(Et2C2B3H2-5-X)]2Co (X ¼ Me, Cl)
[(Z6-MeC6H4CHMe2)Ru(Et2C2B3H2-5-Cl)]2CoH
(Z6-MeC6H4CHMe2)Ru(Et2C2B3H2-5-Me)Co(Et2C2B3H2-5Et)Ru(Z6-MeC6H4CHMe2)
[(Z6-MeC6H4CHMe2)Ru(Et2C2B3Me3)]2Co
Cp*Co(Et2C2B3H4-5-Cl)RhH(Et2C2B3H4-5-Cl)IrCp
Cp*Co(Et2C2B3H4-5-Cl)RhH(Et2C2B3H4-5-Cl)CoCp*
[Cp*Co(Et2C2B3H4-5-R)]2RhH (R ¼ H, Cl)

Tetranuclear 2,3-C2B3 complexes (pentadecker sandwiches)
Cp*Co(Et2C2B3H2Me)CoH(Et2C2B3H3)Co(Et2C2B3H3)CoCp*
Cp*Co(Et2C2B3H2Me)CoH(Et2C2B3H3)CoHn(Et2C2B3H2Me)CoCp*
(n ¼ 0,1)
Cp*Co(Et2C2B3H2Me)CoH(Et2C2B3H3)Ni(Et2C2B3H2Me)CoCp*
Pentanuclear 2,3-C2B3 complexes (hexadecker sandwiches)
Cp*Co(Et2C2B3H2-5-Me)CoHn(Et2C2B3H3)Co(Et2C2B3H3)
Co(Et2C2B3H2-5-Me)CoCp* (n ¼ 0,1)
Cp*Co(Et2C2B3H2-5-Me)CoH(Et2C2B3H3)Co(Et2C2B3H3)
CoH(Et2C2B3H2-5-Me)CoCp*

66a
82
66a

66a
66a
66a
84
84
84
74
74
74

(Continued)


www.pdfgrip.com
Boron-containing Rings Ligated to Metals

Table 3 (Continued)
Compound a
*

Cp Co(Et2C2B3H2-5Me)Co(Et2C2B3H3)Pt(Et2C2B3H3)Co(Et2C2B3H3)CoCp*
Other Co(C2B3)-containing hexadecker sandwiches
Linked sandwiches and multisandwich assemblies
Molybdenum
[(Et2C2B3H5)Mo(CO)2]2(m-Br)2
Cobalt
[(Et2C2B3H5)Co(Z5C5Me4–CUC)]2
[(-CH2C5Me4)Co(Et2C2B3H4-5-X)]2 (X ¼ H, Cl, Br, I)
1,3-[Cp*Co(29,39-Et2C2B3H3)Co(Z5 -C5H4)]2-5-(nidoEt2C2B3H5)Co(Z5 -C5H4)C6H3
1,4-(Et2C2B3H5)Co(Z5-C5Me4–CUC)]2C6H4 (phenylene-bridged)

nido-(Et2C2B3H3-4-R-5-R9)Co(C5Me4)-C6H4(C5Me4)Co[(Et2C2B3H3-4-R0-5-R09)
(R, R9, R0, R09 ¼ H, Cl, Br, Me) phenylene-bridged
[nido-(Et2C2B3H4R)CoL]2(C6H4)LCo(Et2C2B3H2R)M(Et2C2B3H2R)CoL]n(C6H4) (n ¼ 1, 3, 5; L ¼ Z5-C5Me4;
M ¼ Co, Ni; R ¼ Cl, Me)
nido-, closo-M[(Et2C2B3H-4-R-5-R9)Co(C5Me4)-C6H4(C5Me4)Co(Et2C2B3H4-R0)]2
(R, R9, R0 ¼ H, Cl, Me; M ¼ Co, CoH, Ni) phenylene-bridged
nido-, closo-[(Et2C2B3H4Cl)Co(C5Me4)]2[(C5Me4)Co(Et2C2B3H2Cl)Co(Et2C2B3H2Cl)Co(C5Me4)]3(C6H4)4
phenylene-bridged
Other nido-CoC2B3 phenylene-bridged complexes
1,3,5-[(29,39-Et2C2B3H3)Co(Z5-C5H4)]3C6H3 benzene-centered
1,3,5-[Cp*Co(Et2C2B3H4-5-CUC)]3C6H3 benzene-centered
1,3,5-[Cp*2Co2(29,39-Et2C2B3H2-5-CUC)]3C6H3 benzene-centered
tris(triple-decker sandwich)
Other nido-CoC2B3 benzene-centered complexes
[nido-(Et2C2B3H4-5-Me)Co(Z5-C5H4)]2 fulvalene-bridged
nido-, closo-[(Et2C2B3H4-5-Me)Co(Z5-C5H4)2Co3(Et2C2B3H2-5Me)2(Z5-C5H4)]2 fulvalene-bridged
DAB-dendrimer-{[(Z5-C5H4NHC(O))]Co(Et2C2B3H5)}n
(n ¼ 16, 32)
[(Et2C2B4H4)Co(Et2C2B3H3-5-n-C4H9)Ru]4
Theoretical studies
Molecular and electronic structure calculations
CpCo(2,3-C2B3H5)CoCp

