Polynuclear rhodium complexes with dinucleating PNNP ligand: Dynamic and diverse M...M interactions in [(μ-X)Rh2(PNNP)(CO) 2]n+ and [μ4-XRh4(PNNP) 2(CO)4]n+ [X = H, O, C≡C-R, R-C≡C-R, C≡C, CH=CH2, SMe(2); n = 0, 1: PNNP = 3,5-bis(diphenylphosphinomethyl)pyrazolato]

Organometallics 2005, 24, 163-184

163

Polynuclear Rhodium Complexes with Dinucleating

PNNP Ligand: Dynamic and Diverse M‚‚‚M Interactions

in [(µ-X)Rh₂(PNNP)(CO)₂]ⁿ⁺and

[(µ₄-X)Rh₄(PNNP)₂(CO)₄]ⁿ⁺[X ) H, O, CtC-R, R-CtC-R,

CtC, CHdCH₂, SMe₂; n ) 0, 1: PNNP )

3,5-bis(diphenylphosphinomethyl)pyrazolato]

Shukichi Tanaka, Christian Dubs, Akiko Inagaki, and Munetaka Akita*

Chemical Resources Laboratory, Tokyo Institute of Technology,

R1-27, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan

Received July 26, 2004

The dirhodium-tetracarbonyl complex with the PNNP ligand, [Rh₂(PNNP)(CO)₄]BF₄,

1‚BF₄[PNNP ) 3,5-bis(diphenylphosphinomethyl)pyrazolato], serves as a precursor for the

active species [Rh₂(PNNP)(CO)₂]⁺, A, with cis-divacant coordination sites, which can

accommodate a donor species with up to four donor electrons. Interaction of 1‚BF₄with Li-

CtC-R, Li-CHdCH₂, PPN[M(CO)_n], CN-Cy, SMe₂, and Et-CtC-Et leads to the formation

of the corresponding dinuclear adducts [(X)Rh₂(PNNP)(CO)₂]ⁿ⁺(BF₄)_n[X/n ) µ-η¹:η²-CtC-

R/0 (2), µ-η¹:η²-CHdCH₂/0 (3), µ-Co(CO)₄/0 (4a), µ-Mn(CO)₅/0 (4b), (η¹-CtN-Cy)₂/1 (5‚BF₄),

µ-η¹:η¹-SMe₂/1 (6‚BF₄), µ-η²:η²-Et-CtC-Et/1 (7‚BF₄)], respectively. Selective replacement

of the inner CO ligands trans to the P atoms is verified by spectroscopic and crystallographic

characterizations of the adducts. The µ-acetylide (2) and µ-vinyl complexes (3) show dynamic

behavior via the conventional windshield wiper motion of the unsaturated hydrocarbyl ligand.

Systematic structural analysis of a series of the µ-acetylide complexes 2 (R ) H, SiMe₃,

n-Bu, Ph, p-tol) reveals three typical conformations for the Rh₂(PNNP)(CO)₂backbone: C_s-

(type I), C₂- (type II), and C_2v-symmetrical ones (type III). On the other hand, interaction of

1‚BF₄with 1-alkyne, hydrosilanes, and hydroxo anion produces tetranuclear adducts

resulting from 1 (donor):2 [Rh₂(PNNP)(CO)₂] coupling reactions. The µ₄-η¹(C_R):η²(C_RtC_â)-

acetylide complexes, [(µ₄-CtC-R)Rh₄(PNNP)₂(CO)₄]BF₄, 8‚BF₄, consisting of an acyclic,

folded Z-shaped Rh₄linkage are fluxional via a combination of windshield wiper-like motions

between the wingtip Rh centers and between the wingtip and hinge Rh centers, which involve

reversible M-M bond scission and recombination processes. The ethynyl complex 8a‚BF₄

(R ) H) is readily deprotonated by the action of a base to give the µ₄-dicarbide complex

(µ-η¹:η¹:η²:η²-CtC)Rh₄(PNNP)(CO)₄, 9, via cleavage of three Rh-Rh bonds, and this process

is reversed upon protonation of 9 with HBF₄. The reaction of 1‚BF₄with hydrosilanes and

hydroxo anion furnishes the isostructural tetranuclear complexes [(µ₄-X)Rh₄(PNNP)(CO)₄]ⁿ⁺

-

(BF₄)_n[X/n ) H/1 (10‚BF₄), O/0 (11)], in which the four Rh atoms arranged in a tetrahedral

array are connected by the µ₄-bridging ligand (X), as characterized by X-ray crystallography.

The tetranuclear structures are retained mainly by Rh-X interactions even in the case of

the electron-deficient hydride complex 10‚BF₄with 58 valence electrons (cf. 64e for a

coordinatively saturated species), where the filled, spherically distributed 1s orbital of the

hydride ligand interacts with the four Rh centers arranged in a tetrahedral array. Depending

on the size of the bridging ligand X, the structure of the tetranuclear complexes varies from

the encapsulated structure (10‚BF₄and 11) to the one with a folded Z-shaped metal linkage

(8‚BF₄). The present study reveals (i) the high reactivity (electrophilicity) of the active species

A with cis-divacant coordination sites, (ii) M-M bonds being not always essential for a

polynuclear system, (iii) dynamic behavior via reversible M-M bond cleavage and recom-

bination processes, and (iv) flexible coordination properties of the Rh₂(PNNP) system.

Introduction

motif of so-called cluster compounds. On the other hand,

little attention has been paid to polynuclear transition

metal organometallics not supported by metal-metal

bonds, in contrast to relevant inorganic systems such

as polyoxometalates and bioinorganic model complexes,

which have been studied extensively.²

Polynuclear transition metal complexes are expected

to display unique chemical reactivity based on a coop-

erative action of the metal centers.¹Many polynuclear

organometallic complexes are supported by metal-

metal bonds, which are regarded as a key structural

Publication on Web 12/01/2004

164 Organometallics, Vol. 24, No. 1, 2005

Tanaka et al.

Scheme 1

Herein we describe results of the study on polynuclear

rhodium complexes containing the PNNP ligand.⁴Bos-

nich reported the synthesis and some basic reactivity

of Rh- and Ir-PNNP complexes including catalytic

activity for olefin hydrogenation.³They also indicated

that the labile dirhodium tetracarbonyl complex, [Rh₂-

(PNNP)(CO)₄]BF₄(1‚BF₄), served as a precursor for the

cis-divacant active species A, but detailed reaction

features of the intriguing dinuclear system with organic

substrates were left to be studied. This contribution is

divided into three parts, which deal with (I) dinuclear

complexes formed by interaction of 1‚BF₄with hydro-

carbyl fragments and donor molecules, (II) unique

tetranuclear systems resulting from dimerization of the

Rh₂(PNNP) fragment, and (III) coordination features of

the PNNP ligand.

Results and Discussion

I. Dinuclear Complexes. A variety of dinuclear

adducts were obtained upon treatment of 1‚BF₄with

hydrocarbyl fragments and donor molecules.

Dinuclear µ-Acetylide Complexes 2. (i) Synthe-

sis. Treatment of 1‚BF₄with lithium acetylide in THF

readily produced neutral dinuclear complexes with

substituted alkynyl groups 2b-e as yellow crystals, and

the parent ethynyl complex 2a was obtained via desi-

lylation of the SiMe₃derivative 2b in THF in the

presence of a catalytic amount of NBu₄‚F (Scheme 2).

Reaction of 1‚BF₄with 1-alkyne gave tetranuclear

complexes 8‚BF₄as described below (see section II).

We have been interested in the latter type of poly-

nuclear organometallic systems and initiated the re-

search on those containing the PNNP ligand [3,5-

bis(diphenylphosphinomethyl)pyrazolato] developed

twenty years ago by Bosnich (Scheme 1).³Doubly donor-

tethered heteroaromatics such as pyrazole and py-

ridazine derivatives are versatile dinucleating ligands

with two chelating coordination sites,^2cand the PNNP

ligand belongs to this class of ligand. It is expected that

this type of ligand (i) holds the two metal centers

bridged by it at a distance longer than M-M bonding

interaction and (ii) is so flexible as to fit substrates of

various sizes by adjusting the M‚‚‚M separation. Fur-

thermore, (iii) regio- and stereochemistry of other ancil-

lary ligands as well as coordinatively unsaturated sites

can be controlled. For example, if two coordinatively

unsaturated sites can be generated on cis sites (e.g., A

in Scheme 1), they may cooperate to interact with or

activate a substrate in a concerted manner. In addition,

(iv) the chelating coordination suppresses fragmentation

of the dinuclear structure. These features are in contrast

to those of M-M bonded systems, where (i) the M-M

distance is fixed, (ii) M-M bonds may be cleaved during

a reaction course, and (iii) rotation around the M-M

bond may lead to a trans conformation, which is not

suitable for a concerted interaction.

