15 Electron Delocalization Through the Disulfide Bridge Icaro de Sousa Moreira and Douglas Wagner Franco
Downloaded by PRINCETON UNIV on September 27, 2013 | http://pubs.acs.org Publication Date: May 5, 1997 | doi: 10.1021/ba-1997-0253.ch015
1
2
Departamento de Química Orgânica e Inorgânica, Universidade Federal do Ceará, Fortaleza-CE, Brasil 1
Instituto de Química de São Carlos-USP, Caixa Postal 780, 13560-970, São Carlos-SP, Brasil
2
The -S-S- bridge capability for conducting electrons was investigated using 4,4'-dithiodipyridine (DTDP)-type molecules as bridging ligands and the metal centers of ruthenium, iron, and osmium. Acid-base prop-erties of the coordinated DTDP in the monomers and near-infrared and electrochemical data for the binuclear complexes [{Ru(NH ) } DTDP](PF ) and Na [{Fe(CN) } DTDP] and their respective mixed-valence complexes indicate intense electron delocalization between the two metal centers. The Mössbauer spectral data for the mononuclear and binuclear ironpentacyano DTDP complexes also are consistent with the assignment of a valence-delocalized system. 3 5 2
6 4
6
5 2
THE BASIC CHEMISTRY OF SULFUR LIGANDS
has received less attention than the chemistry of the corresponding oxo and nitrogen species, despite the wellrecognized importance of sulfur compounds in biological processes and for defining properties in a wide range of different materials. This sulfur chemistry began to be systematically investigated only in the 1960s (I, 2). Henry Taube recognized the relevance of the sulfur-containing ligands and devoted efforts toward the development of their chemistry. The first wellcharacterized H S transition metal complex was characterized at Taube's labo ratory by C. G. Kuehn (3), and a series of related sulfur ligands were then investigated (3-6). The reduction of the i r a n s - [ R u ( N H ) C l S 0 ] C l to [Ru(NH ) C1S] C1 has been cited (7) as one of the few examples of oxygen removal from coordinated S 0 and demonstrates the Ru-S bond strength. 2
3
3
4
2
4
2
2
2
© 1997 American Chemical Society
In Electron Transfer Reactions; Isied, S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1997.
255
E L E C T R O N TRANSFER REACTIONS
256
QN
S
NQ
|U
[(NH3)5Ru()
Ru('D(NH3)]5+ 5
Downloaded by PRINCETON UNIV on September 27, 2013 | http://pubs.acs.org Publication Date: May 5, 1997 | doi: 10.1021/ba-1997-0253.ch015
I
III
II
The existence of an intervalence transition for the species in structure I led Henry Taube to suggest the availability (8) of 3ά sulfur orbitals for electron derealization between the two centers. No experimental evidence for electron derealization has been obtained in studies with the [isn(NH ) RuS]| ion (4) and related species {9-11). The fact that disulfides react with [ R u ( N H ) ( H 0 ) ] fairly rapidly to produce Ru(III) thiolate (12) almost quantitatively and the advances in the field of superconductors using dithiols (13, 14) as syn thetic starting material led to the interest in investigating ligands where the sulfur atoms are spaced apart by one or more carbon atoms (12,15). As a conse quence, quite remarkable examples of sulfur-to-sulfur through-space interac tion have been described (12,15-20). The reduction of the - S - S - bridge occurs (12, 21) when the 2,2'-dithiodipyridine molecule is allowed to react with [Ru(NH ) (H 0)] . However, this reduction does not occur when the ligand is 4,4'-cuthiodipyridine [see struc ture I I (4,4'-dithiodipyridine) and structure III (2,2'-dithiodipyridine)]. With 4,4'-dithiodipyridine, quite stable mono and binuclear species have been iso lated (22, 23). Furthermore the disulfide bridge, when coordinated to the metal centers (15, 16), becomes resistant to reductive attack (zinc amalgam, ascorbic acid). Therefore, dealing with such simple and easy implementable models, where the spacers are 4,4'-dithiodipyridine (DTDP)-type ligands and the metal centers are ruthenium (22, 23), iron (23), and osmium (24) complexes, we investigate the - S - S - bridge capability for conducting electrons. In this chapter we describe the Môssbauer parameter isomer shift and quadrupole splitting for the [ F e ( C N ) - D T D P - F e ( C N ) ] - and [ F e ^ C N ^ D T D P - F e ( C N ) ] - species, the changes in the basicity of the uncoordinated pyridinic nitrogen (pK ), the determination of the comproportionation con stants (K ), and the near-infrared spectrum characteristics (λ, ε) for the interva lence absorption exhibited by the mixed-valence species. π
3
4
+
3
2
2 +
3
II
III
5
5
II
5
2+
2
5
6
5
a
c
In Electron Transfer Reactions; Isied, S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1997.
