Osmium(VI) nitrido and osmium(IV) phosphoraniminato complexes containing schiff base ligands

Article abstract of DOI:10.1021/ic9814571

A series of osmium(VI) nitrido complexes containing Schiff base ligands, [Os^VI(N)(L)Cl] (L = salophen or salen), have been synthesized by reaction of the ligand with [NBuⁿ₄][Os^VI(N)Cl₄] in the presence of 2,6-dimethylpyridine. The ν(Os≡N) for the salophen complexes occur at around 1070 cm^-1 and are insensitive to the nature of the substituents present on the Schiff base ligand. The structures of [Os(N)(salophen)(MeOH)]ClO₄ and [Os(N)(5,5-Cl₂salophen)(MeOH)]ClO₄ have been determined by X-ray crystallography, and the Os≡N bond distances are 1.651 and 1.66 A?, respectively. The osmium(VI) nitrido complexes react rapidly with triphenylphosphine to produce the corresponding osmium(IV) phosphoraniminato complexes, [Os^IV(NPPh₃)(L)Cl]. The osmium(IV) complexes exhibit reversible Os^V/IV and Os^IV/III couples in cyclic voltammetry. The E_1/2 values show linear correlations with the Hammett constants σ_p of the substituents on the Schiff base ligand. The structure of [Os^IV(NPPh₃)(salophen)-Cl] has been determined by X-ray crystallography. The rather long Os-N(P) bond length (1.92 A) and acute Os-N-P bond angle (149.6°) suggest that there is no significant multiple-bond character in the Os-N bond. The kinetics of nitrogen atom transfer from a series of 5,5′-disubstituted salophen nitrido complexes to PPh₃ have been studied in CH₃CN at 25.0 °C by stopped-flow spectrophotometric method. The following rate law was obtained: -d[Os(VI)]/dt = k₂[Os(VI)][PPh₃]. The reactivities of the complexes were found to follow a Hammett correlation of log(k_X/k_H) with σ_p, with a ρ value of 1.9 ± 0.1. The positive ρ value is consistent with a transition state involving electrophilic attack by the nitrido ligand on the phosphorus atom.

Full text of DOI:10.1021/ic9814571

Inorg. Chem. 1999, 38, 6181-6186
6181
Osmium(VI) Nitrido and Osmium(IV) Phosphoraniminato Complexes Containing Schiff
Base Ligands
Tsz-Wing Wong,† Tai-Chu Lau,*,† and Wing-Tak Wong‡
Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue,
Hong Kong, People’s Republic of China, and Department of Chemistry, The University of
Hong Kong, Pokfulam Road, Hong Kong, People’s Republic of China
ReceiVed December 21, 1998
A series of osmium(VI) nitrido complexes containing Schiff base ligands, [OsVI(N)(L)Cl] (L ) salophen or salen),
have been synthesized by reaction of the ligand with [NBun ][OsVI(N)Cl4] in the presence of 2,6-dimethylpyridine.
4
The ν(OstN) for the salophen complexes occur at around 1070 cm-1 and are insensitive to the nature of the
substituents present on the Schiff base ligand. The structures of [Os(N)(salophen)(MeOH)]ClO4 and [Os(N)(5,5-
Cl2salophen)(MeOH)]ClO4 have been determined by X-ray crystallography, and the OstN bond distances are
1.651 and 1.66 Å, respectively. The osmium(VI) nitrido complexes react rapidly with triphenylphosphine to produce
the corresponding osmium(IV) phosphoraniminato complexes, [OsIV(NPPh3)(L)Cl]. The osmium(IV) complexes
exhibit reversible OsV/IV and OsIV/III couples in cyclic voltammetry. The E1/2 values show linear correlations with
the Hammett constants σp of the substituents on the Schiff base ligand. The structure of [OsIV(NPPh3)(salophen)-
Cl] has been determined by X-ray crystallography. The rather long Os-N(P) bond length (1.92 Å) and acute
Os-N-P bond angle (149.6°) suggest that there is no significant multiple-bond character in the Os-N bond.
The kinetics of nitrogen atom transfer from a series of 5,5′-disubstituted salophen nitrido complexes to PPh3 have
been studied in CH3CN at 25.0 °C by stopped-flow spectrophotometric method. The following rate law was
obtained: -d[Os(VI)]/dt ) k2[Os(VI)][PPh3]. The reactivities of the complexes were found to follow a Hammett
correlation of log(kX/kH) with σp, with a F value of 1.9 ( 0.1. The positive F value is consistent with a transition
state involving electrophilic attack by the nitrido ligand on the phosphorus atom.
Introduction
are known, there have been no reports on the corresponding
nitrido complexes. We report here the synthesis of a series of
Complexes containing Schiff base ligands such as salen and
its derivatives play important roles in oxidation chemistry in
recent years. For example (salen)chromium(V) oxo complexes
are active oxidants for stoichiometric and catalytic epoxidation
of olefins.1 Salen complexes of manganese(III)2 and ruthenium-
(III)6 are also efficient oxidation catalysts for organic substrates.