Informationb

References

S, UV, ESR, MS

85

85

S, H, B, IR, UV

76

S, H, MS
S, H, B, UV, MS
S, H, B, C, IR, UV, MS

55
59
57

S, X, H, B, C, MS
S, H, B

55
87

E

61

S, X(Co); R,R9 ¼ Cl; R0 ¼ H), H, B,
UV, MS(FAB)

87

S, MS(FAB)


87

S, X, H, B, C, IR, UV, MS
E
S, H, B, C, IR, UV, MS
S, X, H, B, C, IR, UV, E, MS

87
57
72
54, 58
58, 54

S, H, B
S, H, UV, MS(FAB)

55
87
87

S, H, B, C, IR, UV, E, MS

60

S, X, H, MS

65

Extended Huăckel


72

a

Metals coordinated to C4B rings are shown in boldface.
Legend: S ¼ synthesis, X ¼ X-ray diffraction, H ¼ 1H NMR, B ¼ 11B NMR, C ¼ 13C NMR, F ¼ 19F NMR, P ¼ 31P NMR, Li ¼ 7Li
NMR, Pt ¼ 195Pt NMR, IR ¼ infrared data, MS ¼ mass spectroscopic data, UV ¼ UV–visible data, E ¼ electrochemical data,
ESR ¼ electron spin resonance data.
b

constructing extended systems having tunable electronic properties. As discussed below, these are of two general
types: multidecker stacks consisting of parallel C2B3 rings coordinated to face-bonded metal atoms, and covalently
linked arrays in which small metallacarborane clusters are linked by hydrocarbon chains or rings.

3.01.4.1 C2B3 Ring Double-decker Sandwiches
One approach to the synthesis of linked MC2B4 or MC2B3 metallacarboranes is via B-substituted derivatives containing reactive substituents such as halogens or alkynyl. Following earlier work,34 systematic methods for regiospecific
halogenation52 and introduction of alkynyl groups53,54 have been published. These methods allow selective syntheses
of C2B3 complexes that are functionalized at the middle boron [B(5)] or the end borons [B(4,6)] (see 45–49).52

19


www.pdfgrip.com
20

Boron-containing Rings Ligated to Metals

B
C


B

H HH

H

B

BH

C

X

TMEDA /H2O
50 °C

C

B

C

B
BH

M

M


45

46

X = Cl, Br, I

X

M = FeCp, CoCp, CoCp*

H

B
HB

C

H H H

H H H

B

C

BH

TMEDA /H2O


B

C

C

BH

N-halosuccinimide

HB

B

C

C

M

M

47

48

B X

e
id

im
in
cc
su
lo
ha
N-

M

HB

H H H

X

B

B

C

C

B X

M

49
The B-halogenated derivatives, in turn, allow direct introduction of alkynyl and other functional groups; desilylation and linkage affords dimers linked by dialkynyl chains:54,55

Other B-substituted derivatives of metal–C2B3 complexes have been prepared by reducing CoIIIC2B4 clusters to
19e CoII species that, in turn, undergo radical reactions with electrophiles and nucleophiles. The MC2B4 clusters
obtained have OH, OR, or NH2 substituents on the center equatorial boron [B(5)]; removal of the apex BH affords
the corresponding metal–C2B3 complexes.56
Species functionalized at boron, such as 50–53, have also been prepared from the corresponding MC2B4 closo
clusters via the removal of the apex BH unit (decapitation) with TMEDA.54–57 The same approach has been
employed to link CoC2B3 units to benzene or other arenes:55,58 As shown in the following subsection, di- and
trinuclear complexes such as 53 and 54 are easily converted to multi-triple-decker sandwich systems.

