Gas phase photodissociation of MC2H2+ (M = Zr, Nb). Determination of D0(M+C2H2

10 February 1995

CHEMICAL PHYSICS LETTERS Chemical Physics Letters 233 (1995) 319-323

ELSEVIER

Gas phase photodissociation of MC2H+2 (M = Zr, Nb) Determination of D°( M +-CzH 2) Don Rufus A. Ranatunga, Ben S. Freiser H.C. Brown Laboratory of Chemistry, Purdue University, West Lafayette, IN 47907-1393, USA

Received 3 October1994; in final form 28 November1994

Abstract

Gas phase photodissociation of M(acetylene) ÷ where M = Nb, Zr is studied with Fourier transform ion cyclotron resonance mass spectrometry to determine metal ion-ligand bond energies. These ions are generated by the dehydrogenation reaction of thermalized metal ions with ethylene and stored in the trapping cell for photodissociation experiments. From the photoappearance thresholds of the simple cleavage product M ÷, experimental bond energy values of D°(Zr+-C2H2) = 59 _ 3 kcal/mol and D°(Nb+-C2H 2) = 57 ___3 kcal/mol are assigned. These energies can be compared to recent theoretical values of De(Zr+-C2H2) = 68 kcal/mol and D e ( N b + - C 2 H 2 ) = 59 kcal/mol.

I. Introduction

Activation of C - C and C - H bonds by transitionmetal ions in the gas phase is an active and interesting area of experimental research providing fundamental information on reaction mechanisms, kinetics and thermochemistry [1-3]. Specifically, the binding interaction of metal ions with simple ligands has been examined using numerous methods including guided ion beam experiments [4], competitive collision-induced dissociation (CID) [5], kinetic energy release distribution (KERD) experiments [6], and ligand-exchange ion-molecule reactions [7,8]. More recently, photodissociation methods have proven effective in obtaining absolute bond strengths for metal ion-ligand complexes and cluster ions [7-16]. The presence of the metal introduces a high density of low-lying electronic states at energies in the bond dissociation region for most transition metal ion complexes [7,8]. Thus, the photodissociation thresh-

olds obtained by monitoring the reaction ML++ hv~ M++ L (1) for these systems in many instances yield accurate absolute bond strengths, since they are determined by the thermodynamics of reaction (1) and not by the absorption characteristics of the ion. In the event that the first allowed excited state lies above the bond dissociation energy, however, the threshold is a spectroscopic one and yields only an upper limit on the bond energy. This is typically the case for organic ions [17], and has been observed for some organometallic ions [18,19]. Among the many systems investigated to date, M(acetylene) ÷ has been one of the most extensively studied. Experimental data for D ° ( M + - C 2 H 2) have been obtained for group 3 [20,21] and some other first- and second-row transition metals [22-25] by guided ion beam mass spectrometry. Group 3 transition metal-acetylene bond energies have also been obtained by photodissociation using Fourier trans-

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form ion cyclotron resonance (FFICR) mass spectrometry [16]. In addition, theoretical values for all first- and second-row transition metals have been computed to establish the trends in bonding for this system [26-28]. In most cases the agreement between the various experimental and theoretical values is quite good. Some discrepancies do occur and, often as not, it is the experimental value which is inaccurate. Ultimately, however, a consensus is reached indicating the importance of using several approaches to determine these values. In this Letter, we report the bond energies for MC2H ~- (M = Nb, Zr) determined by photodissociation threshold measurements using FTICR mass spectrometry and provide additional experimental evidence for the trends in bonding reported in theoretical computations for metal-acetylene complex ions [28].

2. Experimental All experiments were performed on a prototype Extrel FTMS-1000 FTICR mass spectrometer [29]. The 5.2 cm cubic trapping cell, which is situated between the poles of a Walker Scientific Inc. model HF-15H, 15 inch electromagnet maintained at 1.0 T, utilizes two 80% transmittance stainless steel screens as the transmitter plates to permit irradiation of the trapped ions. A Spectra Physics model 2030-18 argon ion laser was used as the irradiation source for photodissociation studies. The beam was directed into the instrument with optical mirrors and finally allowed to pass into the cell through a sapphire window. All chemicals were obtained in high purity from commercial sources and used as supplied except for multiple freeze-pump-thaw cycles to remove noncondensible gases. The reagents were introduced into the cell through General Valve Corporation series 9 pulsed solenoid valves [30] at a maximum pressure of about 10 -5 Torr and then pumped away by a high speed 6 inch diffusion pump in about 400 ms. Pressures were measured with an uncalibrated Granville-Phillips series 280 ionization gauge. Nb ÷ and Zr ÷ were produced by focusing the fundamental output (1064 nm) of a pulsed Quanta Ray Nd:YAG laser onto thin high-purity targets of

