Synthesis and Reactions in the Void Created in Sulfur-Bridged

Chapter 15

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Synthesis a n d Reactions i n the V o i d C r e a t e d in Sulfur-Bridged Dinuclear R u t h e n i u m Complexes Kazuko Matsumoto, T. Koyama, and T. Furuhashi Department of Chemistry, Waseda University, Tokyo 169, Japan

Several sulfur-bridged dinuclear ruthenium compounds have been synthesized and their reactions, especially those of their bridging sulfur-ligands, have been examined. Polysulfide complexes [Ru2(µSn)(µ-S2CNMe2)(S2CNMe )(CO)2PPh3)2](1,n= 5; 2, n = 6) undergo cleavage of the chelating and bridging polysulfide ligand when reacted with NH . The disulfide-bridged dinuclear ruthenium complex[{Ru(CH CN) (P(OMe) )2}2(µ-S2)] (7)is highly reactive and its trans-RuSSRu core easily closes to sandwich small molecules between the two metals, while retaining its core structure. Compound 7 reacts with N2H to give [{Ru(CH CN) (P(OMe)3)2}2(µ-N2H4)2(µ-S2)](CF3SO3)3 (9), while reaction with acetone leads to [{Ru(CH CN)2(P(OMe) )2}(μ-CH COCH2S2){Ru(CH CN)3(P(OMe) ) }] , in which a novel C-S bond is formed. Reaction of[{RuCl(P(OMe) )2}2(μ-Cl)(μ-N2H4)(μ-S2)]( 1 3 )with O2 in CH2C l 2 gives a diazene-coordinated compound [{RuCl(P(OMe)3)2}2(μ-Cl)(μ-N2H2)(μ-S2)](17).TheO2oxidation of 13 in CH CN gave a novelS2O52-bridged complex. 2

3

3

3

4+

3

4

3

3 3+

3

3

2

3

3 2

3

3

2

Transition-metal complexes with S - ligand are widely distributed in nature in ores and in redox active centers of metalloproteins such as ferredoxins and nitrogenases (1-4). Polysulfides (S % x>2) are known to act as chelating or bridging ligands to metals (4), and above all, sulfide (S ) and disulfide (S2 ) are remarkably versatile ligands. Disulfide has also recently been proposed as a possible ligand in the Pcluster of the nitrogenase enzyme system (5, 6). We have attempted to synthesize sulfide-bridged dinuclear ruthenium compounds, whose RuS Ru core structures are robust with respect to ligand substitution. We hoped to construct a stable but reactive space between two Ru atoms; the stability of the space is provided by the RuS Ru cone, while the reactivity is provided by other labile ligands on the Ru atoms. In our RuSSRu core system, small molecules are sandwiched between the two metal centers, thereby acting as another bridging ligands, and undergoing redox reactions. We have unexpectedly discovered that the disulfide ligand is highly 2

x

2

2

x

x

0097-6156/96/0653-0251$15.00/0 © 1996 American Chemical Society In Transition Metal Sulfur Chemistry; Stiefel, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

TRANSITION METAL SULFUR CHEMISTRY

252

susceptible to nucleophilic attack, and even stable molecules such as acetone or O2 reacts with the disulfide ligand to form a stable C-S or O-S covalent bond on the disulfide bridge. The RuSSRu core has also been found to stabilize hydrazine by coordination to the two Ru atoms, and this bridged hydrazine can be oxidized by O2 while its bridging structure is retained. These reactions suggest that the void created between the two Ru atoms in the RuSSRu core is a suitable space, in which a very unstable molecule can be stabilized, and novel redox reaction can occur owing to the extensive electron delocalization in the core.


Syntheses and Reactions of Polysulfide-Bridged Diruthenium Complexes 2

There aie several means by which polysulfide ligands (S - (n>l)) coordinate to a metal. The source of the sulfide can be an alkali metal sulfide such as L i 2 S and Na2S , or simply elemental sulfur. In our effort to synthesize polysulfide complexes, the following reaction was attempted. Although we expected to prepare a disulfidebridged complex, pentasulfide- and hexasulfide-bridged complexes were obtained instead (7). n

n

n

RuH(MeNCS )(CO)(PPh )2 + S 2

3

>

8

benzene

[Ru2(μ-S5)(μ-S2CNMe2)(S2CNMe2)(CO) (PPh3)2] (1) + 2

[Ru2(μ-S )(μ-S2CNMe2)(S2CNMe2)(CO)2(PPh3)2] 6

(2)