Asymmetric epoxidations were also achieved using manganese-
(III) catalysts containing chiral Schiff base ligands.3,4 Recently
(salen)manganese(V) nitrido complexes were found to produce
aziridines from olefins.5 We have initiated a program to
investigate the high-valent chemistry of other transition metal
Schiff base complexes. Although dioxoosmium(VI)7 and di-
alkoxoosmium(IV)8 complexes containing Schiff base ligands
osmium(VI) nitrido complexes containing the tetradentate Schiff
base ligands 5,5′-disubstituted salophen (salophen ) N,N′-bis-
(salicylidene)o-phenylenediamine dianion) and salen (salen )
N,N′-bis(salicylidene)ethylenediamine dianion). These osmium-
(VI) nitrido complexes are readily converted to the correspond-
ing osmium(IV) phosphoraniminato complexes by treatment
with triphenylphosphine.
* Corresponding author. E-mail: bhtclau@cityu.edu.hk.
† City University of Hong Kong.
‡ The University of Hong Kong.
(1) Samsel, E. G.; Srinivasan, K.; Kochi, J. K. J. Am. Chem. Soc. 1985,
107, 7606.
(2) Srinivasan, K.; Michaud, P.; Kochi, J. K. J. Am. Chem. Soc. 1986,
108, 2309.
(3) Zhang, W.; Loebach, J. L.; Wilson, S. R.; Jacobson, E. N. J. Am.
Chem. Soc. 1990, 112, 2801.
(4) Irie, R.; Noda, K.; Ito, Y.; Katsuki, T. Tetrahedron Lett. 1991, 32,
1055.
(5) Du Bois, J.; Tomooka, C. S.; Hong, J.; Carreira, E. M. Acc. Chem.
Res. 1997, 30, 364.
(6) Che, C. M.; Leung, W. H. Inorg. Chem. 1989, 28, 4619.
(7) Che, C. M.; Cheng, W. K.; Mak, T. C. W. Inorg. Chem. 1986, 25,
703.
Experimental Section
Materials. The complexes K[Os(N)(O)3], K[Os(15N)(O)3], [NBun ]-
4
[Os(N)Cl4], and [NBun ][Os(15N)Cl4] were prepared according to
4
literature methods.9,10 The Schiff base ligands were prepared by
condensation of 2 mol of the appropriate salicylaldehyde with 1 mol
of either o-phenylenediamine or ethylenediamine.11 Acetonitrile was
first refluxed over calcium hydride and then distilled under argon.
(9) Griffith, W. P.; Pawson, D. J. J. Chem. Soc., Dalton Trans. 1973,
(8) (a) Che, C. M.; Cheng, W. K.; Mak, T. C. W. Inorg. Chem. 1988, 27,
250. (b) Cheng, W. K.; Wong, K. Y.; Tong, W. F.; Lai, T. F.; Che, C.
M. J. Chem. Soc., Dalton Trans. 1992, 91.
1315.
(10) Cowman, C. D.; Trogler, W. C.; Mann, K. R.; Gray, H, B. Inorg.
Chem. 1976, 15, 1749.
10.1021/ic9814571 CCC: $18.00 © 1999 American Chemical Society
Published on Web 12/04/1999

6182 Inorganic Chemistry, Vol. 38, No. 26, 1999
Wong et al.
Triphenylphosphine (Aldrich) was recrystallized from n-hexane. All
other chemicals were of reagent grade and were used without further
purification.
(MeO)2salophen) (75 mg, 0.2 mmol) and 2,6-dimethylpyridine (0.1 mL)
were added, and the mixture was stirred at 0 °C for 1 h. The resulting
brick-red solid was filtered and washed with diethyl ether. Yield: 49
mg (40%). Anal. Calcd for [OsVI(N)(5,5′-(MeO)2salophen)Cl]: C,
43.21; H, 2.99; N, 6.58. Found: C, 43.21; H, 2.99; N, 6.58. 1H NMR
(δ/ppm, acetone-d6): 3.91 (s, 6H), 7.53 (d, 2H), 7.42 (m, 4H), 7.71
(m, 2H), 8.28 (m, 2H), 9.49 (s, 2H).
Preparations. [OsVI(N)(salophen)Cl] (1a). [NBun ][Os(N)Cl4] (120
4
mg, 0.2 mmol) was dissolved in CH3CN (10 mL). H2(salophen) (64
mg, 0.2 mmol) and 2,6-dimethylpyridine (0.1 mL) were added, and
the mixture was heated to 40 °C for 1 h. After cooling, the resulting
orange-red solid was filtered and washed with diethyl ether. The
compound was recrystallized from methanol/ether. Yield: 92 mg (83%).
Anal. Calcd for [OsVI(N)(salophen)Cl]: C, 43.36; H, 2.53; N, 7.59.
Found: C, 43.56; H, 2.72; N, 7.70. 1H NMR (δ/ppm, CD3OD-d4): 7.19
(t, 2H), 7.53 (d, 2H), 7.73 (d, 2H), 7.89 (t, 2H), 8.03 (d, 2H), 8.51 (m,
[OsVI(N)(5,5′-(MeO)2salophen)(CH3OH)](ClO4) (4b). This was
prepared from 4a by metathesis with LiClO4 in methanol solution.
Yield: 40%. Anal. Calcd for [OsVI(N)(5,5′-(MeO)2salophen)(CH3OH)]-
(ClO4): C, 38.90; H, 3.38; N, 5.92. Found: C, 39.04; H, 3.07; N, 5.77.
1H NMR (δ/ppm, acetone-d6): 3.91 (s, 6H), 7.53 (d, 2H), 7.42 (m,
4H), 7.72 (m, 2H), 8.28 (m, 2H), 9.50 (s, 2H). Conductivity in
2H), 9.87 (s, 2H). Conductivity in CH3OH: Λ ) 68 Ω-1 cm2 mol-1
.