Scheme 2

(ii) Spectroscopic Characterization. Spectroscopic

data for 2 are summarized in Table 1. The dinuclear

µ-acetylide complexes 2 show dynamic behavior, which

is interpreted in terms of the conventional windshield

wiper motion mechanism as characterized by low-

1

temperature H, ³¹P, and ¹³C NMR measurements. As

a typical example, NMR charts for the SiMe₃derivative

2b‚BF₄are shown in Figure 1. The single doublet H

(1) Abel, E. W.; Stone, F. G. A.; Wilkinson, G. Comprehensive

Organometallic Chemistry II; Pergamon: Oxford, 1995; Vols. 3-10.

Dyson, P. J.; McIndoe, J. S. Transition Metal Carbonyl Cluster

Chemistry; Gordon and Breach Science: Amsterdam, 2000. Shriver,

D. F.; Kaesz, H. D.; Adams, R. D. The Chemistry of Metal Cluster

Complexes; VCH: New York, 1990.

(2) (a) See, for example, special issue for polyoxometalates: Chem.

Rev. 1998, 98, 1-388. (b) Yamase, T: Pope, M. T. Polyoxometalate

Chemistry for Nano-Composite Design (Nanostructure Science and

Technology); Kluwer Academic: New York; 2002. (c) Gavrilova, A. L.;

Bosnich, B. Chem. Rev. 2004, 104, 349.

(3) (a) Schenk, T. G.; Downs, J. M.; Milne, C. R. C.; Mackenzie, P.

B.; Boucher, H.; Whelan, J.; Bosnich, B. Inorg. Chem. 1985, 24, 2334.

(b) Schenk, T. G.; Milne, C. R. C.; Sawyer, J. F.; Bosnich, B. Inorg.

Chem. 1985, 24, 2338. See also: (c) Bosnich, B. Inorg. Chem. 1999,

38, 2554.

1

and ³¹P NMR signals for the CH₂P moiety (δ_H3.79 (J_H-P

) 10.2 Hz; δ_P49.7 (J_P-Rh) 142 Hz)). observed at room

temperature separate into two doublets at low temper-

atures (below -60 °C), and in accord with this change,

coupling of the acetylide δ_C(C_R) signal changes from a

triplet-of-triplets (J_C-Rh) 54 Hz, J_C-P) 27 Hz; coupling

with two equivalent RhP fragments) to a doublet-of-

(4) Part of the results were reported as preliminary communications.

Tanaka, S.; Akita, M. Angew. Chem., Int. Ed. 2001, 40, 2865. Tanaka,

S.; Dubs, C.; Inagaki, A.; Akita, M. Organometallics 2004, 23, 317.

Table 1. Selected Spectroscopic Data for [(µ-X)Rh₂(PNNP)(CO)₂]ⁿ⁺and [(µ₄-X)Rh₄(PNNP)₂(CO)₄]^{n+ a}

¹H NMR (δ_H)

¹³C{¹H} NMR (δ_C)

³¹P NMR

IR (cm^-1/KBr)

complex

2a^h

X

CH₂P^b

pz^c

X

CH₂P^d

pz^e

CO^f

194.7

(δ_P)^g

ν_CO

ν_X

CtC-H

4.12

3.85

6.03

87.2

35.4

(27.6)

99.0 (11.1)

153.7 (9.2, 3.7)

48.5

(139.9)

1976,

1964

3235

1859

(1H, m)

(10.8)

(dd, 233,ⁱ18.4,^j

(64.3, 14.8)

9.2,^kC_â)

121.1

(tt, 57.9,^j28.0,^kC_R)

0.94 (s, SiMe₃),

102.7

2b^h

CtC-SiMe₃

0.41

(9H, s)

3.79

(10.2)

6.03

33.7

(25.7)

98.0 (11.9)

152.4 (7.3, 3.7)

193.6

(65.2, 13.8)

49.7

(142)

1966,

1948

1909

(tt, 18.4,^j9.2,^kC_â)

147.3

(tt, 55.1,^j24.8,^kC_R)

106.6 (m, CtC),

107.4 (br, CtC)^l

n

2c^h

2d^h

2e^h

CtC-Bu

CtC-Ph

CtC-tol

l

3.80

(10.9)

3.77

(10.6)

3.80

(10.6)

6.01

5.98

6.04

35.1

(25.7)

98.9 (11.9)

153.1 (9.2, 3.6)

195.3

(66.2, 14.7)

49.8

(141.0)

51.0

(142.0)

51.1

(143.1)

1972,

1944

1975,

1963

1975,

1966

m

a

2.37

21.5 (s, Me),

34.5

(27.5)

99.1 (11.9)

153.1 (9.2, 3.7)

194.6

(67.6, 13. 8)

(3H, s)^a

105.7 (br, CtC),

117.5 (br, CtC)^a

3^o

CHdCH₂

5.67 (1H)^p

6.52 (1H)^p

3.60

(dd, 16.5, 7.3)

3.95

6.05

49.9

(131.5)

1972

(dd, 16.5, 13.5)

3.72

(11.6)

3.81

(11.2)

4a^h

4b^h

Co(CO)₄

Mn(CO)₅

5.99

6.02

210.4

(br, Co-CO)

226.1

33.9

(26)

34.7

(27.5)

100.7 (12),

154.3 (9, 4)

100.5 (11.9)

154.4

(br. d, 7.3)

102.3 (11.0)

156.5 (br. d, 7.3)

192.1

(64.3, 12.9)

193.5

44.0

(172.6)

42.6

1972, 1911,

1856

2024, 1997,

1968, 1915,

1877, 1831

1989

(s, Mn-CO)

(66.1, 12.9)

(171.5)

o

5‚BF₄

(NC-Cy)₂

q

3.70 (dd,

17.0, 5.4, 2H)

4.2-4.3 (m, 2H)

4.25

(11.6)

4.28

6.33

q

30.9

(27.6)

191.0

(65.3, 15.6)

49.4

(129.4)

2187

o

6‚BF₄

SMe₂

3.48

6.30

6.28

32.5

(s, Me)

15.4 (s, Me)

25.8 (s, CH₂)

76.9 (br, tC)

35.4

(30.7)

35.4

101.3 (12.5)

154.2 (m)

101.3 (11.9)

154.4 (br)

191.5

(66.9, 15.1)

191.0

49.0

(d, 159.9)

45.3

1979

1988

(6H, s, Me)

1.60 (t, 7.3,

6H, Me)

3.49 (q, 7.3,

4H, CH₂)

6.82

(1H, s)

0.26

(9H, s)

o

7‚BF₄

3-hexyne

(11.6)

(29.4)

(67.0, 13.8)

(164.1)

h

8a‚BF₄

CtC-H

3.79

(11.2)

3.81

6.13

6.19

93.8 (brd, 237,ⁱC_â)

r

3.64 (s, SiMe₃),

77.6 (br, C_R),

116.8 (br, C_â)

77.4 (br, C_R),

118.3 (br, C_â),

33.8

(29.3)

32.9

99.9 (11.9)

153.4 (9.2, 3.7)

100.0 (11.9)

190.5

(61.5, 15.6)

191.1

54.6

(179.9)

55.1

1977

1982

h

8b‚BF₄

CtC-SiMe₃

(11.6)

(29.4)

152.9 (8.2, 4.6)

(62.4, 14.7)

(183.0)

h

8c‚BF₄

CtC-Bu

s

3.79

(11.2)

6.19

33.5

(29.4)

100.1 (11.9)

153.5 (9.2, 3.7)

191.2

(62.4, 14.8)

56.7

(184.0)

1973

166 Organometallics, Vol. 24, No. 1, 2005

Tanaka et al.

doublets (J_C-Rh) 104 Hz, J_C-P) 47 Hz). The J_C-Rhand

J_C-Pvalues observed at room temperature should be

averaged values of those with the σ- and π-bonded RhP

units observed at -80 °C, and therefore, coupling

constants with the π-bonded unit at -80 °C are esti-

mated to be J_C-Rh) ∼4 Hz and J_C-P) ∼7 Hz,

suggesting an interaction of C_Rwith the σ-bonded Rh

atom much stronger than that with the π-bonded one.