X-ray structural analysis of a single crystal of the product showed the presence of 1 and 2 at 78.5 % and 21.5 %, respectively. In the crystal lattice, the μ-Ss and μ-Sô ligands aie disordered, whereas the remainder of the two molecules are not disordered. Both compounds can be obtained in pure form by recrystallization of the mixture from benzene. The ORTEP drawing of the twligandso molecules are shown separately in Figures I and 2. Both the S5 - and the Se ' ligands act as bridging as well as chelating ligands. The average S-S distance of the S5 chain is 2.06 A , which is slightly shorter than that in [Os (|I-S5)^-S3CNEt2)(S2CNEt2)3] (8), in which the value is 2.09 Â. The average S-S distance of the S6 chain in 2 is 2.03 Â, which is slightly shorter than that of the S chain in 1. Compound (PPh4)[Ru(NO)(NH XS )2] (9) is the first example reported for a polysulfide ligand coordinated to a ruthenium atom, but the S4 ligands only chelate to a ruthenium atom. The present compounds 1 and 2 are the first examples having poly sulfides that are both chelating and bridging. Although 1 is orange, it turns to yellow when it is dissolved in CH2CI2 and excess pyridine (py) is added. Quite interestingly, the yellow crystals obtained from the solution turns to orange again when they aie dissolved in CH2CI2. The P N M R spectra show that the starting compound 1 is recovered on dissolution in CH2CI2. In the CH2C1 solution of 1 with excess py, the chelation of the S5 ligand is ruptured to give a py-coordinated compound 3, in which the S5 ligand acts only as a bridging ligand (10). The reaction is schematically shown in Figure 3. Although the structure of 3 could not be confirmed by X-ray analysis, similar reaction occurs to 2 with other bases such as N H or N H 2 N H 2 in place of py, and the X-ray structures of the products corresponding to 3 have been solved. The ORTEP drawings of the two products [{Ru(S2CNMe2XCO)(PPh3)(NH3)}2(μ-S6)] (4) and [{Ru(S CNMe )^0)(ΡΡη )}2(μ-84Χμ-ΝΗ2ΝΗ2)] (5) are shown in Figures 4 and 5, respectively. In 2

2

2

5

3

4

3 1

2

3

2

3

In Transition Metal Sulfur Chemistry; Stiefel, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

2

Sulfur-Βridged Dinuclear Ruthenium Complexes 253


15. MATSUMOTO ET AL.




254



I

s

S

^ s

I

^

s

S

pph3

S

45

-

Γ1

PPM Γ~~1

i

n

C

2

'

2

S

H CI

i n C H 2 C

2

Me

Me

2

2

Ο

3 1

\

S

\

3

55 2

3

s

(3)

2

50 2

45

5

I ^ C O PPh

Ru

py r

PPM I—ι—ι—ι—ι—ι—ι—ι—ι—ι—j—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—I

S:

,,

3

—

[{Ru (S CNMe )(CO)(PPh )(py)} ( μ -S )l

PPh

Ru

/

Q

Figure 3. Reaction of [Ru2^-S5)^-S2CNMe2)(S CNMe2)(CO)2(PPh3) ] (1) with excess py, and the Ρ N M R spectra of the solution. *unknown. The small peak always exists. wSmall portion of 1 might exist in the chain form like 3.

50

I ι ι ι ι Γ"""I I I I J I ι ι ι ι ι I I I |

P-NMR

55

31

(1)

[Ru'^w-SsKw-SaCNMeaXSîCNMejXCOJ^PPh^J

Ph,R

if^sC'.u

excess py


256



Figure 5. Molecular structure of [{Ru(S CNMe )(CO)(PPh3)}2^-S4)faN H N H ) ] (5). 2

2

2

2



Sulfur-Bridged Dinuclear Ruthenium Complexes

compound 5, the 2% ligand in the starting compound 2 has lost two sulfur atoms and is now a S ligand, whereas in 4, the S6 ligand is retained. All the amine-coordinated complexes are stable in CH2CI2 with excess amine, however without excess amine in the solution, the complexes revert to 2. These reactions reveal that chelates of polysulfide ligands are thermodynamically not as stable as other common chelating ligands. This is perhaps caused by the strain due to the zigzag S-S bonds of the polysulfide ligands. 4


Syntheses and Reactions of Disulfide-Bridged Diruthenium Complexes Formation of trans-DisuIfide-Bridged Complexes and Their reactions. Complex [{RuCl(P(OMe) ) }2(M-Cl)2^-S )] (6) with a disulfide bridging ligand shown in Figure 6 has been prepared by the following reaction (11). 3