[OsVI(N)(salophen)(CH3OH)](ClO4) (1b). This was prepared by the
addition of LiClO4 (ca. 200 mg) to a methanolic solution of 1a (100
mg in 20 mL). The orange-red crystalline solids were washed with a
little methanol and then ether. Yield: 82 mg (70%). Crystals suitable
for X-ray crystallography were obtained by slow evaporation of a
methanol solution of 1b. Anal. Calcd for [OsVI(N)(salophen)(CH3OH)]-
ClO4: C, 38.87; H, 2.27; N, 6.80. Found: C, 39.02; H, 2.51; N, 6.85.
1H NMR (300 MHz, 298 K, CD3OD): δ 7.20 (t, 2H), 7.53 (d, 2H),
7.73 (d, 2H), 7.90 (t, 2H), 8.03 (d, 2H), 8.51 (m, 2H), 9.88 (s, 2H).
methanol: Λ ) 62 Ω-1 cm2 mol-1
.
[OsVI(N)(salen)Cl] (5a). A solution containing [NBun ][Os(N)Cl4]
4
(120 mg, 0.2 mmol) and N,N′-bis(salicylidene)ethylenediimine (H2salen)
(55 mg, 0.2 mmol) in acetone (10 mL) was stirred at room temperature
for 4 h. The resulting red precipitate was filtered and washed with a
little acetone and then ether. Yield: 51 mg (50%). Anal. Calcd for
[OsVI(N)(salen)Cl]: C, 37.98; H, 2.77; N, 8.31. Found: C, 38.35; H,
1
3.02; N, 8.50. H NMR (δ/ppm, CD3OD-d4): 4.88 (s, 4H), 7.08 (m,
2H), 7.41 (d, 2H), 7.76 (m, 4H), 8.98 (s, 2H).
Conductivity in MeOH: Λ ) 72 Ω-1 cm2 mol-1
.
[OsIV(NPPh3)(salophen)Cl] (6). [OsVI(N)(salophen)Cl] (100 mg,
0.18 mmol) was dissolved in CH3CN (10 mL). PPh3 (47 mg, 0.18
mmol) was added, and the mixture was stirred under argon for 0.5 h.
The resulting dark brown solid was collected and washed with diethyl
ether. Yield: 103 mg (70%). Crystals suitable for X-ray crystallography
were obtained by diffusing diethyl ether into a dichloromethane solution
of 6. Anal. Calcd for [OsIV(NPPh3)(salophen)Cl]: C, 55.90; H, 3.55;
N, 5.15. Found: C, 56.13; H, 3.72; N, 5.24.
[OsIV(NPPh3)(5,5′-Cl2salophen)Cl] (7). A mixture of 2a (0.15
mmol) and PPh3 (0.15 mmol) in acetone (10 mL) was stirred at room
temperature under argon for 0.5 h. The resulting dark brown solid was
filtered and washed with diethyl ether. Yield: 80%. The product was
recrystallized from CH3CN/ether. Anal. Calcd for [OsIV(NPPh3)(5,5′-
Cl2salophen)Cl]: C, 51.54; H, 3.28; N, 4.75. Found: C, 50.73; H, 3.13;
N, 4.75.
[OsIV(NPPh3)(5,5′-Me2salophen)Cl] (8). This was prepared from
3a by a procedure similar to that of 7. The product was recrystallized
from CH2Cl2/ether. Yield: 80%. Anal. Calcd for [OsIV(NPPh3)(5,5′-
Me2salophen)Cl]: C, 56.91; H, 3.91; N, 4.98. Found: C, 57.03; H,
3.87; N, 5.01.
[OsIV(NPPh3)(5,5′-(MeO)2salophen)Cl] (9). This was prepared from
4a by a procedure similar to that for 7. Yield: 70%. Anal. Calcd for
[OsIV(NPPh3)(5,5′-(MeO)2salophen)Cl]: C, 51.21; H, 3.64; N, 4.37.
Found: C, 51.42; H, 3.77; N, 4.29.
Warning: Perchlorate salts are potentially explosive. Although we
have not encountered any explosions so far, the amount of perchlorate
salts of osmium nitrido complexes used should be less than 100 mg
each time.
[OsVI(N)(salophen)(NCS)] (1c). NaNCS (30 mg) was added to a
solution of 1a (100 mg) in methanol (10 mL), and the orange solution
was stirred for 1 h. The solid collected was washed with cold methanol
and then diethyl ether. Yield: 73 mg (70%). Anal. Calcd for [OsVI(N)-
(salophen)(NCS)]: C, 43.75; H, 2.43; N, 9.72. Found: C, 43.45; H,
1
2.43; N, 9.55. H NMR (300 MHz, 298 K, CD3OD): δ 7.20 (t, 2H),
7.53 (d, 2H), 7.73 (d, 2H), 7.90 (t, 2H), 8.03 (d, 2H), 8.51 (m, 2H),
9.88 (s, 2H).
[OsVI(N)(5,5′-Cl2salophen)Cl] (2a). [NBun ][Os(N)Cl4] (0.12 g, 0.2
4
mmol) was dissolved in CH2Cl2 (10 mL). H2(5,5′-Cl2salophen) (77 mg,
0.2 mmol) and 2,6-dimethylpyridine (0.1 mL) were added, and the
mixture was stirred at room temperature for 1 h. The resulting orange-
red solid was filtered and washed with diethyl ether. The compound
was recrystallized from methanol/ether. Yield: 75 mg (60%). Anal.
