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Synthesis, Spectroscopic Investigation, and Theoretical Study of the Ruthenium Diazide Complex [Ru(tpy)(PPh3)(N3)2]Won K. Seoka, Sun B. Yima, Heung N. Leea, and Thomas M. Klapötkeb

a Department of Chemistry, Dongguk University,26 Pil-Dong, Chung-Ku, Seoul 100-715, Korea

b Department of Chemistry, University of Munich (LMU),Butenandtstr. 5-13 (D), D-81377, Munich, Germany

Reprint requests to Prof. Dr. T. M. Klapötke. E-mail: tmk@cup.uni-muenchen.de

Z. Naturforsch. 55 b, 462^-66 (2000); received March 10, 2000Ruthenium, Diazide, ZINDO/1 Calculations

The reaction of [Ru(tpy)(PPh3)(Cl)2] (tpy = 2,2':6',2"-terpyridine) with sodium azide in acetone/water afforded [Ru(tpy)(PPh3)(N3)2]. The new Ru-diazide complex was fully char­acterized by elemental analysis, IR, Raman, and multinuclear NMR spectroscopy. The struc­tures, energies and vibrational frequencies of the ruthenium diazide isomers, cis- and trans- [Ru(tpy)(PMe3)(N3)2] (Me=CH0, were calculated at the semi-empirical ZINDO/1 level of theory.

1. Introduction

The six-electron reduction of nitrite to ammonia catalyzed by the nitrite reductase enzyme is an im­portant step in the overall reduction of nitrate to ammonia in green plants [1]. Attempts to model the active site o f the enzyme via a synthetically ac­cessible way have been reported by many research groups [2]. Certain features of the reactivity have been reported: R u -N 0 2+ is reduced to Ru-N H 32+ quantitatively or Ru-N H 32+ is rapidly and quanti­tatively oxidized to give R u-N 02+ [3], The related complexes with various nitrogen-containing ligands have been known as models of intermediates in the multiple electron transfer of nitrogen fixation chem ­istry. It is well known that the metal complex con­taining the nitride ligand is a precursor in the nitrite reduction reaction. From the point o f coordination chemistry, the strong trans effect o f the nitride lig­and plays an important role in determining stereo­chemistry and, presumably, substitution rates [4],

The metal nitride complex containing a metal- nitrogen triple bond can be formed through chem ­ical activation and photolysis of the metal azide complex [5]. Like NCS and NCO, the azide anion is often used as a ligand and its geometry can be lin­ear or non-linear. Recent results on preparation and structural characterization of metal azides [6 , 7] and diazides [8] prompt us to report the synthesis and

spectroscopic investigation o f the ruthenium diazide complex. The semi-empirical calculations of theo­retical models of ruthenium diazide isomers, cis- and rra«5,-[Ru(tpy)(PM e3)(N 3)2] (Me = CH3), were performed at the ZINDO/1 level o f theory using the VSTO-3G* basis set.

2. Results and Discussion

The preparations of six-coordinate ruthenium and osmium complexes have been reported by many research groups [9]. The syntheses reported here represent an alternative procedure for obtain­ing sim ilar complexes. The mono-chloro complex [Ru(tpy)(PPh3)2(C1)](PF6) (tpy = 2,2':6',2"-terpyr- idine) prepared by the reaction of the free ligand PPh3 with [Ru(tpy)(PPh3)Cl2] was useful in obtain­ing ligand PPh3 with [Ru(tpy)(PPh3)Cl2] was use­ful in obtaining the m ono-azide complex [Ru(tpy)- (PPh3)2(N3)](PF6) [6 , 9, 10]. We found that the ruthenium diazide complex was obtained by the reaction of [Ru(tpy)(PPh3)2(Cl)](PF6) with excess N aN 3 albeit in low yield. However, the reaction of fra«s-[Ru(tpy)(PPh3)2(Cl)2] with 2.6 molar equiva­lent o f N aN 3 in acetone/water solution provided the desired product with higher yield (eq. ( 1)).