niobium and zirconium [31], respectively, and were allowed to collide with background argon leaked into the cell through a Varian leak valve at 2 × 10 -5 Torr for at least 1 s to produce thermalized ions. MC2H ~ (M = Nb, Zr) was generated by the dehydrogenation reaction of thermalized M + with pulsed in ethylene and carefully isolated by swept double resonance ejection pulses [32]. Isolated MC2H ~- was trapped in the same background pressure of argon for 2 s and then reisolated prior to irradiation for 4 s with individual lines from the argon ion laser in the wavelength region of 458-529 nm operated at 1 W maximum. The photoproduct intensities were blank subtracted to correct for any background reactions possible in the absence of light, normalized for the number of photons at each wavelength, and plotted as a function of wavelength. A conservatively estimated uncertainty of + 3 kcal/mol is assigned to the bond energy measurements. MC6H ~ (M = Nb, Zr) was prepared by the double dehydrogenation reaction of thermalized M ÷ with pulsed in cyclohexene and isolated as described above. Ligand displacement ion-molecule reactions with the isolated ions were carried out by pulsing in a second reagent through a second pulsed valve. Competitive CID experiments were performed on mixed ligand adduct ions. The collision energy in the laboratory frame corresponds to the maximum translational energy achievable and was varied typically in the range of 0-100 eV. The possibility of getting artificially lower binding energies due to multiphoton absorption through long-lived excited states [33] was studied by increasing the cooling time and the background argon pressure up to 3 s and about 5 X 10 -5 Torr, respectively. The reproducibility of the results obtained under different background pressures and cooling times suggests that the results are representative of singlephoton absorption by thermalized species.

3. Results and discussion Fig. 1 illustrates the photoappearance of Zr ÷ as a function of wavelength when ZrC2H ~- is irradiated with nine individual lines of the argon ion laser. The intensity of the photoproduct remains near zero in the region of 529-497 nm (54-58 kcal/mol) and is

D.RA. Ranatunga, B.S. Freiser / Chemical Physics Letters 233 (1995) 319-323

observed to rise sharply from 477 nm (60 kcal/mol) to a maximum at 458 nm (63 kcal/mol). By taking the wavelength of the last zero intensity and first non-zero intensity points, a threshold is assigned between 477 and 497 nm yielding a bond energy of D°(Zr+-C2H2) = 59 _+ 3 kcal/mol. Theoretical calculations by Bauschlicher and co-workers initially reported a value of 60.5 kcal/mol [28] for D~(Zr +C2H 2) and later increased it to 68 kcal/mol [34]. The higher value was obtained by estimating the corrections for correlation effects using values from the MCzH ~ system [35], which was assumed to have similar binding characteristics as the MCzH ~system. This estimated value is somewhat higher than our experimental value. Ligand displacement ion-molecule reactions were carried out to obtain additional information on D°(Zr+-C2H2 ). ZrCzH ~ was observed to react with benzene by displacement of the C z H 2 ligand to yield ZrC6H~-. For the reverse reaction, only the formation of the adduct, Z r ( C 6 H 6 X C z H 2 )+, was observed. These results indicate that D°(Zr+-C6H6 ) >D°(Zr+-CzH2). Furthermore, competitive CID experiments using the adduct, Z r ( C 6 H 6 ) ( C z H 2 )+, yielded ZrC6H~-, exclusively, over a wide energy range (0-100 eV in the laboratory frame) in agreement with the ion-molecule reaction results. Photodissociation of ZrC6H ~- using the argon ion laser yields two photoproducts, ZrC6H ~- and ZrC4H~-, due to the loss of H 2 and C 2 H 4 , respectively. The Energy ,

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Fig. 2. Photoappearance spectrum of Nb + as a function of wavelength for irradiation of NbC2H ~ with individual lines of the argon ion laser.

