Gas-Phase Thermochemistry of Organometallic and Coordination

Chapter 5

Gas-Phase Thermochemistry of Organometallic and Coordination Complex Ions David E . Richardson, Charles S. Christ, Jr., Paul Sharpe, Matthew F. Ryan, and John R. Eyler

Downloaded by UNIV OF SYDNEY on August 28, 2013 | http://pubs.acs.org Publication Date: June 25, 1990 | doi: 10.1021/bk-1990-0428.ch005

Department of Chemistry, University of Florida, Gainesville, F L 32611

Recent thermochemical results from gas-phase ion cyclotron resonance studies of organometallic compounds and coordination complexes are summarized. Reactions of dihydrogen, alkenes, alkynes, nitriles, and other substrates with d Cp ZrCH +(g) and d Cp Zr (g) lead to estimates for minimum differences in dissociation enthalpies for various Zr-R bonds. Appearance potential studies have been used to provide a preliminary estimate of D(Cp Zr -CH ). Gas-phase charge-transfer bracketing and equilibrium methods have been used to determine the adiabatic free energies of electron attachment to species such as metallocenium ions (Cp M (g)), Cp Ni(g), M(acetylacetonate) (g), and M(hexafluoroacetylacetonate) (g). When combined with other data in thermochemical cycles, these results can be used to obtain values for average gas-phase metal-ligand bond dissociation enthalpies and solvation free energies for organometallic ions and coordination complex ions. 0

1

2

3

+

2

+

2

3

+

2

3

2

3

Relatively few studies of the thermochemistry of gas-phase metal-containing ions with condensed-phase counterparts have been reported. Such data reveal aspects of intrinsic molecular properties in the absence of solvation, thus eliminating a very significant energetic component in the thermodynamics of dissolved ions. A perusal of the important compilation of gas-phase ion thermochemical data by Lias et al. (7) reveals only a few entries for ionic metal species that are also stable in solution or the solid phase. In our research we have focused on the kinetics and thermodynamics of gasphase ion/molecule reactions that often feature metal-containing reactants that can be studied both in gas- and condensed-phases. Thus, the opportunity arises to investigate both intrinsic chemistry and thermodynamics rf such 0097^156/90AM2S-0070$06.00/0 © 1990 American Chemical Society

In Bonding Energetics in Organometallic Compounds; Marks, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

5. RICHARDSON ET A L

Gas-Phase Thermochemistry

71

species as well as the effect erf solvation on the kinetics and energetics of fundamental chemical processes. In this article, we will review some recent results that pertain to the thermodynamics of metal-ligand bonds and chargetransfer processes involving the loss or gain of an electron at a metal center (i.e., oxidation-reduction). We will focus exclusively on reactants with direct condensed-phase counterparts, such as metallocenes and metal tris-chelate complexes.


Experimental We have applied the Fourier transform ion cyclotron resonance mass spectrometiy (FTICR-MS) technique in most of our studies. The basics of the technique have been reviewed (2), and applications erf the method to gasphase metal ion chemistry (3) and coordination compounds have been reviewed recently (4). Ion cyclotron resonance has been used for many years to study ion/molecule reactions involving traditionally organic reactants (5). Indeed, that body of work provides an important survey of the types of information that might be gained through studies of inorganic species. Here we will give only a brief overview erf the capabilities erf the instrument and techniques used in the work summarized. The heart of the instrument is a six-sided ion trap lexated in a high magnetic field. The trap is maintained in a high vacuum chamber at background pressures of ~ 10~ torr, and neutrals are introduced into the system via leak valves or heated solids probes to give total pressures up to ~ 10" torr. We use two FTICR instruments based on the Nicolet FTMS 1000 at the University erf Florida, one with a 2 tesla field (maximum mass -2000 amu) and one with a 3 tesla field (maximum mass -3000 amu). The former instrument has a custom made vacuum system that allows rather accurate determination erf pressures of neutrals in the ion trap. The latter instrument has a high m/z resolution and is particularly well-suited to studies erf highmass ions often encountered in stuelies erf metal compounds. Ions produced in the center erf the trap (usually by electron impact) are constrained by the magnetic field and applied potentials on the trapping plates to follow orbits within the cell and only slowly diffuse toward the walls. During the time the ions are trapped, they may have many collisions with neutral molecules in the trap, and these collisions can lead to thermalization erf the ions and may result in chemical reactions. At a neutral pressure of 10" torr, the approximate pseudo first-order rate constant for ion-molecule collisions is 30 s". The ionic products erf ionization and subsequent reactions in the cell can be detected at any time mass spectrometrically by application of a broad range of radio frequencies and detection erf resonance at cyclotron frequencies corresponding to m/z values erf the ions present. Techniques are available that allow isolation erf one or more m/z value in the ion population by ejection erf all other ions, and this allows the reactions erf a particular ion to be followed directly. Fourier transform methexls, intrexluced by Marshall and 9