1

The H NMR signal for the ethynyl hydrogen atom

in 2a is located at δ_H4.12, and the R- and â-carbon

signals are unequivocally assigned on the basis of the

J_C-Hcoupling constants [at rt: δ_C(C_â) 87.2 (dd, J_C-H

)

233 Hz, J_C-Rh) 18.4 Hz, J_C-P) 9.2 Hz), δ_C(C_R) 121.1

(tt, J_C-Rh) 57.9 Hz, J_C-P) 28.0 Hz; J_C-Hcoupling for

C_Ris obscured by the J_C-Rhand J_C-Pcouplings)]. The

large J_C-Hvalue for the C_âsignal clearly indicates its

sp-character. The acetylide carbon signals for the SiMe₃

derivative 2b are also located at δ_C(C_â) 102.7 (tt, J_C-Rh

) 18.4 Hz, J_C-P) 9.2 Hz) and δ_C(C_R) 147.3 (tt, J_C-Rh

)

55.1 Hz, J_C-P) 24.8 Hz). The deshielding of the δ_C(C_R)

signal could be attributed to contribution of a vinylidene

resonance structure (2′; Scheme 2). Because, however,

the difference in δ_C(C_R) and δ_C(C_â) is not so large and

the δ_C(C_R) signals are located in much higher field

compared with those of typical vinylidene complexes (δ_C

>250),⁵the contribution of 2′ is not significant, as is

consistent with the X-ray results (see below). The

δ_C(C_R) and δ_C(C_â) signals for 2c,e appear in a narrow

range as broad multiplets due to weak couplings

[δ_C106.6, 107.4 (2c); 105.7, 117.5 (2e)] and are tenta-

tively assigned following the trends observed for 2a,b.⁶

The neutral acetylide complexes 2 show two ν_CO

vibrations in the region of 1950-1980 cm^-1, which are

lower in energies than those of the starting cationic

species 1‚BF₄[2086, 2019, 1977 cm^-1(KBr)].

(iii) Crystallographic Characterization. All di-

nuclear µ-acetylide complexes obtained (2a-e) are

characterized by X-ray crystallography. Selected struc-

tural parameters are summarized in Table 2, and

ORTEP views are shown in Figure 2.

The acetylide ligand bridges the two Rh centers in a

µ-η¹:η²-fashion as usually observed for dinuclear µ-acetyl-

ide complexes.⁷The Rh coordination planes are virtually

planar except for 2d (â₂) 24.8°), as judged by the small

dihedral angles (â: 2-9°; see Scheme 3) made by the

P1-Rh1-N1 and C1-Rh1-C01 planes (â₁for Rh1) and

the P2-Rh2-N2 and M-Rh2-C02 planes (â₂for Rh2;

M: midpoint of CtC). The CtC lengths (1.20-1.25 Å)

are comparable to those in previously reported related

complexes.⁸The CtC-R groups are η¹-bonded to Rh1

and η²-bonded to Rh2, and accordingly, the C1-Rh1

distances are substantially shorter than the C1-Rh2

distances, as is consistent with the different coupling

constants with the two Rh centers mentioned above.

Despite the support by the dinucleating PNNP ligand,

the differences in the Rh-C distances (∆₁: 0.2-0.3 Å)

(5) Bruce, M. I. Chem. Rev. 1991, 91, 197. Bruce, M. I.; Swincer, A.

G. Adv. Organomet. Chem. 1983, 22, 59. See also: Bruce, M. I. Chem.

Rev. 1998, 98, 2797.

(6) Low solubility of the Ph derivative 2d hampered obtaining a ¹³

C

NMR spectrum of a sufficient S/N ratio.

(7) Lotz, S.; van Rooyen, P. H.; Meyer, R. Adv. Organomet. Chem.

1995, 37, 219.

(8) Hartley, F. R.; Patai, S.; Rappoport, Z. The Chemistry of the

Metal-Carbon Bond; Wiley: New York, 1982. See also references

therein.

Polynuclear Rhodium Complexes

Organometallics, Vol. 24, No. 1, 2005 167

Figure 1. Variable-temperature ¹H (400 MHz), ³¹P (162 MHz), and ¹³C NMR spectra (100 MHz) for 2b observed in CD₂-

Cl₂.

Table 2. Selected Structural Parameters for (µ-CtC-R)Rh₂(PNNP)(CO)₂, 2^a

complex

R

2a

H

2b^b

SiMe₃

2c

n-Bu

2d

Ph

2e

tol

C1-C2

1.215(3)

2.030(2)

2.399(2)

2.294(2)

2.2825(6)

2.2210(6)

2.028(2)

2.061(2)

1.819(3)

1.820(3)

1.147(4)

1.150(4)

3.7718(2)

172(1)

1.23(1)

2.028(7)

2.395(7)

2.314(7)

2.297(2)

2.234(2)

2.016(5)

2.061(5)

1.827(7)

1.815(7)

1.152(8)

1.145(9)

3.7939(9)

167.6(6)

149.7(6)

30.2(2)

0.367

1.215(9)

2.020(6)

2.399(8)

2.321(7)

2.287(2)

2.229(2)

2.028(5)

2.078(4)

1.807(7)

1.833(6)

1.164(9)

1.127(8)

3.7892(9)

167.1(7)

150.6(7)

29.8(2)

0.379

1.20(2)

2.06(1)

2.358(9)

2.31(1)

2.276(3)

2.253(3)

2.025(7)

2.057(7)

1.81(1)

1.83(1)

1.15(1)

1.14(1)

3.6588(9)

172(1)

161(1)

29.7(4)

0.30

1.252(8)

2.098(5)

2.319(5)

2.313(6)

2.251(2)

2.220(2)

2.029(5)

2.052(5)

1.815(6)

1.824(7)

1.159(8)

1.153(9)

3.6882(5)

156.5(5)

164.3(6)

31.4(2)

0.221

1.205(5)

2.061(3)

2.338(2)

2.388(3)

2.2768(8)

2.2405(7)

2.030(2)

2.066(2)

1.826(3)

1.825(3)

1.145(5)

1.149(4)

3.6169(4)

168.7(3)

164.3(3)

29.5(1)

0.277

-0.050

40.27(6)

20.9(1)

34.0(1)

3.9

2.1

-0.9(4)

Rh1-C1

Rh2-C1

Rh2-C2

Rh1-P1

Rh2-P2

Rh1-N1

Rh2-N2

Rh1-C01

Rh2-C02

C01-O01

C02-O02

Rh1‚‚‚Rh2

Rh1-C1-C2

C1-C2-R

C1-Rh2-C2

128

29.7(4)

0.369

0.105

18.2(4)

-10.55(8)

12.81(9)

5.2

∆

2

1

∆

0.081

0.078

0.05

0.006

θ₁

R₁

R₂

â₁

â₂

γ₁₂

18.7(1)

-19.2(2)

21.5(2)

6.8

5.6

21.5(6)

20.4(1)

-19.5(2)

23.1(2)

7.7

6.7

22.0(6)

38.8(2)

25.1(4)

24.1(3)

3.0

5.3

-1(1)

51.8(1)

-1.6(2)

2.4(2)

9.6

9.4

14.8(2)

24.8

9.8(7)

^aInteratomic distances in Å and bond angles in deg. ∆₁(in Å): d(C1-Rh1) - d(C1-Rh2). ∆₂(in Å): d(Rh2-C1) - d(Rh2-C2). θ₁(in

deg): dihedral angle between the planes defined by Rh2-P2-N2-C02 and Rh2-C1-C2. For R - γ (in deg), see Scheme 3. â₂(in deg):

dihedral angle between the P2-Rh2-N2 and M-Rh2-C02 planes (M: midpoint of C1-C2). ^bTwo independent molecules.

and the Rh‚‚‚Rh separations (3.6-3.8 Å) are similar to

those in dinuclear µ-acetylide complexes without any

additional bridging interaction [e.g., (µ-η¹:η²-CtC-R)-

{Fe(η⁵-C₅Me₅)(CO)₂}₂]⁺BF₄: ∆₁) 0.308 Å (R ) H), 0.223

Å (R ) Ph); M‚‚‚M ) 4.038(1) Å (R ) H), 4.012(1) Å (R

) Ph)].⁹As for the η²-coordination to Rh2, differences

in the Rh2-C1,C2 distances (∆₂) are rather small (0.105

to -0.050 Å), indicating a virtually symmetrical η²-

coordination with a minor contribution of the unsym-

metrical vinylidene form 2′ (Scheme 2).

A difference is found for the η²-coordination geometry

of the CtC moiety with respect to the Rh2 coordination

plane. The dihedral angles (θ₁) made by the Rh2

(9) Akita, M.; Terada, M.; Oyama, S.; Moro-oka, Y. Organometallics

1990, 9, 816.

168 Organometallics, Vol. 24, No. 1, 2005

Tanaka et al.

Figure 2. Molecular structures for 2a-e drawn with thermal ellipsoids at the 30% probability level. For 2b one of two

independent molecules is shown.

