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50.2.9 Alkynyl Ligands Alkynyls (Table 50.7) are anionic strong electron donors, presenting E L values between −0.06 and −0.74 V versus NHE. Phosphonium or ammonium-containing alkynyls, for example, –C ≡C–CPh 2
3 + ) or –C ≡C–C(=CH 2 )(NR 3 + ), are overall neutral ligands and the weakest net electron donors of the series (E L in the −0.06 to −0.28 V range). Alkynyls with electron-acceptor substituents in the aromatic ring (such as NO 2 , CHO, CN, F, or azo group, –N =N–) (E L from −0.24 to −0.37 V) appear next. Moreover, an yne- or ene-type conjugated phenyl substituent appears to promote the net electron-donor character of the alkynyl [28]. Substituent effects can be transmitted along quite extended conjugated systems, which can be of significance for the design of species with nonlinear optical (NLO) properties [89–91]. With regard to the butenyls of the type –C ≡C–C(=CH 2 )X − , the following order of net electron release to the metal is observed: NR 3
< PR 3 + < aromatic < alkyl [28]. Despite the two phenyl groups and due to the electron donation by resonance of the amino group, the aminoalkynyls –C ≡C–CPh 2
2 ) (R
2 = H/Me, Me 2 ) are among the strongest electron donors (E L values
of circa −0.5 V [52]). The most effective electron donors are the alkyl-alkynyls –C≡C–R (E L in the range from −0.5 to −0.7 V).
Alkynyl ligands illustrate a difficulty concerning the possible localization of the HOMO at the ligand instead of the metal, thus leading to anomalous E L values [28]. 50.3 FINAL COMMENTS The redox potential of a coordination or an organometallic compound reflects its structural and electronic features, and the establishment of redox potential– structure relationships enables methods for the quantification of the net electron- donor/acceptor character of ligands and of the electronic properties of the binding metal centers. Systematic studies on relevant ligands in organometallic chemistry have already been reported, but usually still concern a limited number of metal complexes, and thus the proposed values of the electrochemical parameters should be taken 686 REDOX POTENTIAL – STRUCTURE RELATIONSHIPS AND PARAMETERIZATION TABLE 50.6 Values for the E L Ligand Parameters for Selected Carbene Ligands (L) L
L (V) versus NHE a Diphenylcarbene ( =CPh 2
0.51 b , 0.39 Bithiophene-carbenes X S S Y 0.41–0.21 =C(OR)Y R = Me, Et or Ph; Y = 2-methyl-furyl or -thiofuryl, Ph, 4-methylthiazol-2-yl, C 6 H
Cl-4, or C 6 H 4 OMe-4
0.30–0.19 C O 0.27 Hydroxocarbenes (H-bonded), =C(OH···X − )Ph X =HSO 4 , ClO 4 , CF 3 CO 2 0.19 to −0.12
=C(SR)(2-methylfuryl) R = alkyl, Ph 0.17–0.15 =C(OR)Fc R = alkyl
0.14 to −0.04
=C(R-1-yl)Ph R = aziridin, azetidin, pyrrolidin, or piperidin 0.15–0.09 Aminocarbenes, =C(NRR )Y R = R = Me or Et; Y = Ph or Me R = R = H; Y = 2-methyl-furyl or -thiofuryl. R = H, R = Cy or Me; Y = Et or Me 0.09–0.05 Aminocarbenes, =C(pyrrolidin-1-yl)Y Y = 2-methylfuryl, 2-methylthiofuryl 0.06, 0.05 Phosphoylide-aminocarbene, PR 3 = PPh 3 , PPh 2 (CH
2 Ph) or PMe 3 C
N PR 3 0.06 to −0.01
c Fc-aminocarbenes −0.11 to −0.21 {=C(O −
}(NMe 4 + ) R = 4-methylthiazol-2-yl, 2-methylfuryl, CH 2 SiMe 3 , or
Fc −0.16 to −0.73 a Estimated from P L values by using Eq. 50.4; metal centers of the type {M(CO) 5
used; from Reference 28, unless stated otherwise. b From Reference 77. c From Reference 41. cautiously. Nevertheless, common C-ligands can be ordered as follows, according to their net electron π-acceptor minus σ -donor character, as expressed by their E L values: carbynes > aminocarbyne > CO > vinylidenes > allenylidenes > carbenes > alkynyls [28]. The proposed models bear some limitations, namely, those concerning the possible failure of the additivity hypothesis. The risk is higher for the Lever’s model, with maximum additivity, and lower for the Pickett’s approach, with minimum