Highly Electrophilic (Salen)ruthenium(VI) Nitrido Complexes
Wai-Lun Man,†Tsz-Man Tang,† Tsz-Wing Wong,† Tai-Chu Lau,*,†Shie-Ming Peng,‡andWing-Tak Wong§
Department of Biology and Chemistry, City UniVersity of Hong Kong, Tat Che AVenue, Hong Kong, China, Department of Chemistry, National Taiwan UniVersity, Taipei 106, Taiwan, and Department of Chemistry,
UniVersity of Hong Kong, Pokfulam Road, Hong Kong, China Received August 13, 2003; E-mail: [email protected]
Recently, a number of osmium(VI) nitrido species containing nitrogen ligands, such as cis- and trans-[OsVI(N)(tpy)(Cl)
2]+(tpy
) 2,2′:6′,2′′-terpyridine), [OsVI(N)(tpm)(Cl)
2]+(tpm )
tris(1-pyraz-olyl)methane), [OsVI(N)(Tp)(Cl)
2] (Tp )
hydrotris(1-pyrazolyl)-borate anion), and [OsVI(N)(bpy)(Cl)
3] (bpy ) 2,2′-bipyridine), have
been shown to exhibit novel electrophilic properties.1-3They
un-dergo a variety of redox reactions that are initiated by nucleophilic attack at the nitrido ligand to produce osmium species containing N-C, N-N, N-P, or N-E (E ) O, S, Se) bonds. It is anticipated that the corresponding ruthenium(VI) nitrido complexes would be much more electrophilic, because high-valent ruthenium oxo com-plexes are in general much stronger oxidants than that of osmium.4
Surprisingly, the electrophilic chemistry of RutN is largely unexplored, most of the existing ruthenium nitrides contain strongly electron-donating anionic ligands, and they either show nucleophilic or weak electrophilic properties.5 Ruthenium nitrides containing
porphyrin ligands undergo amination reactions with hydrocarbons and silyl enol ethers in the presence of trifluoroacetic anhydride; however, the active electrophilic intermediate is an imido species rather than a nitrido species.6 We report here the synthesis and
reactivities of a series of highly electrophilic, cationic ruthenium-(VI) nitrido species containing the cyclohexyl-bridged salen ligand and its derivatives, [RuVI(N)(L)]+(L ) N,N′
-bis(salicylidene)-o-cyclohexyldiamine dianion). These are also the first examples of high-valent salen complexes of ruthenium. Although salen ligands have played important roles in oxidation chemistry in recent years and they are known to stabilize complexes in high oxidation states,7
high-valent ruthenium salen chemistry has not been explored. Treatment of [NBun
4][RuVI(N)Cl4] with H2L in methanol
follow-ed by the addition of X-produced the diamagnetic [RuVI(N)(L)(CH 3
-OH)]X (X ) ClO4, 1a or PF6, 1b) as orange microcrystalline solids.8
The neutral [RuVI(N)(L)Cl] (1c) can be prepared by adding [NBun
4
]-Cl to 1a or 1b in acetone. Other 5,5′-disubstituted (Cl, Br, MeO) salen complexes can also be prepared by the same procedures. The ν(RutN) stretching frequencies for 1a, 1b, and 1c occur at the same frequency at 1059 cm-1, and as expected this peak is shifted to 1030 cm-1upon15N labeling.
The X-ray structure of 1a is shown in Figure 1. The Ru-N(nitrido) distance is 1.592 Å, which is longer than that of the starting material [RuVI(N)Cl
4]-(1.570 Å),9but is shorter than those
in other ruthenium nitrido complexes (1.594-1.656 Å).5,6
1 is stable to air in the solid state. However, in solvents such as
DMF or DMSO, a facile N‚‚‚N coupling reaction occurs at room temperature. A 0.01 M solution of 1a in DMF turns from orange to green within minutes in an NMR tube, and gas bubbles can be seen. Analysis by GC-MS indicates that the gas released is N2.
When the 50%15N-labeled complex [Ru(15N)(L)(CH
3OH)]ClO4is
used, the gas evolved is found to be a mixture of14N
2,14N15N, and
15N
2, and the ratio of14N15N to15N2 is approximately 2:1. The
electrospray ionization mass spectrum of the resulting green solution shows peaks at m/z ) 422 and 568, which are assigned to [RuIII
-(L)]+and [RuIII(L)(DMF)
2]+, respectively; these assignments are
supported by the excellent agreement between the experimental and theoretical isotopic distribution patterns. These observations are con-sistent with a N‚‚‚N coupling process represented by eqs 1 and 2.
This is the first report of N‚‚‚N coupling between ruthenium nitrides. Previously, coupling of metal nitrides to form dinitrogen has only been observed with Os-Os10and Os-Mo.11Coupling of
OsVI-OsVIis very slow and usually requires heating to over 100
°C. Coupling of OsVI-MoVIis faster, but still requires heating to
50 °C. On the other hand, coupling of OsVI-OsV or OsV-OsV
occurs readily at room temperature. In contrast, facile coupling of
1 occurs with RuVI-RuVI. Cyclic voltammetry of 1a in CH 3CN
shows an irreversible peak at -0.67 V (vs Fc+/Fc), which is assigned to the reduction of RuVI to RuV; this rather low redox
potential suggests that reduction of RuVIto RuVby the solvent prior
to coupling is very unlikely.
