608
J. Phys. Chem. 1980, 84, 608-616
Donor-Acceptor Complexes of Organometals and Iodine. Alkyl Ligands as Probes for Steric Effects In Charge Transfer S. Fukuzuml and J. K. Kochi" Department of Chemistry, Indiana University, Bioomington, Indiana 47405 (Received August 8, 1979) Publication costs assisted by the National Science Foundation
Charge-transfer (CT) absorptions are observed between iodine and a variety of homoleptic alkylmetals including dialkylmercury (R2Hg)and tetraalkylmetals (R4M) of the group 4A elements (where M = lead, tin, germanium, and silicon) in carbon tetrachloride solutions. These alkylmetal-iodine complexes are all classified as weak, the formation constants, K , being generally less than 5 M-' for dialkylmercury and less than 3 M-' for the methylethyllead compounds. The formation constants of tetraalkyltin, -germanium, and -silicon are too small to measure (K < 0.1 M-l). The frequency of the charge-transfer bands ( h v ~varies ~ ) linearly with the vertical ionization potential (ID) of the alkylmetal, determined independently from the photoelectron spectra. However, two separate correlations are required for these alkylmetals-one for the series of two-coordinate, linear dialkylmercury compounds and another for the series of four-coordinate,tetrahedral tetraalkylmetals corresponding to a sterically open and a quasi-spherical configuration of electron donors, respectively. Steric effects in these alkylmetal-iodine complexes may be evaluated in two ways. By the direct method, the role of steric effects in determining the charge transfer transition energy is associated with the Coulombic term (e2/rDn) in the first-order treatment of weak complexes according to the Mulliken theory. The mean separation rDA in the CT complexes of R2Hg and R4M, calculated from the measured values of VCT, ID,and the vertical electron affinity of iodine, shows two parallel trends, both increasing with decreasing values of the ionization potentials. This behavior is the same as that evaluated for the CT complexes of tetracyanoethylene(TCNE)with the same series of alkylmetals, in which K is larger and can be measured for [R2Hg12] as well as [RIM 14 complexes. By the indirect method, steric effects are evaluated relative to a reference alkylmetal (MezHg for RzHg and Me4Sn for RIM). The difference AE,taken as the relative change in steric effects, is shown to be essentially the same in TCNE and iodine complexes. The latter bears on the general question as to whether small (indeterminant) values of K (CO.1 Mu') or -AH (e1kcal mol-') can be used as adequate criteria for contact charge transfer. TABLE I: Steric and Polar Parameters for Alkvl Ligandsu
Introduction Charge-transfer (CT) complexes of iodine are the oldest known and have been extensively studied, both by experiment and from several theoretical points of view, with a variety of organic d ~ n o r s . l - Contact ~ charge transfer (CCT) of iodine with weak donors such as alkyl halides and even alkanes is especially n~teworthy.~ Although there are a few examples of iodine charge transfer with metal comp l e x e ~ ,there ~ * ~ are no reports heretofore of studies with organometals in which the metal is directly bonded to alkyl ligands. Alkylmetals have many desirable properties as electron donors for the study of steric effects in charge-transfer interactions. As relatively volatile and electron-rich compounds, their photoelectron spectra are readily accessible, and the vertical ionization potentials can be accurately measured. Moreover, in alkylmetals of the main group elements, ionization occurs from the highest occupied molecular orbital (HOMO) which has a-bonding character; that is, they are donors.' As a result, alkyl ligands exert a large dominating influence on the ionization potentials and the steric properties of alkylmetals. In this study we have chosen two series of organometals, viz., tetraalkylmetals of the group 4A elements (Si, Ge, Sn, Pb) and dialkylmercury (RzHg),which are geometrically rather different, having tetrahedral (quasi-spherical) and linear (open) configurations, respectively, i.e., r--_
/6,
,R'\,
I,\ R" .M\R ; \--
01
0022-3654/80/2084-0608$0 1.OO/O
E,
0.49 0.55 0.58 0.66
Importantly, alkyl groups are highly tunable ligands for the systematic variation of the steric effects and the electron donor properties of these alkylmetals. Thus both the steric and polar properties of alkyl ligands can be made to vary merely by manipulating the extent of a- and /3methyl branching, as indicated by the magnitudes of the E, and E, parameters, respectively, in Table I. By the proper choice of these alkyl ligands, the stereochemistry as well as the ionization potentials of organometals can be varied to cover wide ranges of molecular properties. [Compare Table I with the PES data in Table 11. Note that the effective size of alkyl groups changes dramatically with both a and p branching, but the ionization potential is strongly influenced only by a branching.] Since these alkylmetals are all substitution stable, any divergence in the behavior toward iodine can be ascribed directly t o differences in their steric properties and ionization potentials.
\--____.
where R represents a homologous series of alkyl ligands.
E,
-0.07 -0.36 -0.93 -1.74 In arbitrary energy units from ref 8 and 9.
.