[R u(tpy)(PPh,)C l2] + 2 N aN , L°;' >

[Ru(tpy)(PPh3)(N3)2] + 2NaCl ( 1)

0932-0776/00/0600-0462 $ 06.00 © 2000 Verlag der Zeitschrift für Naturforschung. Tübingen ■ www.znaturforsch.com K

W. K. Seok et al. • The Ruthenium D iazide C om plex [Ru(tpy)(PPh3 )(N 3 )2] 463

1329

Raman Shift / cm

Fig. 1. Raman spectrum of [Ru(tpy)(PMe3)(N3)2].

Fig. 2. 'H-NMR spectum of [Ru(tpy)(PMe3)(N3)2] in the region of 6.0~ 10.0 ppm.

2.1. Spectroscopy

The new diazide complex of ruthenium was char­acterized by elemental analysis and spectroscopic methods. A broad M LCT band at A max = 550 nm was observed in the UV-vis spectrum. The most intense and characteristic absorption peaks for the asym ­metric in plane and out o f plane diazide stretching modes were found at 2032 and 2014 cm -1 in the FT-IR spectrum.

The Raman spectrum of the ruthenium diazide complex shown in Fig. 1 displays the characteristic symmetric azide stretching mode as a very strong peak at 1329 cm -1 .

The characterization of the complex was corrob­orated with the use of proton, nitrogen, and phos­phorus NMR spectroscopy. The 'H -N M R spec­trum (Fig. 2) showed well resolved peaks in the region between 7.5 and 8.9 ppm for the tpy ligand ring and between 6.9 and 7.3 ppm

for the aromatic protons of the PPh^ ligand rings.

It should be noted that the chemical shift of one of the 6 or 6" protons of the tpy ligand in complexes of the type [(tpy)(PPh3)2Run-L]+/2+ is affected by the nature of the Ligand L and is usually observed far downfield as a doublet o f doublets [6]. This obser­vation can be explained by the m olecular structure, which shows that both 6 and 6" protons of the tpy ligand ring are out of the ring currents of the tpy and PPI13 rings. We observed that the chemical shift o f these particular protons in the [Ru(tpy)(PPh3)2- (N3)]+ complex are slightly shifted from 8.73 to 8.80 ppm in the [Ru(tpy)(PPh3)(N3)2] complex. B e­cause most o f the protons for the tpy ligand are effectively shielded by in plane phenyl groups in the PPh3 ring systems, they are observed in a re­gion upheld from that proton presumably located in a deshielding region. The 'H NM R data sug­gest the trans geometry. The 31P NM R spectrum of

464 W. K. Seok et al. • The Ruthenium D iazide C om plex [Ru(tpy)(PPh3 )(N 3 )2 ]

Table 1. Semi-empirically calculated structural parameters and total energies for m7/25-[Ru(tpy)(PMe3)(N3)2] (la) and c/j,-[Ru(tpy)(PMe3)(N3)2] (lb) obtained using ZINDO/1 with a VSTO-3G* basis set.

Complex E [kcal/mol] NIMAX (N3, asym) [cm-1] (Int.) J(Ru-Na) [A] d(Na-Nb) [Ä] d(Nb-Nc) [A] Z(NaNbNc) [°]

la -15317 0 2094(6) ip 2078(10) oop

1.97 1.24 1.19 174.7

lb -15306 0 2090(10) ip 1.94a 1.24a 1.19“ 173.8a2074(6) oop 1.96b 1.24b 1.18b 170.2b

Expl.c 2032, 2014

a frans-position with respect to the PMe3 group;b c/s-position with respect to the PMe3 group;c [(tpy)Ru(PPh3)(N3)2]; d scaled with the empirical factor of 0.62.

the complex shows a singlet resonance at 32.3 ppm for the coordinated PPh3 group, which suggests a trans complex [6, 11]. It is shifted to slightly lower field than that of ruthenium mono-azide molecule [Ru(tpy)(PPh3 )2(N3)]+ (26.5 ppm).