additivity. However, the former model has a wider scope and has been much more applied, although it is less sensitive to subtle structure or composition changes and, in the initial form, insensitive to isomeric affects. The eventual dependence of the ligand parameters on the properties of the binding metal sites and the (de)localization of the redox orbital at the ligand (instead of being localized at the metal) also constitute difficulties that usually are not easy to overcome and can lead to anomalous parameter values. Extensions from octahedral-type geometries and 18-electron complexes to others have already been achieved, and of particular significance in organometallic chemistry are those concerning sandwich and half-sandwich π-complexes. Different ACKNOWLEDGMENTS 687 TABLE 50.7 Values for the E L Ligand Parameter for Alkynyl Ligands, –C ≡C–R (L) R Metal Center E L (V) versus NHE a C 6 H 3 (C ≡CX) 2 -1,3,5 X = H or Fc {OsCl(dppm) 2 } + −0.20
C 6 H 4 C ≡CPh-4 {OsCl(dppm) 2 } + −0.21
Ph {OsCl(dppm) 2 }
−0.24 CPh
2 (PMe
3 ) + {Fe(NCMe)(depe) 2 } + −0.28
b CPh
2 (C ≡N) {FeBr(depe) 2 } + −0.27
b C( =CH 2 )Ph
{FeBr(depe) 2 } + −0.33
b CHPh
2 {FeBr(depe) 2 }
−0.38 b C( =CHMe)Et {FeBr(depe) 2 }
−0.4 b CPh 2 (NHMe)
{FeBr(depe) 2 } + −0.47
b CPh
2 (NMe
2 ) {FeBr(depe) 2 } + −0.49 b Ph {WH 2 (dppe) 2 } + −0.30 c C( =CH 2 )X {RuCl(dppm) 2 } + −0.06 to −0.19 X = NMe
2 CH 2 C ≡CEt
+ , NMe
3 CH 2 Ph + , NEt 3 + , NEt(C 2 H 4 ) 2 O + , MeN + NMe 2 CH 2 C 6 H 4 OMe-3
+ , NPr
3 + , N(CH 2 CH 2 ) 3 CH + , NC 5 H 4 NMe 2 + C 6 H 4 X {RuCl(dppm) 2 } + −0.51 to −0.24 X = CH=CHPh, NO 2 -4, CHO-4, OMe-4, N =NC
6 H 4 NO 2 -4,4 , CHO-3, C ≡CPh-4, C ≡CC 6 H 4 NO 2 -4,4 , CH =CHC 6 H 4 NO 2 -4,4 , C ≡CC 6 H 4 C ≡CC
6 H 4 NO 2 -4,4 ,4 , HC O O -4 C 6 H 3 (C ≡CX) 2 -1,3,5 {RuCl(dppm) 2 } + −0.39
Ph {RuCl(dppm) 2 }
−0.41, −0.46 CHPh
2 {RuCl(dppm) 2 }
−0.42 Me or
i Pr {RuCl(dppm) 2 } + −0.51 Ph {RuCl(Me 2 bpy)(PPh
3 ) 2 } + −0.62, −0.48 d C 6 H 4 Me-4 {RuCl(Me 2 bpy)(PPh 3 ) 2 } + −0.65, −0.51 d t Bu {RuCl(Me 2 bpy)(PPh
3 ) 2 } + −0.74, −0.60 d a From Eq. 50.3 and Reference 28, unless stated otherwise. b From Reference 52. c From Reference 63. d From Reference 92. metal redox standards have been considered, leading to distinct scales of ligand parameters, but a proposal to maintain the initial model and overcome this limitation has been presented. Nevertheless, the generality has to be further tested. Another perspective that deserves to be further explored concerns the application of the redox potential parameterization models to the prediction (estimate) of the redox potential of complexes. This can be of a relevant identification significance of unknown compounds, by comparing the predicted and the measured values of the redox potential. Important cases of application can include the identification in situ of reaction intermediates or products without requiring their isolation. Hence, a broader application of redox potential parameterization methods in organometallic chemistry, for both characterization and identification purposes, is expected. ACKNOWLEDGMENTS This work has been partially supported by the Fundac¸˜ao para a Ciˆancia e a Tecnologia (FCT), Portugal (project PEst- OE/QUI/UI0100/2013).
688 REDOX POTENTIAL – STRUCTURE RELATIONSHIPS AND PARAMETERIZATION ABBREVIATIONS β Polarizability parameter bpy 2,2 -Bipyridine depe 1,2-Bis(diethylphosphino)ethane dppe 1,2-Bis(diphenylphosphino)ethane dppm 1,2-Bis(diphenylphosphino)methane E L Lever electrochemical ligand parameter E S Electron-richness parameter Et Ethyl
Fc Ferrocenyl HOMO Highest occupied molecular orbital HS High spin i Pr Isopropyl IR Infrared
LS Low spin
Me Methyl
MO Molecular orbital NHE Normal hydrogen electrode NLO Nonlinear optical NMR Nuclear magnetic resonance Ph Phenyl
Pr Propyl
t Bu
THF Tetrahydrofuran Tol Tolyl
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