2

2

iraw5-RuCl (P(OMe) )4 + excess S 2

3

J

8

CH CI 2

2

[{RuCl(P(OMe) ) (^Cl)2^-S )] (6) 3

2

2

Although compound 6 is fairly stable in air, it becomes very reactive when it loses its bridging and terminal chloride ligands as shown in Figure 7. Compound 7 with a trans-di sulfide bridge reacts with acetone to give [{Ru(CH3CN>2(P(OMe)3)20AS CH COCH ){Ru(CH CN) (P(OMe) )2}] (8), in which a novel C H C O C H S ligand bridges the two ruthenium atoms (12). This reaction shows the highly electrophilic nature of the bridging disulfide ligand in 7. In another reaction in Figure 7, 7 easily closes its rraws-RuSSRu core by forming a new bridge in addition to the disulfide. Specifically, 7 reacts with anhydrous N H 2 N H 2 to give [{Ru(CH CN)(P(OMe)3)2}2at-N H4)2ai-S2)] (9) (13). The structures of 8 and 9 are shown in Figures 8 and 9, respectively. Compound 10 in Figure 7 corresponds to a one-electron reduced form of 7, and is obtained from C H C N solution of 7 (14). Reductive Coupling Reaction. Compound 6 and its derivatives undergo reductive coupling to form cluster compounds with higher nuclearity. For instance, [{Ru(CH CN)(P(OMe) ) } 0t-Cl)2^-S2)] (11), which was prepared by addition of 2 equiv of A g C F S 0 to 6 in C H C N , forms the tetranuclear compound [Ru4(p2H)2(^2-S)2(U4-S)2(P(OMe) )g] (12) in the reaction with M g powder as shown in Figure 10 (15). The ORTEP drawing of 12 is shown in Figure 11. Compound 12 is a mixed-valent complex having two R u and two Ru . This reaction occurs via initial reduction of R u in 11 to a mixed-valent state accompanied by 90° rotation of the disulfide bridge to effect octahedral coordination around each Ru atom, which has lost the chloride bridges in 11 as M g C h precipitates in the initial Ru reduction. The disulfide is rotated by 90° and is reduced to two sulfides which coordinate to the vacant sites formeriy occupied by chloride ligands, providing octahedral environment to both Ru atoms. The two independent Ru-H distances are different, one being a normal Ru-H distance (1.65(6) Â), whereas the other is significantly longer (1.95(6) A). The 18 dectron rule is satisfied for a Ru(III) atom, whereas the total number is more than 18 for a Ru(II) atom, which might be the cause of the unusually long R u l H distance in 12. Oxidation of a Hydrazine-Bridged Complex by O 2 . Oxidation of a hydrazinebridged complex by O2 was attempted in the hope of obtaining a diazene-coordinated complex. Such an experiment is especially interesting, since hydrazine and diazene are the intermediates redox states between dinitrogen and ammonia. The recently 3+

2

2

3

3

2

3

3

3

2

3

2+

3

3

3

2

2

3

3

3

11

111

111


2

2

257


258


Figure 6. Molecular structure of [{RuCl(P(OMe) ) }20*-Cl)2^-S2)] (6) (Reproduced with permission from reference 11. Copyright 1992 Japan Chemical Society). 3

2



AN

Ρ

U U

l

(6)

3

(7)

+

η

ΛΑ +

^

^

Solution

λ max = 650 n m

diamagnetic

Blue

3

2

2

2

o ' N H r

\

2

/

AN

+e

AN/ether

N

H

AN

> < f :

Ρ

AN

(9)

I

— ]

(8)

Ru(ll)Ru(lll)

AN C ^ AN

AN

v V

AN \

Figure 7. Reactions of [{RuCl(P(OMe) ) } ^-Cl) ^-S )] (6) and its derivatives.

Ί 4

Ru(lll)Ru(lll)

AN

Ρ = P(OMe)

ρ

Cl

-Ru,.

s—S

Ru^

C

Ru(lll)Ru(lll)

Cl


+

(1

3



260

Figure 9. Molecular structure of [{Ru(CH CN)(P(OMe)3)2}2(R-N2H )2^-S2)] (9). 3

4



15.

MATSUMOTO ET AL.


AN

2+

AN

in MeOH * ρ

S—S

(11)

ρ

V/N; /

excess Mg at r.t. for 30min

/

at50*Cfor12h

>

(under N2)

Ru(3+)

· Ρ

(12) Ru(2.5+)

Figure 10. Reaction of [{Ru(CH3CN)(P(OMe)3)2}2(μ-Cl)2(μ-S2)]

2+

(11).