Calcd for [OsVI(N)(5,5′-Cl2salophen)Cl]: C, 38.55; H, 3.21; N, 6.75.
Found: C, 38.20; H, 3.00; N, 6.94. 1H NMR (δ/ppm, CD3OD-d4): 7.52
(d, 2H), 7.77 (t, 2H), 7.84 (d, 2H), 8.07 (d, 2H), 8.50 (m, 2H), 9.87 (s,
2H).
[OsVI(N)(5,5′-Cl2salophen)(CH3OH)](ClO4) (2b). This was pre-
pared from 2a by metathesis with LiClO4 in methanol. Yield: 60%.
Crystals suitable for X-ray crystallography were obtained by diffusing
diethyl ether into a methanolic solution of 2b. Anal. Calcd for
[OsVI(N)(5,5′-Cl2salophen)(CH3OH)](ClO4): C, 35.07; H, 2.51; N, 5.85.
Found: C, 35.19; H, 2.14; N, 5.82. 1H NMR (δ/ppm, CD3OD-d4): 7.53
(d, 2H), 7.77 (t, 2H), 7.85 (d, 2H), 8.07 (d, 2H), 8.49 (m, 2H), 9.87 (s,
Physical Measurements. IR spectra were obtained from KBr disks
by using a Bomen MB-120 FTIR spectrophotometer. UV-vis spectra
were recorded on either a Hewlett-Packard 8452A diode-array spec-
trophotometer or a Shimadzu UV3100 spectrophotometer. NMR spectra
were recorded on a Varian (300 MHz) FT NMR spectrometer. Cyclic
voltammograms were obtained from a PAR model 273 potentiostat. A
glassy carbon disk working electrode and a Ag/AgNO3 reference
2H). Conductivity in MeOH: Λ ) 80 Ω-1 cm2 mol-1
.
[OsVI(N)(5,5′-Me2salophen)Cl] (3a). These orange-red solids were
prepared by a procedure similar to that for 2a using H2(5,5′-Me2-
salophen). Yield: 70%. Anal. Calcd for [OsVI(N)(5,5′-Me2salophen)-
Cl]: C, 45.40; H, 3.44; N, 7.22. Found: C, 45.83; H, 3.57; N, 7.51.
1H NMR (δ/ppm, CD3OD-d4): 2.27 (s, 6H), 7.25 (t, 2H), 7.54 (d, 2H),
7.56 (d, 2H), 7.63 (s, 2H), 8.30 (t, 2H), 9.61 (s, 2H).
electrode were used. The supporting electrolyte was 0.1 M [NBun ]PF6
4
in CH3CN. Elemental analyses were done on a Elementar Vario EL
analyzer. Magnetic measurements were carried out using a Sherwood
magnetic balance.
[OsVI(N)(5,5′-Me2salophen)(CH3OH)](ClO4) (3b). This was pre-
pared from 3a by a procedure similar to that for 2b using H2(5,5′-Me2-
salophen). Yield: 40%. Anal. Calcd for [OsVI(N)(5,5′-Me2salophen)-
(CH3OH)](ClO4): C, 40.74; H, 3.10; N, 6.20. Found: C, 41.14; H,
X-ray Crystallography. All pertinent crystallographic data and other
experimental details are summarized in Table 1. All measurements were
made on a Rigaku AFC7R diffractometer or a MAR research image
plates scanner with graphite-monochromated Mo KR radiation (λ )
0.710 73 Å). The data were collected at a temperature of 25 ( 1 °C
using the ω-2θ scan technique (AFC7R) or ω-scan technique (MAR).
The data were corrected for Lorentz and polarization effects. Absorption
corrections by ψ-scan method or an approximation by interimage
scaling were applied. The structure was solved by heavy-atom Patterson
methods (6, 1b) or direct methods (2b)12 and expanded using Fourier
techniques. Some non-hydrogen atoms were refined anisotropically,
whereas the rest were refined isotropically. Hydrogen atoms on the
1
2.91; N, 6.31. H NMR (δ/ppm, CD3OD-d4): 2.27 (s, 6H), 7.26 (t,
2H), 7.54 (d, 2H), 7.56 (d, 2H), 7.62 (s, 2H), 8.30(t, 2H), 9.6(s, 2H).
Conductivity in MeOH: Λ ) 80 Ω-1 cm2 mol-1
.
[OsVI(N)(5,5′-(MeO)2salophen)Cl] (4a). [NBun ][Os(N)Cl4] (120
4
mg, 0.2 mmol) was dissolved in ice-cold acetone (10 mL). H2(5,5′-
(11) Pfeiffer, P.; Breith, E.; Lubbe, E.; Tsumaki, T. Liebigs Ann. 1933,
503, 84.