/'
p branching
alkyl E, E, alkyl methyl 0 0 ethyl ethyl -0.07 0.49 n-propyl isopropyl -0.47 0.85 isobutyl tert-butyl -1.54 1.02 neopentyl
-- - - - - (R- H g - R)
M = Si,Ge,Sn,Pb
branching
Results Charge-Transfer Absorption Spectra of Iodine Complexes with Alkylmetals. When a solution of dibutylmercury in carbon tetrachloride is mixed with iodine, a new absorption band with A, 332 nm is immediately observed
0 1980 American
Chemical Soclety
Donor-Acceptor Compiexes of Organometals and Iodine
The Journal of Physical Chemisfry, Vol. 84, No. 6, 1980 809
TABLE 11: Charge-Transfer Absorption Spectra of Alkylmetals with Iodinea __.-. E K ,x A,, hvCT, 1o2M'' no. alkylmetal eV nm eV cm-' --
Me,Sn 9.69 210 4.59 8.8 Et& 290 8.90 4.21 7.8 n-Pr,Sn 8.82 290 4.21 9.9 n-Bu,Sn 8.76 288 4.30 3.1 216 4.49 6.9 EtSnMe, 9.10 215 4.50 7.3 n-PrSnMe, 9.10 276 4.49 7.9 I n-BuSnMe, 9.10 8.0 213 4.54 8 i-BuSnMe, 9.05 7.3 219 4.44 9.01 9 Et,lSnMe, 4.35 10.6 8.80 285 10 n-Pr,SnMe, 285 4.35 5.0 11 n-Bu,SnMe, 8.80 12 i-Pr,SnMe, 6.8 290 4.27 8.56 1 3 t-Bu,SnMe, 8.22 302 4.10 6.5 14 Et,l;nMe 8.95 290 4.21 11.25 15 i-Pr,Sn 8.46 295 4.20 5.3 10.0 299 4.14 16 s-Bu,Sn 8.45 17 i-Bu,Sn 8.68 294 4.22 5.8 18 i-Bu,SnEt, 301 4.00 4.8 1 9 Et,% 3.9 268 4.63 9.78 4.61 3.4 20 Et,Ge 9.41 269 21 n-Bu,Ge 286 4.33 9.4 1.5 291 4.11 22 Me,Hg 9.33 2 3 EtHgMe 3.93 13.3 8.84 312 8.29 3.13 18.7 24 n-Pr,Hg 332 8.35 333 3.12 27.9 25 n-Bu,Hg 3.77 25.3 8.30 329 26 i-Bu,Hg a In carbon tetrachloride solutions at 25 'C, except for Taken from ref 11-13. dialkylmercury at 3 "C.
1 2 3 4 5 6
300
340
260
0
.05
.IO
CRMI
, M
Figure 2. Changes in the charge-transfer absorptions with the varlatlon In the concentrations of various alkylmetals and 8.0 X lo-' M Iodine in carbon tetrachloride solution at 25 OC: (8)Me,Sn, ( 0 )i-BuSnMe3, (0) n-PrSnMe,, (0)f-Bu,SnMe,, (0)i-Pr,Sn, (0) Et4SI.
compounds, the absorption maxima of these tetraalkylmetals are generally shifted further into the ultraviolet and are sometimes partially obscured by the tail absorption of the tetraalkylmetal, particularly with the tin derivatives. However, the difference spectra obtained under calibrated conditions allowed values of A,, in Table I1 to be clearly identified, as described in the Experimental Section. (This technique was also facilitated by A- of the charge-transfer band increasing in parallel with the tail of the alkylmetal absorption.) The energy hvCTfor the iodine complexes of tetraalkylmetals also varies in the manner described above for dialkylmercury. Although there is a general trend for hvCT to parallel ID,there is considerable scatter of points, the deviation from any linear correlation generally being the greatest with sterically hindered alkylmetals. Formation Constants of Alkylmetal-Iodine Complexes. The absorbance change of the CT band was measured at various concentrations of alkylmetal and iodine to evaluate the formation constants K in eq 1and 2. The CT absorbance is represented in terms of the initial concentrations of the alkylmetal donors Do and the iodine acceptor A. by the Benesi-Hildebrand equation,14reexpressed by Person ad6
Wavelength, nm
Flgure 1. Charge-transfer spectra of alkylmetals with 8.0 X M iodine In carbon1 tetrachloride solution at 25 OC: Me,Sn = 4.9 X M, t-Bu,SnMe, = 6.3 X M, n-Bu4Ge = 5.2 X loa2M, n-Pr,Hg = 1.2 X !VI(at 3 O c ) .
in the ultraviolet spectrum. The broad absorption band shown in Figure 1 is characteristic for intermolecular charge transfer complexes,1° e.g.,
5
n-Pr2Hg + I2 [n-Pr2Hg12] (1) Homologous dialkylmercury compounds show the same, behavior, but the absorption maximum varies with the nature of the alkyl ligands. Indeed the energy of the charge-transfer transition, hvCT, increases with the ionization potential of the dialkylmercury compounds listed in Table 11. The series of group 4A metal alkyls listed in Table I1 also exhibit new spectral bands due to charge transfer interaction with iodine, i.e., RIM + Iz& [R4MIz] (2) where M = Si, Ge, and Sn. Compared to dialkylmercury
(3) where the cell pathlength is taken as 1.00 cm, e is the extinction coefficient, and K is the formation constant of the CT complex. However, the plot of the absorbance against either Do or A. as shown in Figure 2 was linear and not curved as expected. From eq 3, linearity would be obtained if KDo