Scheme 3

coordination plane (Rh2-P2-N2-C02) and the (CtC)-

Rh2 plane (Rh2-C1-C2) are variable in the range 18-

52°. As the size of the substituent (R) becomes larger,

the CtC part tends to be arranged perpendicular

(twisted) with respect to the Rh2 coordination plane. It

is reported that, in the case of square-planar η²-alkene

or -alkyne complexes of d⁸-metals, the parallel and

perpendicular arrangements are not so different in

energy, and the orientation of the alkene and alkyne

ligand is dominated by steric repulsions with other

ancillary ligands.¹⁰In the case of 2, steric repulsion

(10) Albright, T. A.; Hoffmann, R.; Thibault, J. C.; Thorn, D. L. J.

Am. Chem. Soc. 1979, 101, 3801.

Polynuclear Rhodium Complexes

Organometallics, Vol. 24, No. 1, 2005 169

between the acetylide substituent and the CO ligand

attached to Rh2 (C02-O02) should cause the deforma-

tion from an in-plane arrangement to a perpendicular

form, although the Rh1-C1 σ-bond forces an in-plane

conformation. The steric repulsion is also released by

twisting the Rh-N-N-Rh linkage (γ: Scheme 3), in

particular, for 2b with the bulky SiMe₃group [γ ∼22°

> 14-1° (2a,c-e)].

The extent of twisting of the Rh-N-N-Rh linkage (γ)

is in the middle of those of the other two categories, but

the Rh2 coordination plane is most distorted, as is

evident from the largest θ₁and â₂values.

Thus the various conformations observed for 2 are a

result of the steric repulsion between the µ-acetylide

ligand and the Rh₂(PNNP)(CO)₂backbone (in particular,

CO2-O02) under the constraint of the Rh‚‚‚Rh separa-

tion being ca. 3.7 Å. The steric repulsion is released by

a combination of (i) puckering of the azaphospharho-

dacyclopentenes (R), (ii) distortion of the Rh coordination

structure from a square-planar geometry (â), (iii) twist-

ing of the Rh-N-N-Rh linkage (γ), and (iv) twisting

of the CtC moiety with respect to Rh2 (θ₁). The

conformation of the Ph rings should also be influenced

by intermolecular interactions, including π-π stacking

and C-H π-interaction,¹³because the aryl derivatives

2d,e with essentially the same steric and electronic

demands adopt considerably different backbone struc-

tures.

Taking into account the conformations observed for

2, their dynamic behavior can be discussed in detail

(Scheme 4). Averaging of all four CH₂P signals (¹H

NMR) at the first exchange limit could be explained by

a combination of two intermediates, B and C. The

windshield wiper motion proceeds via the C_2v-sym-

metrical intermediate B (process a), and through this

process, all four CH₂P protons become equivalent at

room temperature. The decoalescence of the CH₂P

signals of 2b at low temperatures mentioned above

(Figure 1a: single doublet f two doublets) is consistent

with the situation, where process a is frozen out, but

flipping of the acetylide ligand above and below the

PNNP plane (process b) is still ongoing (H_a) H_b* H_c

) H_d), because (i) at the slow exchange limit of the two

processes, all four CH₂P protons should be inequivalent

and (ii) slowing process a causes the appearance of two

³¹P NMR signals at low temperatures (Figure 1), which

is the case (cf. one signal for slowing process b). This

result reveals that process a is higher in energy than

process b. The acetylide ligand in B loses the π-interac-

tion in contrast to that in C, in which the µ-η¹:η²-

coordination of the acetylide ligand is retained.

In contrast to the similar (µ-CtC-R)Rh₂core struc-

tures in 2, the structures of the Rh₂(PNNP)(CO)₂

backbone, in particular, the arrangement of the phenyl

rings attached to the P atoms, are considerably different

and can be classified into three groups: apparently C_s-

(type I: 2c,e), C₂- (type II: 2a,b), and C_2v-symmetrical

structures (type III: 2d) (Scheme 3).¹¹The three

structural types can be characterized on the basis of the

conformation of the five-membered azaphospharhoda-

cyclopentenes fused to the central pyrazolyl ring. The

azaphospharhodacyclopentene in the Rh₂(PNNP) part

may adopt an envelope-like conformation in a manner

similar to cyclopentene,¹²where the sp³-hybridized

phosphorus atom is bent out of the virtually planar Rh-

NdC-CH₂part containing the CdN functional group,

and the extent of the bending can be estimated by the

dihedral angle between the Rh-P-CH₂plane and the

Rh-NdC-CH₂plane (R₁for Rh1 and R₂for Rh2;

Scheme 3). The ( sign refers to the direction of the

puckering of the metallacycle with respect to the

orientation of the µ-acetylide ligand. A positive value

corresponds to the situation where the metallacycle is

puckered in the direction opposite the projection of the

acetylide, whereas a negative value refers to the con-

formation where the metallacycle is puckered in the

same direction as the projection of the acetylide. On the

basis of the R₁and R₂values, the three structural types

are characterized as follows: type I (R₁, R₂>0), type II

(R₁<0, R₂>0), and type III (R₁, R₂∼0). In the case of

type I complexes (2c,e), the two metallacycles are folded

away from the projection of the acetylide ligand. As a

result, two phenyl groups are projected in the downward

axial direction and the other two phenyl groups occupy

the equatorial positions, leading to a C_s-symmetrical

backbone structure. Because the strain may be relieved

by the puckering of the metallacycles, no significant

distortion is observed for the Rh-N-N-Rh moiety, as

judged by the very small γ values, indicating a planar

conformation for the central Rh₂moiety. Contrastingly,

in the case of type II complexes (2a,b), the two metal-

lacycles are puckered in the opposite directions; the

Rh2-metallacycle is puckered downward as observed for

type I complexes, whereas the Rh1-metallacycle is

puckered upward. As a result, the Rh-N-N-Rh link-

age (γ) is twisted to a considerable extent to lead to the

C₂-symmetrical Rh₂(PNNP)(CO)₂backbone, which causes

projection of two phenyl groups to the opposite axial

directions. The type III complex (2d) contains essentially

planar metallacycles, which cause projection of the four

phenyl groups in the diagonal directions, leading to the

apparent C_2v-symmetrical Rh₂(PNNP)(CO)₂backbone.

Vinylation and Metalation of 1‚BF₄. The success-

ful synthesis of the µ-acetylide complexes 2 was followed

by attempts at introduction of sp²- and sp³-hybridized

hydrocarbyl fragments. While an unstable µ-vinyl com-

plex 3 was obtained and characterized as described

below, attempted alkylation (e.g., with n-BuLi) resulted

in intractable mixtures of products. Then metalation of

1‚BF₄with [Co(CO)₄]^-and [Mn(CO)₅]^-, which are

14

isolobal with the alkyl anion, was examined.

(i) µ-Vinyl Complex [(µ-CHdCH₂)Rh₂(PNNP)-

(CO)₂], 3. Reaction of 1‚BF₄with vinyllithium generated

by treatment of Sn(CHdCH₂)₄with n-BuLi gave the

yellow product 3 quantitatively (Scheme 5). It was

essential to use a halide-free alkyllithium reagent.¹⁵

Otherwise 1‚BF₄readily reacted with the halide anion

present in the reagent to give the µ-halo complex (µ-X)-

(11) Conformation of chelating ligands has been discussed in detail

for many complexes. See, for example, for asymmetric catalysis:

Brown, J. M.; Chalonaer, P. A. In Homogeneous Catalysis with Metal

Phosphine Complexes; Pignolet, L. H., Ed.; Plenum: New York, 1983.

(12) McMurry, J. Organic Chemistry, 6th ed.; Brooks/Cole: Belmont,

2003.

(13) Jorgensen, W. L.; Severance, D. L. . J. Am. Chem. Soc. 1990,

112, 4768.

(14) Hoffmann, R. Angew. Chem., Int. Ed. 1982, 21, 711. Albright,

T. A.; Burdett, J. K.; Whangbo, M.-H. Orbital Interactions in Chemistry;

Wiley: New York, 1985.

(15) Juenge, E. C.; Seyferth, D. J. Org. Chem. 1961, 26, 563.

170 Organometallics, Vol. 24, No. 1, 2005

Tanaka et al.

Scheme 4

Scheme 5

(4a), Mn/7 (4b)]. Reaction of 1‚BF₄with metalates

PPN[M(CO)_n] readily produced trinuclear adducts 4a

and 4b (Scheme 5). Composition of the adducts (PNNP)-

Rh₂M(CO)_n+2[M/n ) Co/4 (4a), Mn/5 (4b)] is supported

by FD-MS data, and semibridging CO vibrations (∼1850

cm^-1) suggest formation of metal-metal bonds. X-ray

crystallography (Figure 3 and Table 3) reveals the

trinuclear structures consisting of an open Rh-M-Rh

Rh₂(PNNP)(CO)₂preferentially. Low solubility of 3 in

organic solvents and decomposition upon attempted

dissolution in CH₂Cl₂and acetone hampered further

purification of 3, which was characterized by NMR (¹H,

³¹P) and IR.