Os(VI) and Os(IV) Complexes
Inorganic Chemistry, Vol. 38, No. 26, 1999 6183
Table 1. Summary of Crystal Data, Details of Data Collection Solution, and Refinement Parameters for Compounds 1b, 2b, and 6
1b
2b
6
empirical formula
M
OsC21H18N3O7Cl
650.04
OsC21H16N3O7Cl3
718.93
OsC38H31N3O3PCl
834.31
cryst color, habit
cryst size/mm
cryst syst
space group
a/Å
b/Å
c/Å
brown, block
0.21 × 0.22 × 0.22
monoclinic
P21/n (no. 14)
11.552(1)
14.856(1)
13.554(2)
110.91(2)
2172.9(5)
4
red, block
0.14 × 0.16 × 0.21
monoclinic
P21/n (no. 14)
10.738(2)
11.019(2)
19.645(2)
99.86(1)
2290.2(5)
4
orange, block
0.21 × 0.22 × 0.24
monoclinic
P21/n (no. 14)
14.722(1)
14.241(2)
16.038(2)
98.39(2)
3326.5(6)
4
â/deg
U/Å3
Z
Dc/g cm-3
1.987
2.085
1.666
F(000)
1256
60.37
20 128
4225
3385
0.046
0.055
1384
59.64
4041
3817
2559
0.062
0.046
1648
40.02
28 147
5106
3121
0.067
0.084
µ(Mo KR)/cm-1
no. of reflns collected
no. unique reflns
no. of obsd reflns, I > 1.5σ(I)
Ra
b
Rw
goodness of fit
max. ∆/σ
no. of params
max., min. density in
∆F map/e Å-3
2.36
0.06
298
0.64, -0.24
2.18
0.07
306
1.87, -0.95
2.31
0.03
204
0.61, -0.66
a R ) ∑×e7×e7Fo×e7 - ×e7Fc×e7×e7/∑×e7Fo×e7. b Rw ) [∑w(×e7Fo×e7 -×e7Fc×e7)2/∑wFo2)]1/2
.
organic moieties were generated in their ideal positions (C-H, 0.95
Å), while hydrogen atoms on the hydroxyl groups were located from
difference Fourier synthesis. Hydrogen atoms were included but not
refined. All calculations were performed on a Silicon-Graphics
computer, using the program package TEXSAN.13
Kinetics. The kinetics of the reaction of osmium(VI) nitrido
complexes with PPh3 were performed under pseudo-first-order condi-
tions ([PPh3] . [OsVI]) in CH3CN using an Applied Photophysics DX-
17MV stopped-flow spectrophotometer. The progress of the reaction
was monitored by measuring absorbance changes at λmax of OsVI.
Pseudo-first-order rate constants, kobs, were obtained by nonlinear least-
squares fits of At to time t according to the equation At ) Ao + (Ao -
A∞) exp(-kobst), where Ao and A∞ are the initial and final absorbances,
respectively.
in CH3OH, suggesting both species are present either as the
five-coordinate[OsVI(N)(L)]+ orasthesix-coordinate[OsVI(N)(L)-
(CH3OH)]+ species. Treatment of 1a with excess NaNCS in
methanol produced [OsVIN(salophen)(NCS)] (1c). The IR
spectrum shows the ν(CtN) at 2068 cm-1, which is shifted
from the value of 2053 cm-1 for NaNCS, suggesting that the
NCS- is coordinated to the osmium. Magnetic measurements
indicated that all the complexes are diamagnetic, as found in
other osmium(VI) nitrido complexes.14 The V(OstN) for the
salophen chloro complexes can be assigned to a medium peak
at around 1070 cm-1, as verified by the expected shift of ∼35
cm-1 to lower frequency upon 15N labeling (Table 2). (Assign-
ment of V(OstN) for the perchlorate salts was unsuccessful
-
due to strong absorption by ClO4 in that region.) The
Results and Discussion
frequencies are lower than that of the starting [NBun ][OsVI(N)-
4
Cl4] (1126 cm-1 15
)
and [NBun ][Os(N)(CH2SiMe3)4] (1100
Synthesis of and Characterizations of Osmium(VI) Nitrido
4
Complexes. Treatment of [NBun ][OsVI(N)Cl4] with the pro-
cm-1),16 but are comparable to that of [Os(N)(dmbipy)Cl3] (1084
4
cm-1 17 and [NBun ][Os(N)(S2C6H4)2] (1063 cm-1).18 The
)
tonated Schiff base ligand (H2L) and 2,6-dimethylpyridine in
acetonitrile, acetone, or dichloromethane afforded [OsVI(N)LCl]
as air-stable orange-red crystals. The 15N-labeled complexes
4
V(OstN) of the salophen complexes are, however, insensitive
to the nature of the substituents present on the Schiff base ligand.
The UV-vis spectra of the nitrido complexes in CH3CN (Table
2, Figure 1) show an intense peak at 430-470 nm, which
probably arises from salophen2- to OsVI ligand-to-metal charge
transfer (LMCT) transitions. Cyclic voltammetry of the nitrido
complexes shows an irreversible peak between -0.60 and -0.79
V (vs Fc/Fc+), which is assigned to the reduction of OsVI to
OsV.
were similarly prepared using [NBun ][Os(15N)Cl4]. No product
4
could be isolated in the absence of 2,6-dimethylpyridine nor
when stronger bases such as triethylamine were used instead.
The salen complex was obtained in lower yields than the
salophen complexes; it was also more difficult to purify.