Figure 11. Molecular structure of [Ru ^2-H)2^2-S)2^4-S)2(P(OMe) ) ] (12). 4

3


8

261


262


revealed X-ray structure of nitrogenase cofactor (16) shows a possible reaction site for dinitrogen reduction on the iron molybdenum cofactor, which contains a symmetrical cavity surrounded by six Fe atoms and three S atoms. This geometry suggests that N2 molecule may be surrounded by the six Fe atoms in the cavity. If this is the actual situation for the enzyme, this interaction mode of N2 molecule with metals is quite beyond what researchers had imagined. The X-ray structural work on the nitrogenase enzyme has highlighted the importance of a multi-metal center. Our diruthenium system, although much simpler than the natural system, should stabilize diazene between the two metals with cis-coordination. This mode of diazene bonding is totally different from that reported for other diazene complexes, and is expected to show redox behavior different from that of previously reported synthetic compounds (17, 18). Our first attempt to oxidize coordinated hydrazine by O2 was performed on [{Κηα(Ρ(ΟΜβ) )2}2(μ^1)(μ-82)(μ-Ν Η4)] (13) (19) in C H 3 C N . Although several compounds were isolated as crystals from the reaction solution, none of them contains an oxidized from of hydrazine (Figure 12). A l l the compounds in Figure 12 have been confirmed structurally by X-ray crystallography. Compound 16 is noteworthy, as an unprecedented S2O5 - anion is produced by O2 oxidation of the disulfide ligand in 13 (20). In our second attempt, the same reaction was carried out in C H C 1 and the diazene-bridged complex [{RuCl(P(OMe) ) }2^-Cl)^-S )(uN2H2)] (17) was isolated together with 16 (20). The ORTEP drawing of 17 shown in Figure 13, clearly shows that Rul-Nl-N2-Ru2 and the two hydrogen atoms HI and H2 are almost coplanar. TheN-N distance of 1.33(2) Â is relatively long compared with that of trans-diazene (1.301 Â) in the previously reported diruthenium complex (21), but is distinctly shorter than those of hydrazine (1.43-1.46 À) (19, 22, 23) or hydrazide N H " (1.410(9) and 1.391(15) À) complexes (23, 24). These structural features are definitive evidence that the N2H2 ligand in 17 is diazene. Compound 17 deserves special attention, since it is the first reported example of cis diazene coordination. Free diazene is cis, trans or iso, and so far only a few diazene complexes are known. All of these contain trans diazene, either as a bridge between two metal centers (25) or as a terminal ligand (20). 3

2

2

2

3

2

2

2

2

2

2

Spectroscopic Characterization of the Mixed-Valence Ru(II)SSRu(HI) Core (27) UV-Vis and Resonance Raman Spectra. A l l the compounds with a RuSSRu core described above have a strong absorption band in the region 600-800 nm, while compound 16 with a S 0 - bridge is coloriess. Compound 10 has a Ru(II)SSRu(IIl) mixed-valence core, while all others have a Ru(III)SSRu(III) core. The UV-vis spectra of compounds 6 and 10 are shown in Figure 14. None of the compounds absorbs in the near-IR up to 2000 nm. Compound 6 shows a strong absorption band at 737 nm, and exhibits resonance Raman bands at 385 cm- (v(Ru-S), strong), 456 cm; (v(S-S), very weak), and 769 cm- ( double harmonic of 385 c m ) . The assignment was made based on the reported values for analogous compounds (4, 2831). The 385 c m (v(Ru-S)) and 456 cm- (v(S-S)) bands of 6 can be compared to the reported values: 384 and 372 cm- (v(Ru-S)) and 536 and 525 cm" (v(S-S)) in the two isomers of [(μ-S2){Ru(III)(PPh ) S4 }2], where 'S^ is l,2-bis[(2morcaptophenyl)thio]ethane(2-) (29), 415 c m (v(Ru-S)) and 519 c m (v(S-S)) in [{Ru(III)(NH )5}20i-S )]Br4 (30), and 409 cnr (v(Ru-S)) and 530 cm- (v(S-S)) in [{CpRu(III)(PPh )2}2^-S2)](BF )2 (31). These compounds are the only transRu(III)SS(III) cores, for which UV-vis and resonance Raman spectra have been reported; no Raman spectra data are available for c*w-RuSSRu compounds other than 2

2

5

1

1

1

1

1

1

1

1

3

,

,

1

1

1

3

1

2

3

4


15. M A T S U M O T O E T A L .

Sulfur-Bridged Dinuclear Ruthenium Complexes 263

ci

ci

CI

,Ru


p

/

\

_

s

/

\ p

atr.t.