5
15.
MOREIRA A N D FRANCO
Disulfide Bridge Electron Dehcalization
257
Isomer Shift and Quadrupole Splitting The Môssbauer spectra of mononuclear ironpentacyano complexes are dou blets due to the presence of an electric field gradient with noncubic symmetry in the F e nucleus (25). The isomer shift δ reflects the changes in the elec tronic density at the nucleus, which are caused by modifications in the electron populations of valence orbitals of the Môssbauer atom. In the iron pentacyano complexes the noncubic electronic configuration of the central ions comes from the asymmetric σ- and π-bonding involving only the axial ligand L . The σ-donation mechanism increases the s-electron population causing conse quently an increase of the electronic density at the metal. The σ-acceptance mechanism also increases the s-electron density at the metal due to the decrease i n the shielding effects by the donation from metal dn orbital to ligand ρπ* orbital.
Downloaded by PRINCETON UNIV on September 27, 2013 | http://pubs.acs.org Publication Date: May 5, 1997 | doi: 10.1021/ba-1997-0253.ch015
5 7
For iron pentacyano complexes the σ- and π-bonding involving the axial ligand L are responsible for the appearance of the electric field gradient (EFG) at the iron nucleus and consequendy for the quadrupole sph'tting. In this case the quadrupole splitting is caused by the interaction of the nuclear quadrupole moment of the iron atom with the Ζ component of the E F G (25, 26). The isomer shift and quadrupole splitting values for the mononuclear and binuclear species are shown in Table I. F e ( I I I ) - D T D P has a d low-spin configuration decreasing the shielding effect of the s-electrons compared with Fe(II)-DTDP d low-spin configura tion. O n the other hand, d low-spin configuration leads to a bigger E F G due to the asymmetric configuration. Thus the isomer shift is much smaller, and the quadrupole splitting of the Fe(III)-DTDP monomer is much bigger, than those observed for the Fe(II)-DTDP mononuclear complex. The Môssbauer spectra of Fe(II)-DTDP-Fe(II) and Fe(II)-DTDP-Fe(III) complexes, illustrated in Figure 1, showed two absorption lines indicating identical coordination sites of D T D P to the F e . Since there is no evidence of either Fe(III) or Fe(II) quadrupole splitting (at least at room temperature), it is an indication of an electronic delocalized system. The bigger value of quadru pole splitting (AQ ) observed for the mixed-valence complex compared with that of the fully reduced species is consistent with the existence, for the metal 5
6
5
57
S
Table I. Môssbauer Parameter Data
Complex
δ± 1CH (mm/s)
A
QS
± 10- (mm/s) 3
Na3[(CN) Fe(DTDP)]-4H 0
0.291
Na [(CN) Fe(DTDP)Fe(CN) ]-5H 0 Na [(CN) Fe(DTDP)Fe(CN) ]-6H 0 Na [(CN) Fe(DTDP)]-3H 0
0.740
5
2
0.864
5
5
5
2
0.278
6
5
5
2
0.300
0.705
2
5
0.220
1.709
2
NOTE.- δ means isomer shift; AQ§ means quadrupole splitting. Τ — 300 Κ; sodium nitroprusside was used as standard. DTDP is 4,4'-Dithiodipyridine.
In Electron Transfer Reactions; Isied, S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1997.
E L E C T R O N TRANSFER REACTIONS
Downloaded by PRINCETON UNIV on September 27, 2013 | http://pubs.acs.org Publication Date: May 5, 1997 | doi: 10.1021/ba-1997-0253.ch015
258
0.97Η
,
-4.00
-2.00
1
1
1
2.00
000
VELOCITY
4.00
(mm/s)
Figure 1. Môssbauer spectrum ofNa^(CN) FeOTO?Fe(CN)J'6
H 0.
5
2
centers in the Fe(II)-DTDP-Fe(III) species, of an intermediate formal valence state between Fe(II)-DTDP-Fe(II) and Fe(III)-DTDP-Fe(III). The 3d-3d electronic population in the mixed-valence species is smaller than that of the Fe(II)-Fe(II) complex, decreasing the shielding effect. Thus, the smaller isomer shift observed i n the spectrum of the mixed-valence species not only is i n agreement with this observation but is also consistent with the assignment of a valence-delocalized system.