H

C

B HB
C B

Co

50

I

H
H

i, ClZn-C≡C-SiMe3
ii, Pd(PPh3)4
THF

C B B

C B

Co

51

SiMe3

C

Bu4NF

C

B

H
H

H

BB

Co

52

ClCH2C(O)Me
Et3N


Co

H

C
C

B = B or BH

B

H

B

Co

53

B

B B B CC
H

H


www.pdfgrip.com
Boron-containing Rings Ligated to Metals


C

Co

C

B
B H
B

H

H

B

C
C

H

B

C6H3I3

H

B

Pd(PPh3)2Cl2/CuI


Co

Et3N

THF
H

H B

Co

B B
C

C

B = B or BH

B B
H B C
H
C

Co

54
Other modes of linkage of metal–C2B3 complexes have been demonstrated. For example, metal-promoted
connection of monomeric species yielded dimers connected by a single B–B bond 55 or in more complex fashion
56–58 as revealed by X-ray crystallography.59 Other polymetallacarborane assemblies that incorporate planar C2B3

end rings have been constructed by linking metal-bound cyclopentadienyl ligands.57

–

Co
0.5 CoCl2

H H

C

B

C

B

BH

Cp2Co

H

H

H

C

B

C

H

B

H

B

B

B

H
H

H

C
B

C

C

+

H


B

–

C

B

H

C

B

B

I

NiBr2

H

Co

THF

Co
C
C


Co

H

H

B

B
B

H

H

H

C
C

B

B

B
H

57
H
H


HH

C B B
C
B
H

Co

Cl
Na
THF

H

B

H

B

B H
C BH
Co C B

H
H H

B


B

H

58

C

H H

Co

H

C B BH
C
B

BH
C

H

+

Co

B
H


56

55

H

B

Co

Co

Co

Co

H H

C

B
H

C

C

21



www.pdfgrip.com
22

Boron-containing Rings Ligated to Metals

Organic functional groups can also be placed on the Cp ligands in CpMC2B3 complexes, e.g., 59,60 and this
approach has been exploited to generate metallodendrimers such as 60 [DAB-32 ¼ diaminobutane-dend(NH2)32].60

B
B C
C Co
B

H

H

C

C

B

B

Co

B BH
C CB


H H

B
B
B CB
C

DAB-32

Co
O
C
Cl

H B B
BC C

Co

HB B
BC C

O

Co

C

NH


NH

H

Co

O C

O C

C O

H

B
HB C
B C

C

O

NH

Co
H

N


H

N

N

H B
B C
BC

N

H

C

H

N

N

O

N

N
H

N


C

N

N

O

H

N

Co

N

N

N

O

C
O

HN

HN


C

Co

O

CB
C
B BH

Co

H

CB
C BH
B H

C O

NH

C O

Co

B
C H
C B
B H


Co

C H
B
C H
B

NH

NH

C

O C

B

O C

O

B

H

C
B
C
B


Co

B
H

H

C Co
B C
B

H

Co

BC
B C
H B

O

C
O

Co

CB
C B H
B H


Co
C CB
B B H
H

C

NH

HN

O

H

N

N

HN

B

H

N

C


N

C C
B
H B

H

N

H

C

Co

C
O

N

C
O

N

N

N


N

H

N

N

H

CB
C B H
B H

C
O

N

H

N

C

Co

N

O


H

Co C B

H

H

Co

B H
C B

C

N

H

BC C
H B B

Co

N

N

N


Co

C

H

N

O

BC
H B C
H B

B H
C B H
CB

O
N

N
N

B C Co
H
B C
H B


Co

N

N

N

N

H

C

B B H
C
CB

O
C

O

Co

H

C

N


N

H

B B B
C C

O

NH

C

H

B
H B C
B C Co

Co

B H
C
B
C H
B

H


HN

N

N

H

B
C
B
H
C
B
H

HN
H

O

Co

59

HN

HN

C


O
C

H

C O

C
O

Co

Et3N
CH2Cl2

O

Co

H

H

B H
C B
Co C B H

B H
C BH

CB

Co
C C
B B B
H
H

B C C
H B B
H

H

60
The electrochemical54,61 and chemical redox behavior of metal–C2B3 complexes has been studied in detail. The
oxidation of [Cp*Co(Et2C2B3H3X2)]À anions to the closo-Cp*Co(Et2C2B3X2Br) products 61, predicted from electroncounting theory,62,62a provided the first experimental demonstration of a nido–closo conversion in a six-vertex cluster
system.63 The process shown is reversible; reduction of 61 (X ¼ Br) with lithium naphthaleneide regenerated the
open CoC2B3 cluster.63