/\

Ru

„

/ \

P

P S S 6 Ru(lll)Ru(lll)

Reaction Solution

\ 6%

13 Ru(ll)Ru(lll) ( P=P(OMe) ) 3

Cl

3RIT— ^Ru P

/\ P

HS0 "

D

/\

4

P

N

14

S S P Ru(lll)Ru(lll) 29%

Ρ

/

P

ο 16 Ru(ll)Ru(ll)

Ρ

2

CI

15 Ru(lll)Ru(lll)

19%

22%

Figure 12. Reaction of [{^€1(Ρ(ΟΜ6)ι)ο}2(μ-α)(μ-82)(μ-Ν Η4)] (13) with 0 2

in C H 3 C N .

Figure 13. Molecular structure of [{RuCl(P(OMe)3)2}2^-Cl)^-S2)^-N H )] (17). 2


2

2

264



ο

300

600

900

1200

Wavelength (nm) 4

Figure 14. UV-Vis spectra of 6 ( — 1.44 χ Ι Ο M), and 10 ( Ι Ο M) i n C H C N . 4

3

Figure 15. The π-ΜΟ scheme for a Ru(III)SSRu(III) core.


1.33 χ



6. The v(S-S) frequencies in disulfide complexes generally range from 480 to 600 c m (4), which should be compared to free S (725 c m ) (32), S " (589 cm" ) (33, 34) and S (446 cnr ) (4). The emission band of 6 at 385 c m is more strongly enhanced by 647.1 nm radiation than by 568.2 nm. Therefore, the UV-vis absorption at 737 nmis assigned to an electronic transition within the R u S core. Compound 10 exhibits strong visible absorption at 646 nm, which is assigned analogously. The resonance Raman spectrum of 10 shows a strong v(S-S) band at 561 c m . A v(Ru-S) band could not be observed for 10 in in C H 3 C N , while a strong v(S-S) band was observed at 561 c m . The v(Ru-S) band should be in the range 370-400 c m , which is obscured by the Raman band of the C H 3 C N solvent. Even if the v(Ru-S) band is present in the area, its intensity may be very weak. Acetonitorile has to be used as the solvent, in order to avoid the release of the coordinated C H 3 C N . It should be noted that for complex 6 with a cw-RuSSRu core, v(Ru-S) is strongly enhanced, whereas v(S-S) is only very weakly observed. This is in remarkable contrast with complex 10 with a ira/w-RuSSRu core, which exhibits only a strong v(S-S) in C H 3 C N ; v(Ru-S) is very weak or is not enhanced. The resonance Raman spectrum of [(H N) RuSSRu(NH3)5] with a planar ira/w-Ru(III)SSRu(III) core (30) exhibits strong v(Ru-S) but no v(S-S) when excited at 647.1 nm. This excitation wavelength is close to the visible absorption maximum of the complex at 715 nm. The v(S-S) at 514 cnr is observed only when it is excited by the shorter wavelength i.e., by 568.2 nm or less (30). The resonance Raman and electronic bands can be reasonably explained by a qualitative MO description of the Ru SSRu core (27). The electronic transitions in the visible region correspond to a L M C T (ligand to metal charge transfer) from S - to Ru(III). A resonance Raman band with significant magnitude should be observable only for symmetric v(Ru-S) when the solution is irradiated in the visible band. The theory also predicts that the intensity of v(S-S) band is zero (30). A basically similar but simpler explanation can be given for the electronic absorption bands of [{Ru(PPh )'S }2^-S )]*CS (29). In order to obtain a clear image of the electronic states and to explain the Raman and ESR spectra (see later section) of the present compounds, the π-ΜΟ scheme for a RuSSRu core is given in Figure 15, which is basically similar to what is described in Ref. 29. The strong visible absorptions of compounds 6 and 10, and all other compounds with RuSSRu cores, are the transitions from to π , which is L M C T . Compound 6 is diamagnetic, since the two unpaired electrons of the two low-spin Ru(III) ions are paired as shown in Figure 15. Compound 10 with a Ru(II)SSRu(III) core is paramagnetic, since its one unpaired electron is in the π orbital. It is noteworthy that, for the three compounds with a /rarts-Ru(III)SSRu(III) core, [{CpRu(III)(PPh ) } ai-S )](BF4) (31), [{Ru(III)(NH3)5}2(μ-S2)]Cl ·2H 0 (30), and [{Ru(PPh3)'S '} ^-S )]

Synthesis and Reactions in the Void Created in Sulfur-Bridged

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