1

The vinyl-methylene H NMR signals are located at

δ_H5.72 (ddt, J ) 21.0, 6.0, 1.4 Hz) and 6.52 (apparently

sextet, J ) 5.8 Hz), which appear as complicated signals

owing to coupling not only with each other but also with

the Rh and P nuclei, but the vinyl-methine proton signal

should overlap with the aromatic proton signals. At

room temperature only one ³¹P NMR signal is observed,

suggesting the occurrence of a dynamic behavior. Be-

1

cause, however, the CH₂P H NMR signals appear as

two separate dd signals with geminal CH₂coupling [δ_H

3.95 (J_H-H) 16.5 Hz, J_H-P) 13.5 Hz), 3.60 (J_H-H

)

16.5 Hz, J_H-P) 7.3 Hz)] even at room temperature (at

the first exchange limit), a windshield wiper motion

should be responsible for the dynamic process and

flipping of the vinyl ligand (similar to process b in

Scheme 4) is not working.

(ii) Trinuclear Rh₂M Complexes with a Bent

Rh-M-Rh Array, (PNNP)Rh₂M(CO)_n[M/n ) Co/6

Figure 3. Molecular structures for 4a and 4b drawn with

thermal ellipsoids at the 30% probability level.

Polynuclear Rhodium Complexes

Organometallics, Vol. 24, No. 1, 2005 171

Table 3. Selected Structural Parameters for

(PNNP)Rh₂M(CO)_n+2, 4^a

Scheme 7

4a

4b

M(CO)_n

Rh1-M

Rh2-M

Rh1-P1

Rh2-P2

Rh1-N1

Rh1-CO

Co(CO)₄

Mn(CO)₅

2.6688(8)

2.747(1)

2.647(1)

2.7381(7)

2.255(1)

2.262(2)

2.252(2)

2.282(1)

2.070(5)

2.070(4)

1.840(8) (C1)

2.345(6) (C5)

2.057(5)

1.835(6) (C1)

2.407(7) (C6)

2.068(6)

Rh2-N2

Rh2-CO

1.842(7) (C2)

2.308(6) (C6)

1.790(6) (C3)

1.778(8) (C4)

1.794(6) (C5)

1.807(6) (C6)

1.847(9) (C2)

2.255(5) (C7)

1.848(6) (C3)

1.825(7) (C4)

1.814(9) (C5)

1.853(7) (C6)

1.884(6) (C7)

1.13-1.17

isolobal M(CO)_nfragments gives 4 with a dative M-Rh

interaction, which would correspond to an agostic

interaction¹⁷in a putative alkyl complex.

Introduction of Neutral Donor Molecules into

the Rh₂(PNNP) System. In addition to the reactions

of 1‚BF₄with anionic species described above, interac-

tion with neutral donors was also examined, and the

reaction products are shown in Scheme 7.

M-CO

C-O

M-C-O (µ-CO)

1.12-1.15

163.4(6) (CO5)

162.2(6) (CO6)

117.4(4) (CO5)

118.8(5) (CO6)

79.1(2) (CO5)

79.0(2) (CO6)

3.8131(6)

166.4(7) (CO6)

160.7(4) (CO7)

114.4(6) (CO6)

117.0(4) (CO7)

79.1(2) (CO6)

82.3(2) (CO7)

3.8846(5)

Rh-C-O (µ-CO)

M-C-Rh

Reaction of 1‚BF₄with cyclohexylisonitrile readily

afforded the substituted product 5 in quantitative yield.

An IR spectrum of 5 contains a ν_CtNvibration (2187

cm^-1) in addition to a ν_CtOband (1989 cm^-1), and FD-

MS data are consistent with the composition. ¹H NMR

signals for the CH₂P moiety appear as two dd signals

[2H × 2; J_H-P, J_H-H(geminal)], in contrast to the single

doublet signal (4H; J_H-P) observed for 1‚BF₄, while a

symmetrical structure is suggested by the single ³¹P

NMR signal. These NMR features are consistent with

a structure resulting from replacement of the inner CO

ligands trans to P in 1‚BF₄. The sterically congested

situation brought about by introduction of the bulky

R-NC ligands should be relieved by taking a C₂-

symmetrical twisted conformation to make the CH₂P

protons diastereotopic, and the bulky R-NC ligands

should hinder switching of their sites, which should lead

to NMR features consistent with a C_2v-symmetrical

structure. On the other hand, replacement of the outer

CO ligands trans to N would not affect the situation of

the central Rh₂(CO)₂part leading to a single doublet

CH₂P signal (¹H NMR) as observed for 1‚BF₄, but this

is not the case, as is evident from the NMR data. It is

therefore concluded that the inner CO ligands are more

labile than the outer CO ligands, as mentioned in the

Introduction, and this should be attributed to the trans

effect of the P atom, which is stronger than that of the

N atom of the pyrazolyl moiety.¹⁸Thus the carbonyl

complex 1‚BF₄can be regarded as a precursor for A with

cis-divacant sites (Scheme 1).

Rh1‚‚‚Rh2

Rh1-M-Rh2

91.67(3)

90.19(3)

^aInteratomic distances in Å and bond angles in deg.

Scheme 6

linkage with bent angles of 91.67(3)° (4a) and 90.19(3)°

(4b) and the Rh‚‚‚Rh separations of 3.8131(6) Å (4b) and

3.8846(5) Å (4b), which are much longer than a Rh-

Rh bond (<3.3 Å; see below). The two complexes are

virtually isostructural except the number of η¹-CO

ligands attached to M and the bulky M(CO)_nfragments

lead to type I conformation for the PNNP ligands. The

semibridging CO ligands weakly interact with the Rh

centers, as judged by (i) the Co-C-O angles (160-166°)

larger than the Rh‚‚‚C-O angles (114-119°) and (ii) the

Rh‚‚‚C separations (>2.2 Å) much longer than the Co-C

lengths (∼1.8 Å). Accordingly, ¹³C NMR signals for the

CO ligands attached to Rh and M are observed sepa-

rately, and the M(CO)_nmoieties show single broad

resonances owing to site exchange between the terminal

and semibridging CO ligands reaching the fast exchange

limit at ambient temperature. It is also notable that the

symmetrical structures of 4 result from resonance

hybridization of two canonical structures with a dative

MfRh interaction,¹⁶which leads to an electron-precise

46e configuration [18e (M) + 16e (square-planar Rh) ×

2 - 2e (Co-Rh bond) × 2 ) 46e] (Scheme 6). Although

an alkyl complex cannot be obtained, introduction of the

Dimethyl sulfide, SMe₂, bearing two lone pair elec-

trons, also readily reacted with 1‚BF₄to give a brown

product 6‚BF₄(Scheme 7). Replacement of CO ligands

is evident from (i) the single CO vibration, (ii) appear-

ance of NMR signals for SMe₂, and (iii) the ESI-MS peak

for the cationic part [m/z ) 788: (SMe₂)Rh₂(PNNP)-

(CO)₂]. Taking into account the result of the reaction

with isonitrile and the stoichiometry [1 (SMe₂):1 (Rh₂-

(PNNP))], the two inner CO ligands in 1‚BF₄should be

replaced by the two lone pair electrons of SMe₂. It is

(17) Brookhart, M. S. Prog. Inorg. Chem. 1988, 36, 1.

(18) Yamamoto, A. Organotransition Metal Chemistry; Wiley-Inter-

science: New York, 1986. Collman, J. P.; Hegedus, L. S.; Norton, J.

R.; Finke, R. G. Principles and Applications of Organotransition Metal

Chemistry, 2nd ed.; University Science Books: Mill Valley, CA, 1987.

Crabtree, R. H. The Organometallic Chemistry of the Transition Metals,

3rd ed.; Wiley-Interscience: New York, 2001.

(16) See, for example: Jiang, F.; Yap, G. P. A.; Pomeroy, R. K.

Organometallics 2002, 21, 773. See also references therein.

172 Organometallics, Vol. 24, No. 1, 2005

Tanaka et al.

Scheme 8

Scheme 9

also revealed that the coordinated SMe₂ligand ex-

changes with an external SMe₂molecule at room

temperature, because (i) a single Me₂S resonance (¹H

NMR) is observed even in the presence of excess SMe₂

and (ii) addition of an excess amount of SMe₂causes a

shift of the SMe₂signal to the signal for free SMe₂.