X
B
–

H H

X B

B
C


C

Co

B

X
X
Me2CBrNO2

X = Cl, Br

C

B

B

Br

C

Co

61
Metal-promoted fusion or linkage of anionic CoC2B3 complexes affords exceedingly rich and varied chemistry.
Thus, FeCl2-promoted oxidative face-to-face fusion gave Co2C4B6 clusters 62. With NiBr2, the product was a B–B



www.pdfgrip.com
Boron-containing Rings Ligated to Metals

linked dimer 57 as noted above, while CoX2 (X ¼ Cl, Br, I) generated tetradecker sandwich products (see Section
3.01.4.3).64

H H

C B B
B
C
H

Co

C

–

C
C

I

FeCl2

B

THF
O2


Co

C

B

B

B

Co

B
B

B = B–H
C = C–Et

62

Oxidative fusion of the nido-, closo-cobaltacarborane 63, which has both C2B3 and C2B4 ligands, yielded the Ru4Co4
tetramer 64 whose structure was confirmed crystallographically.65

C
B

C
CC


B

C

B

H

CB

Co
B

C

B
B

C
B

2–

Bu

C

B

63


i, [(COD)Ru(MeCN)4]2+

B

B
B

Co

B

Ru

B

B B

Co
B

Bu

C

B

Bu

C


B

C

B
B

Ru

Ru

ii, O2, silica
B
C = C–Et
B = BH, B

B

Bu

B
B
C B
Co BB
C
C
C

Bu


B

C
BB

B

Co C

Ru
B

B

C

B

C

64

The extensively developed chemistry of cobalt–C2B3 complexes has been extended to the remaining
cobalt group metals. Following an earlier preparation of Cp*Rh(Et2C2B3H5),66 the iridium analogs
Cp*Ir(Et2C2B3H4–X) (X ¼ H, Cl) have been synthesized66a and used to construct mixed metal cobalt group
tetradecker sandwiches (see below). Mixed sandwich cobaltocenium–metallacarborane salts 65 have been
prepared and characterized.67

H


B

B

C

H

Co –
B

C

+

CB
C

B
B

B

Co

C = C–C2H5
B = BH

65


Several metal complexes of planar C2B3 ligands have been shown to have significant cytotoxicity against lymphoma
and leukemia cells in both cultures and solid tumors.68

23


www.pdfgrip.com
24

Boron-containing Rings Ligated to Metals

3.01.4.2 C2B3 Ring Triple-decker Sandwiches
The study of isolable, robust transition metal multidecker sandwiches began with the synthesis and isolation of
isomeric CpCo(RR9C2B3H3)CoCp complexes, which were the first examples of neutral, air-stable triple-decker
compounds.69 Extensive development of this area continued through the period covered by COMC (1982)
and COMC (1995), and more recently has been extended in new directions, as is apparent from the compounds
listed in Table 3. These studies include detailed electronic probes, new structural modes such as sandwiches
endcapped by open C2B3 rings and linked triple-deckers, and the synthesis of extended chains. Paramagnetic
triple-deckers such as the FeIIICoIII species 66, whose NMR spectra are ordinarily uninformative because the signals
are shifted over very broad ranges of frequencies, have been characterized by NMR methods in which the neutral
paramagnetic species are gradually reduced and/or oxidized to diamagnetic anions or cations, allowing the paramagnetic spectra to be interpreted via correlation diagrams.70,71 This is illustrated in Figure 2, where the 1H NMR signals
of neutral paramagnetic Cp*Fe(Et2C2B3H3)CoCp* (29 ve) vary from ca. ỵ20 to 10 ppm; on stepwise reduction to
the diamagnetic 30 ve monoanion via exposure of the solution to a potassium mirror, the signals are compressed to a
much smaller range, and are readily assigned to the ethyl (A,C), FeCp* (D), and CoCp* (B) protons. Further reduction
of the anion, or oxidation of the neutral complex, yields the paramagnetic dianion and the paramagnetic monocation,
respectively. The slopes of the correlations are proportional to the degree to which the various proton environments
are influenced by the addition or subtraction of electron density to the complex. Thus, it is clear that in the FeIIICoIII
neutral species the unpaired electron resides mainly on Fe, whereas in the FeIICoII dianion it is primarily located on


+

0

–1

AgBF4
28e

K

–2
K

29e

30e

31e

D

30
C

CH3

A
Fe
CH3 CH2


B B B
Co

B
CH3

20
B

A

δ 10
A

A
D
B

B

C

0
C

C

D


D

A

–10
0

0.5

1

0.5
fp

0

0.5

1

Figure 2 Correlation diagram for 1H NMR spectra of Cp*Fe(Et2C2B3H3)CoCp* in CDCl3 (for the oxidation) and THF-d8
(for the reductions), showing d plotted vs. mole fraction of the paramagnetic component (fp).70 Reproduced with permission of
the American Chemical Society.