5

6

1



BONDING ENERGETICS IN ORGANOMETALLIC COMPOUNDS

Comisarow (6), allow the rapid detection and signal averaging for a complete mass spectrum of the ion population of the trap. We have also applied more traditional mass spectrometric methods in the study of organometallic ions. For preliminary appearance potential determinations, compounds were admitted into a nearly field-free source and ionized by variable energy electron impact. Detection of ions is by acceleration into a quadrupole mass spectrometer equipped with a sensitive detector. Ramping of the electron ionization energy rapidly through the range of the appearance potential (AP) allows determination of the AP for constant source conditions. AP values determined on the current instrument are in good agreement ( ± 0.1 - 0.2 eV) with literature values when comparisons are available (Berberich, D. W.; Hail, M. E.; Johnson, J. V.; Yost, R. A. Int. J. Mass Spectrom. Ion Ρ roc, in press). Chemistry of Cp ZrCH + (g) and Zr-R Bond Dissociation Enthalpies 2

?

+

C p Z r C H (g) (I, Cp = cyclopentadienyl) can be produced by electron impact ionization of Cp Zr(CH ) , and we have characterized the gas-phase reactions of this highly reactive ion with a number of substrates, including dihydrogen, alkenes, alkynes, and nitriles (7; C. Christ, J. R. Eyler, and D. E. Richardson, J. Am. Chem. Soc, in press; C. Christ, J. R. Eyler, and D. E. Richardson, submitted). Interest in this particular ion derives from a number of factors, some of which are summarized here. First, the study of the solution chemistry of highly electrophilic d° and d ? organometallic complexes has been rapidly expanding over the last several years (for leading references, see Ref. 8-12). In particular, these types of complexes, containing Ti(IV), Sc(IlI), or lanthanide(III) for example, are often efficient catalysts for C-H activation and the coordination polymerization of ethylene, and they are believed to be excellent homogeneous models for Ziegler-Natta catalysts (8). Second, Cr^ZrCT^ is thought to be the reactive complex in the solution chemistry of [Cp ZrCH (S) ] (S= solvent, n = 1,2), which has been extensively characterized by Jordan and coworkers (12). Third, the mechanistic chemistry of a d° ion is limited in the sense that oxidative addition is not generally an energetically viable option; thus, its reactivity is not expected to include the oxidative addition/reductive elimination pathways commonly encountered in the chemistry of bare metal ions with hydrocarbons (13). The overall reactivity of I in the gas-phase is very similar to the solution chemistry of related d° early transition metal compounds. 2

3

2

3

2

0

1

+

+

2

3

n

4

Relative Cp^Zr " -R Bond Energies. Thermochemical information from the study of ion/molecule reactions of I can be obtained through identification of thermoneutral or exothermic reactions with partially known thermochemistry. Although not all exothermic reactions are observed in the FTICR due to large kinetic barriers or competing pathways, all reactions that are observed must be exothermic or near thermoneutral (assuming that the reactant ions are thermalized). Endoergicity of more than a few kcal mol" will usually lead to 1


5. RICHARDSON ET AL.

73

Gas-Phase Thermochemistry

reactions with efficiencies too low for detection in the ion trap prior to ion loss (kforward/ collision < - M>"). Although this method is only semiquantitative in nature, it has provided some insight into Zr-ligand bond dissociation enthalpies for products of reactions of I with various substrates. As a simple example, consider the reaction of I with dihydrogen (Equation 1) (7). k