absence of benzene loss over the entire energy range used in the experiment suggests, but not unequivocally, a lower limit for D°(Zr+-C6H6) > 63 kcal/mol. NbC2H ~ photodissociates to produce Nb ÷, exclusively, when the ions are irradiated with the individual lines of the argon ion laser. As shown in Fig. 2, the photoproduct yield is observed to rise sharply at 502 nm (57 kcal/mol) to a peak at 497 nm (58 kcal/mol) with a threshold between 497-515 nm, implying D ° ( N b + - C 2 H 2 ) = 57 _+ 3 kcal/mol. This result is in excellent agreement with the theoretical value of De(Nb+-CzH2) = 59 kcal/mol reported by Bauschlicher and co-workers [34] after correcting their previous computations for correlation effects [28]. Interestingly, the peak at 497 nm is reproducible and apparently corresponds to a specific transition. NbC2 H+ reacts with benzene, again by ligand displacement to f o r m NbC6H~-. The absence of the reverse reaction to displace the benzene ligand by C 2 H 2 implies that D°(Nb+-C6H6 ) > D°(Nb +C2H2). Competitive CID of the adduct formed in the reverse reaction, N b ( C 6 H 6 X C 2 H 2 )+, yields predominantly NbC6H ~- in accordance with the ion-molecule reaction results. Photodissociation of N b C 6 H ~using the argon ion laser again produces two photoproducts, N b C 6 H ~- and NbC~H~-, due to the loss of H 2 and C 2 H 4 , respectively. The loss of complete ligand to yield Nb + was not observed, again suggest-

322

D.R.A. Ranatunga, B.S. Freiser / Chemical Physics Letters 233 (1995) 319-323

ing D ° ( N b + - C 6 H 6 ) > 6 3 k c a l / m o l . This lower limit is in agreement with D ° ( N b + - C 6 H 6 ) = 64 + 3 k c a l / m o l obtained from previous photodissociation threshold measurements on NbC6 H+ using a H g - X e arc lamp as the light source and cutoff filters to access energies up to ultraviolet region [15], and 66 + 7 k c a l / m o l derived from thermochemical cycles involving observed i o n - m o l e c u l e reactions of NbFe ÷ [36]. The theoretical binding energy for N b + - f 6 n 6 , 52.1 _+ 5 k c a l / m o l [37], is considerably lower than the above experimental values. This calculation, however, did not take into account the large correlation effect involved and the distortion of Nb + from the C6v symmetry axis. Thus, according to the results from the photodissociation of NbC6H ~ with both the argon ion laser and the arc lamp, and the i o n - m o l e c u l e reaction of NbC6H ~ with C2H2, we believe that the assignments D ° ( N b + - C z H 2) = 57 _ 3 k c a l / m o l < D ° ( N b + - C 6 H 6 ) = 64 _+ 3 k c a l / mol are the most reasonable. Recent theoretical calculations on the interaction of first- and second-row transition metal ions with acetylene have provided detailed analysis and bonding mechanisms for each metal ion [28]. For a given transition metal row, the strength of the covalent bond is calculated to decrease with increasing atomic number ( Z ) due to: (i) a growing discrepancy between the radial extent of the s and d orbitals of the metal making sd-hybridization less favorable and (ii) an increasing loss of d - d exchange energy accompanying the formation of the bond for the first half of the metals. In contrast, the strength of electrostatic bonding increases with increasing Z in relation to the decreasing size of the metal ion. Therefore, one can expect more favorable covalent bonding on the left side of a given row and a cross over to an electrostatic bonding mechanism at a certain metal proceeding to the right in a transition metal row. Because the covalent bond is much stronger than the electrostatic interaction, a sudden decrease in the bond strength from a certain metal to the next is observed. Our experimental bond energies for M + - C 2 H 2 (M = Nb, Zr) are in good agreement with the theoretical values computed for covalent bonding for these two systems, and considerably larger than the theoretical values calculated by assuming the electrostatic interaction of these two metal ions with acety-

lene, 34 and 32 k c a l / m o l for Zr + and Nb +, respectively [34]. Thus, the experimental values confirm that in these systems, the bonding is primarily covalent in origin.

Acknowledgements This work was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences in the US Department of Energy under the grant DE-FG02-87ER13766.

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