Conductivity measurements indicated that [OsVI(N)(L)Cl] is
essentially a 1:1 electrolyte in CH3OH, in accord with the strong
trans-influence of the nitrido ligand, although the Cl- is probably
ligated to the osmium in the solid state. [OsVI(N)(L)Cl] can be
readily converted to [OsVI(N)(L)(MeOH)]ClO4 (1b) by metath-
esis with LiClO4 in methanol. Conductivity measurements
indicated that 1b is also present as a 1:1 electrolyte. The UV-
vis spectra of the chloride and the perchlorate salts are identical
Structures of [OsVI(N)(salophen)(MeOH)]ClO4 (1b) and
[OsVI(N)(5,5′-Cl2 salophen)(MeOH)]ClO4 (2b). The molecular
structure of 1b has been determined by single-crystal X-ray
diffraction method, and a perspective view of the cation is shown
(14) Che, C. M.; Yam, V. W. W. AdV. Inorg. Chem. 1992, 39, 233.
(15) Collin, R. J.; Griffith, W. P.; Pawson, D. J. Mol. Struct. 1973, 19,
531.
(12) Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.; de
Gelder, R.; Israel, R.; Smits, J. M. M. The DIRDIF 94 program system,
Technical Report of the Crystallography Laboratory; University of
Nijmegen, 1994.
(13) TEXSAN, Crystal structure analysis package; Molecular Structure
Corporation: Houston, TX, 1985 and 1992.
(16) Marshman, R. W.; Shapley, P. A. J. Am. Chem. Soc. 1990, 112, 2,
8369.
(17) Chin, K. F.; Cheung, K. K. Yip, H. K. Mak, T. C. W.; Che, C. M. J.
Chem. Soc., Dalton Trans. 1995, 657.
(18) Sellmann, D.; Wemple, M. W.; Donaubauer, W.; Heinemann, F. W.
Inorg. Chem. 1997, 36, 1397.

6184 Inorganic Chemistry, Vol. 38, No. 26, 1999
Wong et al.
Table 2. UV-Vis (CH3CN), IR (KBr), and Electrochemical Data (CH3CN) for [OsVI(N)L(Cl)]
Ep,c (OsV/VI
)
L
λ
max (nm (ꢀ, M-1, cm-1))
V(Ost14N)/cm-1
V(Ost15N)/cm-1
(V vs Fc/Fc+)a
salophen
430 (7600), 340 (20470), 300 (15130),
234 (26500)
440 (7450), 336 (19800), 302 (16600),
282 (15400), 242 (34000), 210 (29700)
438 (9770), 344 (29300), 304 (21800),
238 (37300), 210 (38500)
470 (8720), 344 (26500), 312 (22200),
246 (34600), 210 (31600)
530 (1320), 370 (3600), 308 (4870),
276 (5070), 222 (9180)
1072
1037
-0.79
-0.60
-0.75
-0.73
5,5′- Cl2salophen
5,5′-(Me)2salophen
5,5′-(MeO)2salophen
salen
1067
1067
1070
1094
1036
1035
a Glassy carbon working electrode, Pt counter electrode, Ag/AgNO3 reference electrode, 0.1 M [NBun ]PF6 supporting electrolyte.
4
Figure 3. Cyclic voltammogram of [OsIV(NPPh3)(salophen)Cl] (6) (in
CH3CN, vs ferrocenium/ferrocene).
The crystal structure of 2b was also determined in an attempt
to study the effects of the chloro substituent on the osmium-
nitrido bond. The coordination geometry around the osmium is
very similar to that of 1b. In this case the molecules do not
Figure 1. UV-vis spectra of [OsVI(N)(salophen)(CH3OH)]ClO4 (1b)
and [OsIV(NPPh3)(salophen)Cl] (6) in CH3CN.
form an infinite chain; instead the oxygen atom of the
coordinated CH3OH is hydrogen bonded to the ClO4-. The
osmium-nitrido bond distance of 1.66 Å is virtually identical
to that in 1b, indicating that the chloro substituent has little or
no effect on the osmium-nitrido bond. These bond distances
are, however, among the longest in osmium nitrido complexes;
they are comparable to that of trans-[Os(N)(tpy)Cl2] (1.663 Å),19
but are longer than that of [Os(N)Cl5]2- (1.614 Å),20 [Os(N)-
(CH2SiMe3)4]- (1.631 Å),16 and [Os(N)(L1)Cl] (1.634 Å, H2L1
) 1,2-bis(2-hydroxy-2,2′-diphenylethyl)pyridine).21
Synthesis and Characterizations of Osmium(IV) Phos-
phoraniminato Complexes. The nitrido ligand in [OsVI(N)-
(L)Cl] is electrophilic, as evidenced by its rapid attack by
triphenylphosphine to produce the corresponding phosphoran-
iminato complex [OsIV(NPPh3)(L)Cl], which readily crystallizes
from acetone as brown, air-stable solids. The IR spectrum of
[OsIV(NPPh3)(salophen)Cl] (6) shows a medium peak at 1089
cm-1 that is assigned to ν(PdN); this assignment is supported
by the expected shift of this peak to 1065 cm-1 in the 15N-
labeled complex. The ν(PdN) is slightly lower than the value
of 1112 cm-1 in trans-[OsIV(tpy)(Cl)2(NPPh3)](PF6)2.21 In
contrast to the corresponding nitrido complex, [OsIV(NPPh3)-
(L)Cl] is essentially a nonelectrolyte in CH3CN, suggesting that
there is little trans-influence of the phosphoraniminato ligand.