3-Hexyne, an internal acetylene, did not react with

1‚BF₄at ambient temperature, but refluxing in acetone

led to formation of the brown product 7‚BF₄(Scheme

7), which was so labile that attempted purification by

crystallization caused elimination of the 3-hexyne ligand

to give an intractable mixture of products. Spectroscopic

data of 7‚BF₄consistent with apparent C_2v-symmetrical

structural features (a single set of NMR resonances for

the CH₂P and C-CH₂CH₃moieties) suggest symmetri-

cal µ-η²:η²-coordination of 3-hexyne, and the coordinated

alkyne carbon signal is located at δ_C76.9, which is

comparable to that in (µ-η²:η²-H-CtC-H)Co₂(CO)₆(δ_C

70.8).¹⁹Reaction of 1‚BF₄with R-CtC-R (R) Ph,

COOMe) was slow and sluggish, and after a prolonged

reaction period complicated mixtures were obtained. On

the other hand, 1-alkynes afforded tetranuclear product

8‚BF₄resulting from tC-H cleavage as described

below.

Complex 1‚BF₄was inert toward alkenes such as

1-hexene and cyclopentene, whereas catalytic transfor-

mations were observed for dienes (Scheme 8). 1,3-

Cyclohexadiene underwent disproportionation at am-

bient temperature to give an equimolar mixture of

benzene and cyclohexene in the presence of a catalytic

amount of 1‚BF₄. Dimerization of norbornadiene (NBD)

was also catalyzed by the action of 1‚BF₄. The reaction

in THF at room temperature caused quantitative con-

version of 13 equiv of NBD into “Binor-S” (a 4+4 adduct)

within 4 days. When the reaction was carried out in

refluxing THF, more than 150 equiv of NBD was

consumed within 10 h, but a mixture of isomeric

products containing exo- (46%) and endo-isomers (8%)

of the 4+2 adduct in addition to Binor-S (48%) was

obtained. Because no characterizable rhodium species

could be detected, the reaction mechanism remains to

be studied. Many complexes were reported to catalyze

the dimerization of NBD,²⁰but we found that Rh-diene

complexes such as [Rh(η⁴-NBD)₂]⁺and [(η⁴-NBD)₂Rh₂-

(PNNP)]⁺(precursors for 1‚BF₄) were not effective.

Reactions with butadiene, isoprene, and cyclopenta-

diene gave mixtures of unidentified organometallic

species, whereas unconjugated diene such as 1,5-cy-

clooctadiene left 1‚BF₄unaffected.

II. Tetranuclear Complexes. Unexpected formation

of unique tetranuclear adducts [(µ₄-X)Rh₄(PNNP)₂]ⁿ⁺

,

which resulted from a 1 (X):2 [Rh₂(PNNP)] coupling, was

noted for interaction of 1‚BF₄with 1-alkyne, hydro-

silane, and water, and their structures were dependent

on X.

Interaction of 1‚BF₄with 1-Alkyne Leading to

Tetranuclear µ₄-Acetylide Complexes [(µ₄-CtC-

R)Rh₄(PNNP)₂(CO)₄]BF₄, 8‚BF₄. In contrast to the

above-mentioned reactions of 1‚BF₄with substrates

containing a CtC functional group, which afforded

dinuclear products, i.e., µ-acetylide complex 2 (with

lithium acetylides) and µ-η²:η²-alkyne complex 7‚BF₄

(with 3-hexyne), interaction with 1-alkynes led to the

formation of the tetranuclear µ₄-acetylide cluster com-

plexes 8‚BF₄.

(i) Synthesis and Characterization of 8‚BF₄.

Reaction of 1‚BF₄with 1-alkyne (R ) SiMe₃, n-Bu, Ph,

p-tol) in acetone afforded deep purple-red tetranuclear

µ₄-acetylide complexes 8b-e‚BF₄as sole products in

good yields (Scheme 9).²¹Even in the presence of an

excess amount of 1-alkyne, 8b-e‚BF₄were formed as

single products. The tetranucelar complexes 8a-e‚BF₄

were also obtained by addition of the Rh₂fragment

(1‚BF₄) to the dinuclear µ-acetylide complex 2.

¹H NMR and the ESI-MS spectra of 8‚BF₄reveal

formation of cationic tetranuclear acetylide complexes

with the composition [(R-CtC)Rh₄(PNNP)₂(CO)₄]⁺

through a 1 (R-CtC):2 [Rh₂(PNNP)] coupling, but

complicated dynamic behavior is observed for them. For

example, the p-tolylethynyl complex 8e‚BF₄shows a

single ³¹P NMR resonance (δ_P56.3) at 25 °C, which

separates into two signals below -42 °C (Figure 4a).

(19) Aime, S.; Milone, L.; Rossetti, R.; Stanghellini, P. L. Inorg.

Chim. Acta 1977, 22, 135.

(20) (a) Mitsudo, T.; Suzuki, T.; Zhang, S.-W.; Imai, D.; Fujita, K,

Manabe, T.; Shiotsuki, M.; Watanabe, Y.; Wada, K.; Kondo, T. J. Am.

Chem. Soc. 1999, 121, 1839. (b) Binger, P.; Albus, S. J. Organomet.

Chem. 1995, 493, C6. (c) Gol’dshleger, N. F.; Azbel, B. I.; Isakov, Y. I.;

Shipiro, E. S.; Minachev, K. M. J. Mol. Catal. A: Chem. 1996, 106,

159. (d) Acton, N.; Ronald, J. R.; Katz, T. J.; Frank, J. K.; Maier, C.

A.; Paul, I. C. J. Am. Chem. Soc. 1972, 94, 5446. (e) Azbel, B. I.;

Gol’dshleger, N. F.; Khidekel, M. L. J. Mol. Catal. 1987, 40, 57. (f)

Kanai, H.; Watanabe, Y.; Nakayama, T. Bull. Chem. Soc. Jpn. 1986,

59, 1277. (g) Schrauzer, G. N.; Bastian, B. N.; Fosselius, G. A. J. Am.

Chem. Soc. 1966, 88, 4890. (h) Schrauzer, G. N.; Ho, R. K. Y.;

Schlesinger, G. Tetrahedron Lett. 1970, 8, 543.

(21) We also attempted synthesis of the Ir₄and Rh₂Ir₂derivatives.

Although a dinuclear acetylide complex, (µ-η¹:η²-CtC-SiMe₃)Ir₂-

(PNNP)(CO)₂, was obtained successfully, subsequent treatment with

1‚BF₄gave a mixture of hetero- and homometallic tetranuclear

complexes with the metal compositions of Ir₄, Ir₂Rh₂, and Rh₄as

observed by ESI-MS and ³¹P NMR but could not be separated owing

to their lability. This result also supports the dissociation equilibrium

of 8‚BF₄and its iridium analogue. On the other hand, reaction of [Ir₂-

(PNNP)(CO)₄]BF₄with hydrosilane resulted in Si-H oxidative addi-

tion. Dubs, C.; Inagaki, A.; Akita, M. Unpublished results.

Polynuclear Rhodium Complexes

Organometallics, Vol. 24, No. 1, 2005 173

Figure 5. Molecular structures of the cationic part of 8e‚

BPh₄drawn with thermal ellipsoids at the 30% probability

level: (a) an overview and (b) the core part.

for 8a‚BF₄(237 Hz) reveals its sp-hybridization. Similar

spectroscopic features are noted for the other products.

These spectroscopic features suggest that (i) the Rh₄

metal framework is unsymmetrical with respect to the

R-CtC moiety, but exhibits dynamic behavior, which

averages all Rh coordination environments at higher

temperatures, and (ii) the acetylide part is coordinated

to the Rh₄framework in a µ₄-η¹(C_R):η²(C_RtC_â)-fashion.

The p-tolylethynyl complex 8e⁺was characterized by

X-ray crystallography after conversion to the BPh₄salt.

ORTEP drawings for an overview and the core part of

the cationic part are shown in Figure 5, and its selected

structural parameters are listed in Table 4. Although

the four rhodium atoms are arranged in a butterfly-like

array,¹it should be noted that the two Rh‚‚‚Rh distances

[Rh1‚‚‚Rh2: 3.6767(4) Å; Rh3‚‚‚Rh4: 3.7592(6) Å] ex-

ceed the range of bonding interaction, in contrast to the

other Rh-Rh distances of ca. 2.9 Å (Table 4). The metal

array observed for 8a⁺is best described as a folded

Z-shaped linkage. As is suggested by the spectroscopic

data, the C_Ratom of the acetylide ligand forms σ-bond-

ing interactions with all four Rh centers and the CtC

moiety is π-bonded to one of the wingtip metal atoms

(Rh2), leading to the µ₄-η¹(C_R):η²(C_RtC_â)-coordination.²²

Figure 4. NMR spectra for 8a‚BF₄observed in CD₂Cl₂:

(a) variable-temperature ³¹P NMR spectra observed at 81

MHz; (b) ¹³C NMR spectra for the CtC part observed at

25 °C at 100 MHz.