14N NMR spectroscopy was also used to char­acterize the prepared diazide complex. From earlier l4N NMR data for coordinated azides [12, 13], indi­vidually well resolved resonances of Na, Nb, and Nc (connectivity: Ru-Na-Nb-Nc) have been assigned in the 14N NMR spectrum of the complex. Each res­onance at -337, -136, and -225 ppm corresponds to azide coordinated to Ru in the [Ru(tpy)(PPh3)2- (Nß)]+ complex [6], Although the three N atoms of the tpy ligand system in the [Ru(tpy)(PPh3 )2 ( ^ 3 )2] complex could not be observed, the three nitrogen chemical shifts for the diazide ligands coordinated to the ruthenium atom in the complex were obtained at -335, -132, -278 ppm. It should be noted that the relative line widths in the l4N NMR spectra for the Ru-azide complex are related to the local symmetry around the nitrogen atom [14- 16].

2.2. Semi-empirical calculations

The structure and total energy of the cis and trans isomers of the ruthenium diazide complex containing PM e3 instead of PPI13 were calcu­lated at the semi-empirical ZINDO/1 level o f the­ory. These cis- and trans- [(tpy)Ru(PM e3)(N3)2] molecules have been used instead of the correspond­ing [(tpy)Ru(PPh3)(N3)2] complexes, because the trim ethylphosphine ligand is electronically similar and closely related to the triphenylphosphine lig­and. Reducing the number of parameters can also save cpu time.

The isomers are very close in their total energy with the trans form being thermody-

Fig. 3. Structure of a formula unit of (a) rä-[Ru(tpy)- (PMe3)(N3)2] and (b) rratts-[Ru(tpy)(PMe3)(N3)2] based on semi-empirical calculations with HyperChem. Grey circles denote carbon atoms.

namically slightly favored over the cis isomer [AE(cis-trans) = 11 kcal mol-1 ], as is shown in Table 1.

W. K. Seok et al. • The Ruthenium D iazide C om plex [R u(tpy)(PPh3 )(N 3 )2] 465

Because the calculations were performed for iso­lated molecules and ions in the gas phase, the sta­bilities in the condensed phase might be reversed for cis and trans isomers due to cation-anion inter­actions, neutral molecule-ion interactions, solvent interventions, and crystal packing effects [17].

For the both cis and trans isomers the stationary points were characterized as true m inim a (within the ZINDO/1 model) by frequency analyses (in all cases NIMAG = 0; NIMAG, number of imaginary frequencies) [18]. The assignments o f the experi­mentally observed IR frequencies of the complex [Ru(tpy)(PPh3)2(N3)2] were done in accord with those of the previously reported trans-[Ru(tpy)~ (PPh3)2(N3)]+ complex [6]. The asym m etric N3 stretching mode, which is very strong in the IR spec­trum, was also calculated (Table 1). M ore important than the numbers, however, is the fact that trans- and ci\y-[(tpy)Ru(PMe3)(N3)2] are expected to show two absorption peaks for the asymmetrical N 3 stretch­ing mode. These two bands were calculated to be of comparable intensity and to appear very close in wavenumbers. For the asymmetric N3 stretching mode of the trans- and c/s-[Ru(tpy)(PPh3)(N3)2] complex, one absorption was observed [6].

3. Experimental

RuCb, 2,2':6',2"-terpyridine(tpy), NaN3, and NH 4 PF6

were purchased from Aldrich Chemical Co. and used without further purification. Triphenylphosphine (Aldrich Chemical Co.) was recrystallized twice from ethanol [19]. Dimethylsulfoxide- ^ 6 (99.9 atom% D), and dichloromethane-di (99.6% D) (Aldrich Chemical Co) were used as received. Acetonitrile was purified by distil­lation over P4 O 10 under an Ar atmosphere. Distilled water was used after passing through a Nanopure™ (Bamstead) water system.