Figure 2. ORTEP diagram of [OsVI(N)(salophen)(CH3OH)]ClO4 (1b).
in Figure 2. The osmium center is in a distorted octahedral
environment; it is displaced ∼0.4 Å above the mean salophen
plane, a feature that is also found in several other nitrido
species.14 The nitrido ligand and the oxygen atom of the
coordinated methanol are trans to one another with an angle of
176°. The cations are joined together by intermolecular hydrogen
bonding between the O-H group of methanol and a phenoxy
oxygen (O(2)) to form an interesting infinite chain structure.
The osmium-nitrido bond distance is 1.651 Å. The Os-
O(MeOH) distance of 2.346 Å is rather long, reflecting the
strong trans effect of the nitrido ligand.14
(19) Pipes, D. W.; Bakir, M.; Vitols, S. E.; Hodgson, D. J.; Meyer, T. J. J.
Am. Chem. Soc. 1990, 112, 5507.
(20) Bright, D.; Ibers, J. A. Inorg. Chem. 1969, 8, 709.
(21) Demadis, K. D.; Bakir, M.; Klesczewski, B. G.; Williams, D. S.; White,
P. S.; Meyer, T. J. Inorg. Chim. Acta 1998, 270, 511.

Os(VI) and Os(IV) Complexes
Inorganic Chemistry, Vol. 38, No. 26, 1999 6185
Table 3. Selected Bond Lengths (Å) and Angles (deg) for the
Complexes 1b and 2b
1b
2b
Os-N(3)
1.651(7)
2.043(6)
2.036(7)
1.997(5)
1.984(6)
2.346(7)
98.4(3)
1.66(1)
2.02(1)
2.02(1)
1.979(9)
1.981(8)
2.27(1)
96.8(5)
99.7(5)
103.6(5)
102.0(5)
175.3(5)
81.1(5)
75.9(4)
78.5(4)
82.3(4)
Os-N(1)
Os-N(2)
Os-O(1)
Os-O(2)
Os-O(3)
N(1)-Os-N(3)
N(2)-Os-N(3)
O(1)-Os-N(3)
O(2)-Os-N(3)
N(3)-Os-O(3)
N(1)-Os-O(3)
N(2)-Os-O(3)
O(1)-Os-O(3)
O(2)-Os-O(3)
97.7(3)
103.6(3)
104.3(3)
176.2(3)
79.1(2)
79.1(2)
78.8(2)
Figure 4. Plots of E1/2 vs σp for the OsV/IV and the OsIV/III couples of
[OsIV(NPPh3)(5,5′-X2salophen)Cl].
78.8(2)
Table 4. Selected Bond Lengths (Å) and Angles (deg) for Complex
6
Os-N(3)
1.92(1)
2.02(1)
1.99(1)
2.03(1)
2.03(1)
2.424(5)
1.56(1)
96.1(6)
93.7(6)
89.5(5)
88.7(5)
176.1(4)
85.0(4)
90.2(4)
86.7(3)
90.3(3)
149.6(10)
Os-N(1)
Os-N(2)
Os-O(1)
Os-O(2)
Os-Cl(1)
N(3)-P(1)
N(1)-Os-N(3)
N(2)-Os-N(3)
O(1)-Os-N(3)
O(2)-Os-N(3)
N(3)-Os-Cl(1)
N(1)-Os-Cl(1)
N(2)-Os-Cl(1)
O(1)-Os-Cl(1)
O(2)-Os-Cl(1)
Os-N(3)-P(1)
Figure 5. ORTEP diagram of [OsIV(NPPh3)(salophen)Cl] (6).
Complex 6 has a room-temperature magnetic moment of 1.82
µB, consistent with its formulation as a d4 phosphoraniminato
complex of osmium(IV).21 The UV-vis spectra data of the
osmium(IV) complexes are shown in Table 4. (See also Figure
1.) The intense bands in the visible region are most probably
ligand-to-metal charge transfer (LMCT) in origin; possible
sources include Cl-, PPh3dN-, and salophen2- to OsIV transi-
tions. Cyclic voltammetry of the Os(IV) complexes in CH3CN
exhibits reversible OsV/IV and OsIV/III couples (Table 5 and
Figure 3), except for complex 9, where the OsV/IV couple cannot
be observed. Similar reversible couples were also observed for
Os(IV) phosphoraniminato complexes containing terpyridine
ligands.21 As shown in Table 5, there is a general trend of
increased stability for OsV over OsIV, and for OsIV over OsIII,
as the substituents on the 5,5′-positions of the salophen ligand
are changed from electron withdrawing to donating. The effect
of a substituent on the reduction potential is best demonstrated
by the correlation of E1/2 and σp, the Hammett constant.
Reasonable straight lines were obtained when E1/2 was plotted
against σp, with slopes of 0.20 ((0.02) and 0.33 ((0.05) for
the OsV/IV and the OsIV/III couples, respectively (Figure 4). The
smaller slope for the OsV/IV couples means that the reduction
potentials are less sensitive to substituent effects than the OsIV/III
couples. A possible explanation is that at higher oxidation states
multiple bonding between Os and NPPh3 dominates, and
electron doanation by the Schiff base ligand is less important.