In nonpolar solvents such as CH2Cl2or CHCl3, 1 is stable with
respect to N‚‚‚N coupling for over 1 day at room temperature. Dilute solutions of 1 (<1× 10-4M) in CH3CN and CH3OH are also
rea-sonably stable at room temperature (t1/2> 1 h). Thus, it is possible
to study the reactivities of the ruthenium nitrides in these solvents. These ruthenium nitrides are found to react rapidly with a variety of nucleophiles, including amines, phosphines, alkenes, thiols, azide, and cyanide. Their reactions with secondary amines are described below to illustrate that these complexes are highly electrophilic.
1a reacts rapidly with secondary amines such as pyrrolidine
(HNC4H8), to produce a RuIV-hydrazido(1-) species, [RuIV
(N(H)-NC4H8)(L)(HNC4H8)]+, isolated as the PF6-salt (2).122 is found
to be diamagnetic (solid sample, Gouy method), consistent with its RuIVformulation. The1H NMR spectrum in CD
3CN exhibits a †City University of Hong Kong.
‡National Taiwan University. §University of Hong Kong.
Figure 1. ORTEP diagram of the cation of 1a (H atoms are omitted). Selected bond lengths (Å): N(1) 1.592(4), N(2) 2.018(4), Ru-N(3) 2.030(3), Ru-O(1) 1.977(3), Ru-O(2) 1.972(3), Ru-O(3) 2.431(3).
2[RuVI(N)(L)]+f [(L)RuIIINtNRuIII(L)]2+ (1)
[(L)RuIIINtNRuIII(L)]2++ 4DMF f
2[RuIII(L)(DMF)2]++ N2 (2)
Published on Web 12/24/2003
478 9 J. AM. CHEM. SOC. 2004,126, 478-479 10.1021/ja037899f CCC: $27.50 © 2004 American Chemical Society
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N-H resonance at 13.3 ppm that disappears upon addition of D2O;
this is assigned to the hydrazido N-H. For Mo(VI)-hydrazido-(1-) complexes, the N-H resonances also occur at around 13 ppm.13The IR spectrum (KBr) of 2 shows a strong and a weak
band at 3217 and 3276 cm-1, respectively, which are assigned to ν(N-H) stretching.
The structure of 2 has been determined by X-ray crystallography (Figure 2). Although there is no evidence for N(3)-H and N(4)-H in the X-ray determination, the rather long Ru-N(hydrazido) dis-tance of 1.940 Å and the very acute Ru-N-N angle (129.4°) are consistent with a RuIV-hydrazido(1-) rather than a RuV
-hydra-zido(2-) species.14In the case of osmium, the Os-N(hydrazido)
distances in [OsV(tpm)(Cl)
2(NN(CH2CH3)2)]BF4 and [OsIV
(tpm)-(Cl)2(N(H)N(CH2)4O)]PF6are 1.855 and 1.904 Å, respectively.15
The N-N distance (1.279 Å) lies within the range found in other transition metal hydrazido complexes (1.17-1.42 Å) and is indic-ative of a double bond.15,16
The kinetics of the reaction between 1a and morpholine have been studied by UV-vis stopped-flow spectrophotometric methods. The reaction is first order in RuVIand second order in morpholine
with k(CH3CN, 25.0°C) ) 2.08 ((0.07)× 106M-2s-1. This rate
constant is over 4 orders of magnitude larger than that of the corresponding reaction of the electrophilic osmium nitride trans-[OsVI(N)(tpy)(Cl)
2]+with morpholine, which has a similar rate law
with k ) 58.1 ((1.2) M-2s-1.15
In conclusion, we have prepared a series of highly electrophilic (salen)ruthenium(VI) nitrido complexes which are probably the most electrophilic transition metal nitrido complexes reported so far. The facile N‚‚‚N coupling in polar solvents and the reaction with amines have not previously been observed with other ruthenium nitrides. N‚‚‚N coupling for Ru occurs readily in the +VI oxidation state, while that of Os occurs readily only in the +V oxidation state; this difference could have implications on the mechanism of the reverse NtN splitting process. The reaction of R2NH with RuVItN results in the formation of RuIV-N(H)NR
2, while with OsVItN the product is OsV-NNR
2; because the products are different, it is likely that
the mechanisms are also different.
Acknowledgment. The work described in this paper was substantially supported by a grant from the Research Grants Council of Hong Kong (CityU 1117/00P), partially supported by the City University of Hong Kong, the National Taiwan University, and the University of Hong Kong.
Supporting Information Available: NMR, kinetics, and
crystal-lographic data (PDF and CIF). This material is available free of charge via the Internet at http://pubs.acs.org.
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JA037899F Figure 2. ORTEP diagram of the cation of 2. (H atoms are omitted.)
Selected bond lengths (Å) and bond angles (deg): Ru(1)-N(1) 2.002(5), Ru(1)-N(2) 2.009(5), Ru(1)-O(1) 2.024(4), Ru(1)-O(2) 2.038(4), Ru-(1)-N(3) 2.157(5), Ru(1)-N(4) 1.940(5), N(4)-N(5) 1.279(6), N(5)-N(4)-Ru(1) 129.4(4).
C O M M U N I C A T I O N S
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