4

Cp ZrCH 2

+ 3

+H

Cp ZrH

2

+

+ CH

2

(1)

4

The hydrogenolysis reaction proceeds at a near collisional rate, but the activation of methane, the reverse reaction, does not proceed at a measurable rate in the FTICR. Thus the equilibrium constant (=k /k ) for Equation 1 is > 1, and the product Zr-H homolytic bond dissociation enthalpy, D(Cp2Zr H), is therefore greater than D(Cp2Zr -CH ) since D(H-H) = D(H C-H) = 104 kcal mol" . (Note that entropy changes in Equation 1 and following transformations are assumed to be negligible.) The difference in the Zr bond dissociation enthalpies cannot be derived from the data, but the order is typical for known condensed-phase organometallics and is opposite that usually observed for M r C H and M - H ions in bare metal ion chemistry (13) . In particular, for the closely related zirconium(IV) complex Cp* ZrR^ (Cp*=pentamethylcyclopentadienyl), D(Zr-H) - D(Zr-CH ) - 10 kcal mol" (14) . Another example involves the reaction of C p Z r C H with benzonitrile to produce the phenyl complex (Equation 2). The organic part of this


Î

T

+

+

3

3

1

+

+

3

2

f

3

+

2

Cp ZrCH 2

+ 3

+ C H CN 6

5

+

Cp ZrPh 2

3

+ CH CN

(2)

3

1

transformation (Equation 3) is endothermic by 8 ± 2 kcal mol" . Thus, for CH

3

+ C H C N -* Ph + C H C N 6

5

(3)

3

+

+

the homolytic Zr-R bond enthalpies, D(Cp Zr -Ph) - D(Cp Zr -CH ) > ca. 8 kcal mol" . This result is consistent with other thermochemical differences between M-Ph and M-CH bond dissociation enthalpies in d ? complexes. For example, Marks and co-workers found D(Th-Ph) - D(Th-CH ) = 13 ± 3 kcal mol" in C p ^ T h l ^ (15). From other observed reactions and thermochemical data, D(Cp2Zr -R) > DiCp^Zr* -CH ) for the following R groups in C p Z r R (minimum derived difference in kcal mol" in parentheses): -OH (-14), -C=CH (-20), r^-alryl (-2), -CH=C=CH (-4). The results are generally in accord with experience for other organometallic thermochemistry, but the method is clearly incapable of giving absolute values of dissociation enthalpies for the various metal-ligand bonds. 2

2

3

1

0

1

3

3

+

3

+

1

2

2

Absolute Zr-R Bond Dissociation Enthalpies. It is desirable to determine the absolute Zr-R bond dissociation enthalpies in gas-phase C p Z r R ions. We have used two methods to estimate these quantities. +

2


74

BONDING ENERGETICS IN ORGANOMETALLIC COMPOUNDS 1

The first approach is to investigate the reactivity of the d radical ion Cp Zr (g) (II), which is also formed in the electron impact ionization of Cp Zr(CH ) (g). The general reaction of interest is given in Equation 4. +

2

2

3

2

Cp Zr

+

2

+

+ RX ^ C p Z r X + R

(4)

2

The abstraction of a radical X from the substrate will only occur efficiently if the reaction is exothermic or near thermoneutral (in the absence of significant entropy change). II was found to abstract CI from CC1 , C C l ^ , and HCCI3, thereby placing a lower limit on the homolytic D(Cp Zr -CI) (rf -81 kcal mol" . By observation (rf other atom and group abstraction reactions analogous to Equation 4, the following lower limits were established (kcal mol' ): D(Cp2Zr -Br) > 83; D(Cp2Zr -OCH ) > 57; D(Cr>>Zr SCH ) > 67; D ( C p 2 Z r - N 0 ) > 70. Many (rf these bond enthalpies are likely to be much larger than the lower limits, but an absence erf atom and group transfer substrates with higher bond dissociation enthalpies and multiple reaction pathways found for many substrates limits the applicability

Gas-Phase Thermochemistry of Organometallic and Coordination

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