Structure of [OsIV(NPPh3)(salophen)Cl] (6). The structure
of 6 was determined by X-ray crystallography, and a perspective
view of the complex is shown in Figure 5. The coordination
around the osmium is a distorted octahedron, the osmium atom
lies in the plane of the Schiff base ligand, in contrast to the
situation in the osmium(VI) nitrido complexes. This is probably
Table 5. UV-Vis and Electrochemical Data for [OsIV(NPPh3)(L)Cl] in CH3CN
max (nm (ꢀ, M-1 cm-1))
E1/2 (OsV/IV
)
E1/2 (OsIV/III
)
L
λ
(V vs Fc/Fc+)a
(V vs Fc/Fc+)a
salophen
566 (4140), 455 (11500), 430 (12300),
340 (18200),287 (25500)
+0.14
-0.21
5,5′-Cl2salophen
581 (5860), 540 (7000), 442 (13800), 343 (21600),
329 (24600), 314 (25600), 279 (29400), 245 (37500)
581 (4500), 538 (5520), 436 (13200), 348 (19500),
332 (21700), 313 (23800), 290 (26800)
514 (3720), 435 (13300), 408 (14700), 346 (15100),
328 (15400), 290 (24000), 260 (24100), 223 (36300)
+0.20
+0.10
b
-0.12
-0.27
-0.30
5,5′-Me2salophen
5,5′-(MeO)2salophen
a Glassy-carbon working electrode, Pt counter electrode, Ag/AgNO3 reference electrode, 0.1 M [NBun ]PF6 supporting electrolyte. b Not observed.
4

6186 Inorganic Chemistry, Vol. 38, No. 26, 1999
Wong et al.
Table 6. Second-Order Rate Constants at 25.0 °C for the Reaction
of PPh3 with [OsVI(N)(L)(CH3OH)]ClO4
L
k2/M-1 s-1
salophen
5,5′-Cl2salophen
5,5′-Me2salophen
5,5′-(MeO)2salophen
(2.53 ( 0.11) × 104
(1.05 ( 0.02) × 105
(1.16 ( 0.02) × 104
(1.51 ( 0.08) × 104
the result of a substantial decrease in osmium-nitrogen bond
order on going from Os(VI) to Os(IV). The Os-N(PPh3) bond
length is 1.92 Å, which is longer than the normal range of 1.65-
1.86 Å.22 The Os-N-P bond angle is also rather acute (149.6°).
These data suggest that there is no significant multiple-bond
character in the Os-N bond, although the bond order is probably
somewhat larger than that in trans-[OsIV(tpy)Cl2(NPPh3)], which
has a Os-N(PPh3) bond length of 2.093 Å and a Os-N-P
bond angle 132°.21 The Os-Cl distance of 2.424 Å is
comparable to the Os-Cl(trans) (2.365 Å) and Os-Cl(cis)
(2.394 Å) in cis-[OsIV(tpy)(Cl)2(NPPh2Me)]PF6.21
Figure 6. Plot of log(kX/kH) vs the Hammett substituent constant σp
for the reaction of PPh3 with [OsVI(N)(5,5′-X2salophen)(CH3OH)]ClO4.
Kinetics of the Reaction of Osmium(VI) Nitrido Com-
plexes with Triphenylphosphine. Rapid spectral changes were
observed when a solution of an osmium(VI) schiff base nitrido
complex was mixed with an excess of triphenylphosphine in
CH3CN. Analysis of the final spectrum indicated quantitative
formation of the corresponding osmium(IV) phosphoraniminato
complex. The rates of the reaction were followed in acetonitrile
at λmax of the osmium(VI) complex, using excess PPh3; [OsVI]
) (1.0-2.0) × 10-5 mol dm-3 and [PPh3] ) (0.10-2.0) ×
10-3 mol dm-3. Pseudo-first-order kinetics were obeyed for over
three half-lives, and plots of the pseudo-first-order rate constants
kobs with concentrations of triphenylphosphine were linear,
indicating the following rate law: rate ) kobs[OsVI] ) k2[OsVI]-
[PPh3]. Values of k2 at 25.0 °C are given in Table 6. These rate
constants are comparable to the value of 1.36 × 104 dm3 mol-1
s-1 obtained for trans-[OsVI(N)(tpy)Cl2]Cl. The rate constants
are sensitive to the nature of the substituent on the Schiff base
ligand; complexes with electron-withdrawing substituents react
more rapidly than those with electron-donating substituents. A
good Hammett correlation can be obtained; the plot of log(kX/
kH) versus σp is linear with F ) 1.9 ( 0.1 (Figure 6). The
positive F value is consistent with a transition state involving
electrophilic attack by the nitrido ligand on the phosphorus atom.
A similar Hammett correlation with F ) 2.9 was also obtained
in the epoxidation of olefins by chromium(V) oxo complexes
containing salen ligands.1
Conclusion
A series of osmium(VI) nitrido complexes have been
synthesized, and their structures are established by X-ray
crystallography. This work extends the chemistry of high-valent
transition metal complexes containing Schiff base ligands. The
nitrido ligands in these complexes are electrophilic, as evidenced
by the rapid attack by triphenylphosphine to produce the
corresponding osmium(IV) phosphoraniminato complexes.
Acknowledgment. We thank the Hong Kong Research
Grants Council, City University of Hong Kong, and the
University of Hong Kong for financial support.
Supporting Information Available: Tables of crystallographic data
and data collection parameters, atomic coordinates, anisotropic dis-
placement parameters, all bond lengths and angles, nonbonded contacts,
and least-squares planes for 1b, 2b, and 6. This material is available
free of charge via the Internet at http://pubs.acs.org.
(22) Dehnicke, K.; Stra¨hle, J. Polyhedron. 1989, 8, 707.
IC9814571