The two signals with different P-Rh coupling constants

[8a⁺: 195 Hz (δ_P64.5); 171 Hz (δ_P61.4)] reveal that

8a⁺contains two types of rhodium atoms located in

different environments. No further change is observed

even when the sample is cooled to -90 °C. The fact that

the J_P-Rhvalue at 25 °C (183 Hz) is precisely the

average of the J_P-Rhvalues observed at -90 °C [(195 +

171)/2 ) 183] confirms that the single δ_Psignal at 25

°C results from coalescence of the two signals observed

at low temperatures. In accord with the changes in the

³¹P NMR spectrum, the acetylide δ_C(C_R) signal, which

appears at 25 °C as a quintet-of-quintets due to coupling

with four equivalent RhP units (Figure 4b), changes into

an irresolvable multiplet at low temperatures. The

δ_C(C_â) signal appears as a broad one irrespective of

temperatures presumably due to weaker interaction

with the [Rh₂(PNNP)]₂moiety compared with that of

1

the C_Ratom. The lack of J_C-Hcoupling for the δ_C(C_R)

Taking into account the structural features of 8‚BF₄,

a canonical resonance structure 8′‚BF₄, which is appar-

ently formed from 2 and A, can be depicted as shown

in Scheme 10. Interaction of the Rh_C-C_Rbonding

signal clearly indicates formation of an acetylide cluster

compound resulting from cleavage of the tC-H bond

in 1-alkyne, and the large J_C-Hvalue for the C_âsignal

174 Organometallics, Vol. 24, No. 1, 2005

Table 4. Selected Structural Parameters for [(µ₄-X)Rh₄(PNNP)₂(CO)₄]^{n+ a}

Tanaka et al.

complex

X

8e‚BF₄

CtC-p-tol

9

C₂

10

H

11

O

Rh1-X (X)

2.151(4) (C1), 3.257(5) (C2)

2.309(4) (C1), 2.386(5) (C2)

2.223(4) (C1), 3.194(5) (C2)

2.335(4) (C1), 2.555(5) (C2)

2.264(1), 2.235(1)

2.04(1) (C1)

2.438(8) (C1), 2.408(7) (C2)

2.06(1) (C2)

1.70(5) (H1)

2.101(8) (O0)

Rh2-X (X)

2.05(5) (H1)

2.069(8) (O0)

2.089(9) (O0)

2.085(8) (O0)

2.210(3), 2.209(3)

2.204(4), 2.210(3)

2.014(9), 2.059(7)

2.024(8), 2.046(9)

1.82(1), 1.15(2)

1.84(1), 1.15(1)

1.83(1), 1.16(1)

1.82(1), 1.14(2)

3.644(1), 3.113(1)

3.234(1), 3.149(2)

3.542(1), 3.651(2)

91.5

Rh3-X (X)

Rh4-X (X)

1.99(4) (H1)

1.91(5) (H1)

2.396(7) (C1), 2.441(8) (C2)

2.259(3), 2.230(2)

2.258(3), 2.219(2)

2.027(6), 2.079(7)

2.042(7), 2.069(8)

1.815(9), 1.15(1)

Rh1-P1, Rh2-P2

Rh3-P3, Rh4-P4

Rh1-N1, Rh2-N2

Rh3-N3, Rh4-N4

Rh1-C01, C01-O01

Rh2-C02, C02-O02

Rh3-C03, C03-O03

Rh4-C04, C04-O04

Rh1‚‚‚Rh2, Rh1‚‚‚Rh3

Rh1‚‚‚Rh4, Rh2‚‚‚Rh3

Rh2‚‚‚Rh4, Rh3‚‚‚Rh4

θ₂

2.259(2), 2.238(2)

2.238(2), 2.256(2)

2.030(6), 2.037(6)

2.021(6), 2.020(6)

1.832(9), 1.14(1)

1.838(8), 1.15(1)

1.827(8), 1.15(1)

1.856(9), 1.12(1)

3.4730(8), 2.9083(9)

2.8890(7), 3.1022(6)

2.8969(8), 3.4153(6)

86.9

14.6(2), 20.6(3)

14.9(2), 12.9(2)

14.6, 20.6

14.9, 12.9

2.8(7), 3.5(7)

2.237(1), 2.218(1)

2.062(4), 2.042(4)

2.059(4), 2.052(4)

1.846(5), 1.152(6)

1.844(5), 1.138(7)

1.83(1), 1.14(1)

1.847(5), 1.146(6)

1.81(1), 1.16(1)

1.854(7), 1.118(8)

1.81(1), 1.16(1)

3.6767(4), 2.8925(4)

2.9474(5), 2.9732(5)

4.5943(6), 3.7592(6)

3.815(2), 5.162(1)

3.7615(8), 3.7826(8)

3.173(2), 3.849(1)

R₁, R₂

15.4(2), 16.3(2)

25.4(2), 7.8(2)

18.9(4), 39.2(4)

-14.1(4), 28.1(5)

1.7, 11.0

4.5, 3.1

0(1), 15(2)

4.5(4), 8.0(4)

1.4(4), 7.7(4)

6.2, 4.5

5.0, 7.3

4(1), 8.3(9)

R₃, R₄

â₁, â₂

â₃, â₄

γ₁₂, γ₃₄

11.8(5), -7.1(6)

^aInteratomic distances in Å and bond angles in deg. For R-γ (in deg), see Scheme 3. θ₂(in deg): dihedral angles made by the Rh1-

X-Rh2 and Rh3-X-Rh4 planes.

Scheme 10

Scheme 11

above (Scheme 9). The limiting structure 8′‚BF₄indi-

cates that (i) all metal centers are Rh(I) and (ii) the Rh_A,

Rh_C, and Rh_Dparts and the Rh_Bpart are regarded as

four-coordinate structures with 16 valence electrons

(VEs) and a five-coordinate structure with 18 VEs,

respectively. Because the number of VEs in 8‚BF₄(60

VEs) is equal to that of a Z-shaped tetranuclear metal

array with three M-M bonds consisting of three 16 VE

and one 18 VE fragments, the µ₄-acetylide complex

8‚BF₄is concluded to be electron-precise.

The Z-shaped tetranuclear metal linkage is rather

rare, and to the best of our knowledge, 8‚BF₄is the first

example of a (µ₄-X)M₄-type species with such a metal

linkage. The Rh1-Rh3 connection causes a severe

distortion of the structure, as is evident from the large

R and γ values, and the conformation of the PNNP

ligand in 8‚BF₄is type I, as judged by the R values

(Table 4). The interaction mode of the acetylide ligand

in 8‚BF₄is similar to µ₄-η¹(C_R):η²(C_RtC_â)-acetylide

cluster compounds with a spiked triangular (E1) and

butterfly metal array (E2: Scheme 11).²¹Cleavage of

one or two M-M bond(s) apparently leads to the

framework observed for 8‚BF₄.

electrons with the Rh_Aand Rh_Batoms and that of the

Rh_D-C_Rbonding electrons with the Rh_Batom lead to

the tetranuclear structure 8′‚BF₄, and the interactions

should originate from the highly electrophilic nature of

A. The bonding scheme of 8′‚BF₄suggests a formation

mechanism of 8‚BF₄from 1‚BF₄and 1-alkyne as shown

in Scheme 10. At the first stage of the reaction, 1-alkyne

should be incorporated into the dirhodium system as a

µ-η²:η²-coordinated ligand (D), as observed for 7‚BF₄.

Deprotonation of the acidic tC-H proton in the cationic

intermediate D should form the dinuclear µ-acetylide

complexes 2, which further interact with 1‚BF₄to

produce 8‚BF₄. In accord with this mechanism, (i) the

reaction proceeds cleanly in a basic solvent such as

acetone when compared with the reaction in a nonbasic

solvent such as CH₂Cl₂and (ii) treatment of an isolated

sample of 2 with 1‚BF₄produces 8‚BF₄, as described

The temperature-dependent NMR behavior of 8⁺

described above can be interpreted in terms of a

combination of flipping of the acetylide ligand (process

a), reorganization of the folded Z-shaped metal linkage

(process b), and switching of the interactions of Rh-C_R

bonding electrons (process c) (Scheme 12), although a

spectrum at the slow exchange limit, which is consistent

with the X-ray structure of 8e⁺with no element of

symmetry, is not obtained. The former two processes

can be regarded as modified versions of the well-

(22) Related µ₄-η¹(C_R):η²(C_RtC_â)-acetylide cluster complexes with a

butterfly or spiked triangular metal array are known. Sappa, E.;

Tiripicchio, A.; Braunstein, P. Chem. Rev. 1983, 83, 203. Raithby, P.