Acid-Base Properties and pK
a
Acid-base properties of the coordinated ligands, when compared with those of the uncoordinated ligand {27-30), can provide information on the interplay between the σ and π components of the M - L bond. Thus the p K for the reac tion types shown i n Scheme I has been extensively (25-28) used to estimate how the σ and π components of the R u » - L bond change with the change in the metal-center oxidation state. As can be observed in Table II, for L = pz (pyrazine), as a consequence of the coordination to the Ru(II) center, the p z H fragment exhibits an increase in its p K value of 1.9. units. This is interpreted {27-30) as due to the Ru(II) - » pz, 4ά —> π* backbonding interaction, which will be responsible for transfer ring electron density from the metal center to the heterocyclic ring, and then a
n
m
+
a
π
In Electron Transfer Reactions; Isied, S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1997.
15.
M ORE IRA A N D FRANCO
Disulfide Bridge Electron DelocalizaUon
259
Scheme L Table II. pK Data for Coordinated and Uncoordinated DTDPH+ and pzH+ Acids at 25 °C
Downloaded by PRINCETON UNIV on September 27, 2013 | http://pubs.acs.org Publication Date: May 5, 1997 | doi: 10.1021/ba-1997-0253.ch015
a
Acid
Réf.
[Ru(NH3) DTDPH] [Ru(NH ) DTDPH] [Os(NH ) DTDPH] [Os(NH ) DTDPH] DTDPH+ pzH tRu(NH3)epzHp+ [Ru(NH ) pzH]^ 5
3
5
3
5
3
5
3+
5.25
23
4+
3.20
23
3+
5.50
24
4+
3.20
24
4.80 0.6
23
2.5
27
-0.8
27
+
3
NOTE:
5
27
DTDP is 4,4'-dithiodipyridine; pz is pyrazine.
to the uncoordinated nitrogen atom. The polarization effects i n this case are overcome by the backbonding (27-30). However, for the Ru(III) complex, i n the absence of the 4d electrons available for backbonding to the p z H ligand, the inductive polarization effects of the good Lewis acid Ru(III), 4d , dominates. Thus, electron density is with drawn from the ligand, and the electron pair of the uncoordinated nitrogen becomes less available for the proton. Accordingly, a decrease of 1.4 p K units is exhibited for the acid p z H when coordinated to Ru(III). A comparison between the D T D P H and the p z H system p K values showed that the same tendency is observed for both cases (22, 31). However, it should be emphasized that on the D T D P H the protonated nitrogen is located on another pyridinic ring separated from the ( N H ) R u p z moiety by the - S - S - bridge. Thus, the electronic effects are transmitted to the protonated nitrogen, through the - S - S - bridge, distant from the coordinated nitrogen at least by 10 Â. n
+
5
a
+
+
+
a
+
3
5
Comproportionation Constants and Near-Infrared Data Comproportionation constants (K ) and near-infrared band characteristics (λ, ε) are well-accepted as good parameters (32-35) to analyze the mediator ability of c
In Electron Transfer Reactions; Isied, S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1997.
E L E C T R O N TRANSFER REACTIONS
260
a ligand (L) that bridges two metal centers on different oxidation states M -I^M . Based on the voltammetric spectra (23) of the [ { R u ( N H ) } D T D P ] ions n
m
3
[(£1/2)1 = -
1
3
0
v
a
n
d
(^1/2)2 =
0
1
6
0
y
]
a
n
d
o
f
t
h
5
4+
2
[{FeiCN^DTDP] "
e
6
species [ ( E ^ i = 0.155 V and ( E ) = 0.275 V], the comproportionation con stants Κ = 8 χ 10 and 1 χ 10 have been calculated for the reaction in eq 1: 1/2
4
2
2
2(III,II)
(II, II) + (III, III) -C N
N
Q- -C Q-C C,C
N
N
Oi-O KCN) Fe] 5
2
/==\ Ν Ν
O-G*
ε (M- cm- ) 1
1
1570
5000
4xl0
1500
4260
δχΙΟ
920
1010
δχΙΟ
960
760
2x10
920
640
14
1030
920
2x10
810
30
9.8
1200
2200
δχΙΟ
1195
900
lxlO
1300
600