Routine UV-visible spectra were recorded on a Hewlett-Packard 8452A Diode Array spectrophotometer. FT-IR spectra were obtained on a Bomen MB 100 FT-IR spectrophotometer either on nujol mulls or in solutions us­ing NaCl plates. All 'H and31P NMR data were obtained with a Varian Gemini-200 FT-NMR spectrometer using (CD^bSO as solvent at 25 °C. The chemical shift param­eters are presented in parts per million ( 6 ) downfield from internal reference tetramethylsilane (TMS) while the MP chemical shifts were referenced to external 85% H3 PO4 . The 14N NMR spectrum was recorded in a 10 mm NMR tube using CD 2CI2 as solvent at -2 0 °C using a Brucker SY 200 spectrometer operating at -14 .462 MHz and was referenced to external CH3 NO 2 in CD 2 CI2 solution. The

Raman spectrum was obtained on a Perkin Elmer Sys­tem R 2000 at 25 °C. Elemental analyses were performed by the analytical laboratory at Basic Science Institute o f Korea using a Carlo Erba E A 1180.

3.1. Preparation o f [Ru(tpy)(PPhj)(N3)?]

First method: Previously prepared [Ru(tpy)(PPh3 )Cl2] (100 mg, 0.15 mmol) [9c)] and N aN3 (25 mg, 0.39 mmol) dissolved in 10 ml o f acetone and 1 ml of distilled water were heated under reflux under a stream of Ar for 2 h. Dur­ing this period, a precipitate developed under a deep pur­ple solution. The hot reaction mixture was cooled down in the refrigerator to r. t. overnight. The precipitate was filtered off in air and washed with cold acetone and wa­ter. The deep purple-colored product was vacuum-dried. Yield: 61 mg (60%).

Second method: The same procedure as for the prepa­ration o f [Ru(tpy)(PPh3 )2 (N 3 )](PF6 ) was applied [6], ex­cept using 0.10 mmol of [Ru(tpy)(PPh3)2(Cl)](PF6) and 0.43 mmol o f sodium azide. Yield: 20 mg (30%).

Elemental analysis for RUPC3 3 H2 6 N 9 (680.67): Anal. Calcd C 58.23, H 3.85, N 18.52. Found: C 58.19, H 3.95, N 18.56%.

IR (c m -1): 3054 (w), 2032 (vs), 2014 (vs), 1600 (m), 1482 (m), 1446 (s), 1386 (s), 1280 (w), 1094 (w), 771 (s), 747 (m), 700 (s), 527 (s), 516 (s), 500 (m). - Raman (1064 nm, 200 mW, 500 scans, cm -1 , (Int)): 3060 (4), 20 4 0 (1 ), 1596 (12), 1484(3), 1463 (5), 1329 (17), 1280(5), 1178 (4), 1038 (8), 1007 (12), 725 (2), 646 (4) 526(4). - 'H NMR ( 6 (multiplicity)): 8.80 (d), 8.26 (d), 8.13 (d), 7.91 (t), 7.69 (t), 7.57 (t), 7.32 - 6.88 (m). - 31P NMR(6): 32.3. 14N NMR ( 6 ) : -3 3 5 , -1 3 2 , -278 .

3.2. Semi-empirical calculations

All semi-empirical calculations were performed with the program package HyperChem (version 4.0 and 5.0) at the ZINDO/1 level o f theory using a VSTO-3G* basis set [20, 21], The ZINDO/1 is a variation o f INDO extended to transition metals, which is equivalent to the most re­cent version o f the INDO/1 method that differs from the original by using constant orbital exponents. The model that is within the unrestricted Hartree-Fock formalism is characterized by inclusion o f one-center exchange terms as well as an accurate description o f integrals involving 3d atomic orbitals.

Acknowledgements

Financial support o f this research by the Korea Sci­ence and Engineering Foundation, MOE, the DFG and the Fonds der Chemischen Industrie is gratefully acknow­ledged.

466 W. K. Seok et al. • The Ruthenium D iazide C om plex [Ru(tpy)(PPh3 )(N 3 )2]

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