R.; Rosales, M. J. Adv. Inorg. Chem. Radiochem. 1985, 29, 169. Sappa,

E.; Tiripicchio, A.; Carty, A. J.; Toogood, G. E. Prog. Inorg. Chem. 1987,

35, 437.

Polynuclear Rhodium Complexes

Organometallics, Vol. 24, No. 1, 2005 175

Scheme 12

Scheme 13

documented windshield wiper motion of the acetylide

ligand:⁷between the wingtip metal atoms (process a)

and between the intra-PNNP-ligand wingtip-hinge metal

atoms (process b). Canonical structures are also shown

in Scheme 12. In the interconversion a (8F T 8G and

8H T 8I), switching of the η²-coordination to the wingtip

Rh centers (Rh_Aand Rh_D) is associated with reorganiza-

tion of the η¹- and η²-interactions as well as switching

of the Rh-C σ-bonding interaction (C_R-Rh_Cf C_R-Rh_B).

On the other hand, process b (8F T 8H and 8G T 8I)

involves switching of the η¹- and η²-interactions in the

part corresponding to 2 (Scheme 10), as observed for 2

(Scheme 4). It is notable that, in process a, the relative

arrangement of the four metals is retained, whereas

process b involves not only simultaneous rotation of the

two PNNP ligands but also Rh-Rh bond cleavage and

recombination processes to exchange the wingtip and

hinge Rh centers. Furthermore process a + b cannot

account for the CH₂P proton signals appearing as a

single resonance at higher temperatures, because H_a

atoms (proximal to the acetylide ligand) and H_batoms

(distal from the acetylide ligand) are left distinguish-

able. The H_a-H_bsite exchange can be explained in

terms of, for example, switching of interaction modes

of the Rh_C-C_Rσ-bonding electrons with Rh centers

(from Rh_A(8K) to Rh_B(8J): process c), and accompanied

rotation of the PNNP ligand renders H_aand H_bequiva-

lent. An intermolecular mechanism via dissociation into

A + 2 (process d) can be excluded on the basis of the

following reasons. Although a related process was

observed as described below (Scheme 13), its rate (∼30

min) was much slower than the time scale of coalescence

of NMR signals and the negative activation entropy

value (∆S^q) -20.8 eu, ∆H^q) 5.3 kcal/mol) estimated

from the VT measurement of 8e⁺also supports an

intramolecular mechanism. A C_2v-symmetrical inter-

mediate 8K cannot account for the H_a-H_baveraging

process but cannot be excluded, because a combination

with other rotation processes such as process c makes

it possible to account for the dynamic behavior of 8‚BF₄.

The subtle difference in the C2-Rh2 and C2-Rh4

distances (∼0.17 Å) in 8e‚BPh₄suggests that the lower

energy process, which cannot be frozen out even at -90

°C, should be ascribed to the least motion process

(process a). It is notable that the higher energy process

b and the rotation process c involve reversible cleavage

and recombination of the metal-metal bonds.

(ii) Exchange between Di- (2) and Tetranuclear

Acetylide Complexes (8‚BF₄).²²To check the viability

of a dissociation process for the dynamic behavior of the

tetranuclear acetylide complex 8‚BF₄(process d in

Scheme 12), interaction between the dinuclear acetylide

complex (2) and a tetranuclear complex containing a

different acetylide substituent (8‚BF₄) was followed by

¹H NMR (Scheme 13). As a result, a 1:1 mixture of 2c

and 8e‚BF₄reached an equilibrium with the composi-

tion of 8e⁺:8c⁺) ∼2:1 during the course of 30 min. This

result clearly indicates participation of a slow dissocia-

176 Organometallics, Vol. 24, No. 1, 2005

Tanaka et al.

Scheme 14

tion process, but the rate is much slower than the time

scale of coalescence of NMR signals.

(iii) Deprotonation of 8a‚BF₄Leading to a µ₄-

Dicarbide Cluster Compound, 9.²³The NMR data

for the tC-H part of the ethynyl complex 8a‚BF₄(δ_H

6.82; J_C-H) 237 Hz) suggest its sp-hybridized, acidic

character.²⁴Actually, 8a‚BF₄was acidic enough to be

readily deprotonated upon treatment with alumina or

NEt₃to give the neutral C₂(dicarbide) complex 9,²⁵and

the deprotonation process could be reversed by addition

of HBF₄‚OEt₂to 9 (Scheme 14).

The simple NMR features for 9 (single δ_C(CtC) and

δ_Psignals), which do not change even when the sample

is cooled as low as -90 °C, suggest formation of a

symmetrical structure, and the ¹³C NMR signal for the

C₂bridge is located at δ_C98.9 (multiplet). X-ray crystal-

lography of 9 (Figure 6 and Table 4) reveals the

tetranuclear µ₄-dicarbide structure,²⁵in which the C₂

bridge is interacted with each Rh₂(PNNP) fragment in

a µ-η¹:η²-fashion. The CtC length [1.22(1) Å] is com-

parable to those in 2 and 8e‚BF₄, and the structure of

the dinuclear moiety (µ-C₂)Rh₂(PNNP) is found to be

similar to that of the dinuclear µ-acetylide complexes

2. Complex 9 is a rare example of a (µ₄-η¹:η¹:η²:η²-C₂)-

M₄-type dicarbide complex,²⁶but, when compared with

the previous examples, the (µ₄-C₂)M₄moiety is folded

to a considerable extent presumably (i) to avoid steric

repulsion among CO ligands and (ii) to overlap with the

two orthogonal π-orbitals of the central C₂ligand in an

effective manner. The dihedral angle between the C1-

C2-Rh2 plane and the C1-C2-Rh4 plane is 94.8°,

which is substantially more acute than the correspond-

Figure 6. Molecular structures of the cationic part of 9

drawn with thermal ellipsoids at the 30% probability

level: (a) an overview and (b) the core part.

ing dihedral angle in [(µ₄-C₂)Cu₄(Ph₂Ppzpy)₂]²⁺(129.0°).^26b

In addition, because the Rh2-Rh4 separation

[3.1723(7) Å] falls in the longer end of Rh-Rh bonds,

the folding should result from an attractive interaction

between the Rh2 and Rh4 atoms. The two Rh₂(PNNP)

backbones adopt different conformations (type I for

Rh1-Rh2 and type II for Rh3-Rh4). The inconsistency

between the virtually C₂-symmetrical solid state struc-

ture with two different P-environments and the single

³¹P NMR signal is due to dynamic behavior via oscilla-

tion of the C₂ligand, i.e., concurrent switching of the

η¹- and η²-coordination (Scheme 14),^26cwhere the σ-bond-

ed metal centers in one structure (Rh_A,C) move to the

π-bonded sites in the other structure. Of two viable

transition states, the planar (L) and twisted one (M),

the latter should be plausible, because (i) in the case of

the related complexes 2, steric hindrance between the

CO ligand and the acetylide substituent (R) causes

distortion of the structure (see above) and the twisted

conformation M can minimize such interactions and (ii)

the two Rh₂(PNNP)(CO)₂fragments can effectively

overlap with the two orthogonal π-orbitals of the CtC

moiety. The activation barrier for the fluxional process

is very low, because the process cannot be frozen out

even at -90 °C. Notable is that (i) the fluxional behavior

of 9 involves a cleavage process of the Rh-Rh interac-

tions and (ii) the deprotonation-protonation sequence

is associated with reversible M-M bond cleavage and

recombination processes (Scheme 14).

(23) The p-tolyl complex 8e‚BF₄was subjected to reaction with

nucleophiles. Reaction with Li-CtC-p-tol and KOH gave 2e and a

mixture of 11 and 2e, respectively, which were apparently formed via

nucleophilic attack followed by dissociation into dinuclear fragments.

The reaction with H₂SiPh₂gave a mixture containing 10‚BF₄.

(24) Akita, M.; Terada, M.; Oyama, S.; Sugimoto, S.; Moro-oka, Y.

Organometallics 1991, 10, 1561. Akita, M.; Takabuchi, A.; Terada, M.;

Ishii, N.; Tanaka, M.; Moro-oka, Y. Organometallics 1994, 13, 2516.

Article Doi

Polynuclear rhodium complexes with dinucleating PNNP ligand: Dynamic and diverse M...M interactions in [(μ-X)Rh2(PNNP)(CO) 2]n+ and [μ4-XRh4(PNNP) 2(CO)4]n+ [X = H, O, C≡C-R, R-C≡C-R, C≡C, CH=CH2, SMe(2); n = 0, 1: PNNP = 3,5-bis(diphenylphosphinomethyl)pyrazolato]

DOI: 10.1021/om049427v

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Article abstract of DOI:10.1021/om049427v

Full text of DOI:10.1021/om049427v

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