Protonation and deprotonation of TpOs(NHPh)Cl2: An unusually inert amido ligand

Article abstract of DOI:10.1021/ic001177x

Protonation of the Os(IV) amido complex TpOs(NHPh)Cl₂ (1) to give the aniline complex [TpOs(NH₂Ph)Cl₂]-OTf (2) requires excess triflic acid (HOTf). Complex 1 is unreactive with HCl and other moderately strong acids. Consistent with the low basicity of 1, the aniline complex 2 is extremely acidic and is deprotonated by stoichiometric addition of weak bases such as Cl^- or H₂O. No reaction is observed between 1 and methyl triflate (CH₃OTf) at ambient temperatures. Upon heating, CH₃OTf removes the chloride ligands from 1 to give CH₃Cl and the amidobis-(triflate) complex TpOs(NHPh)(OTf)₂ (3). Attack at the amido nitrogen is not observed. Complex 1 is thus very inert to protonation and electrophilic attack at nitrogen. A deprotonated form of 1, TpOs[NPh(MgBr)]Cl₂ (4), is generated on reaction of PhMgBr with TpOs(N)Cl₂. Complex 4 is extremely basic and will protonate to 1 with weak acids such as CH₃CN, DMSO, and acetic anhydride. Thus, 1 has a low acidity as well as a low basicity; it is both less acidic and less basic than aniline. The inertness of 1 is ascribed to partial Os-N π bonding and to the oxidizing nature of the Os(IV) center.

Full text of DOI:10.1021/ic001177x

1888
Inorg. Chem. 2001, 40, 1888-1893
Protonation and Deprotonation of TpOs(NHPh)Cl2: An Unusually Inert Amido Ligand
Jake D. Soper, Brian K. Bennett, Scott Lovell,† and James M. Mayer*
Department of Chemistry, Campus Box 351700, University of Washington,
Seattle, Washington 98195-1700
ReceiVed October 24, 2000
Protonation of the Os(IV) amido complex TpOs(NHPh)Cl2 (1) to give the aniline complex [TpOs(NH2Ph)Cl2]-
OTf (2) requires excess triflic acid (HOTf). Complex 1 is unreactive with HCl and other moderately strong acids.
Consistent with the low basicity of 1, the aniline complex 2 is extremely acidic and is deprotonated by stoichiometric
addition of weak bases such as Cl- or H2O. No reaction is observed between 1 and methyl triflate (CH3OTf) at
ambient temperatures. Upon heating, CH3OTf removes the chloride ligands from 1 to give CH3Cl and the amidobis-
(triflate) complex TpOs(NHPh)(OTf)2 (3). Attack at the amido nitrogen is not observed. Complex 1 is thus very
inert to protonation and electrophilic attack at nitrogen. A deprotonated form of 1, TpOs[NPh(MgBr)]Cl2 (4), is
generated on reaction of PhMgBr with TpOs(N)Cl2. Complex 4 is extremely basic and will protonate to 1 with
weak acids such as CH3CN, DMSO, and acetic anhydride. Thus, 1 has a low acidity as well as a low basicity;
it is both less acidic and less basic than aniline. The inertness of 1 is ascribed to partial Os-N π bonding and to
the oxidizing nature of the Os(IV) center.
Introduction
Scheme 1. Formation of TpOs(NHPh)Cl2 (1) via Addition
of Ph- to TpOs(N)Cl2
Transition metal amide complexes, LnM(NRR′), are inter-
mediates in a number of catalytic processes and bond activation
reactions.1-4 Amide ligands are often quite reactive, particularly
when bound to later, less electropositive metals. They undergo
insertion, hydrogenation, and elimination and are especially
reactive toward electrophilic attack.5-9 The high reactivity is
due to facile heterolytic cleavage of the metal-nitrogen bond
rather than this bond being weak in a homolytic sense.10-11
The osmium(IV) amido complex TpOs(NHPh)Cl2 (1) is an
exception to these generalizations, being remarkably inert to
protonation and electrophilic attack [Tp ) hydrotris(1-pyra-
zolyl)borate]. Complex 1 is formed by addition of phenyl anion
to the electrophilic nitrido ligand in the osmium(VI) nitrido
complex TpOs(N)Cl2, followed by aqueous workup (Scheme
1).12,13 Described here are reactions that delineate the low
† UW Chemistry staff crystallographer.
basicity, acidity, and nucleophilicity of 1, as well as the synthesis
and structure of the even less basic triflate analogue TpOs-
(NHPh)(OTf)2 (3) (OTf ) triflate ) OSO2CF3). The unusual
properties of the amide ligands in 1 and 3 are discussed in terms
of Os-N π bonding and the oxidizing nature of the Os(IV)
center.
(1) Schrock, R. R.; Glassman, T. E.; Vale, M. G. J. Am. Chem. Soc. 1991,
113, 725-726.
(2) Nugent, W. A.; Ovenall, D. W.; Holmes, S. J. Organometallics 1983,
2, 161-162.
(3) Cowan, R. L.; Trogler, W. C. Organometallics 1987, 6, 2451-2453.
(4) Boncella, J. M.; Villanueva, L. A. J. Organomet. Chem. 1994, 465,
297-304.
(5) Lappert, M. F.; Power, P. P.; Sanger, A. R.; Srivastava, R. C. Metal
and Metalloid Amides; Wiley: New York, 1980.
(6) Cotton, F. A.; Wilkinson, G. AdVanced Inorganic Chemistry; Wiley:
New York, 1988; pp 367-373.
Results
Protonation. TpOs(NHPh)Cl2 (1) is unreactive toward excess
HCl (g), HCl/Et2O, acetic acid, acetyl chloride, methyl iodide,
phenol, toluidine, PhNH3+, and H2O at ambient temperatures
(7) Fryzuk, M. D.; Montgomery, C. D. Coord. Chem. ReV. 1989, 95, 1-40.
(8) Bryndza, H. E.; Tam, W. Chem. ReV. 1988, 88, 1163-1188.
(9) Sharp, P. R. J Chem. Soc., Dalton Trans. 2000, 2647-2657.
(10) Bryndza, H. E.; Fong, L. K.; Paciello, R. A.; Tam, W.; Bercaw, J. E.
J. Am. Chem. Soc. 1987, 109, 1444-1456.
(11) Mayer, J. M. Comments Inorg. Chem. 1988, 8, 125-135.
(12) Crevier, T. J.; Mayer, J. M. J. Am. Chem. Soc. 1998, 120, 5595-
5596.
1
in CDCl3 solution. The characteristic paramagnetic H NMR
spectrum of 1 is unchanged on addition of these reagents, even
after extended reaction times. To our knowledge, this is the first
amido complex that is inert to HCl. The lack of reaction is not
due to any instability of the anticipated product because
(13) Crevier, T. J.; Bennett, B. K.; Soper, J. D.; Bowman, J. A.; Dehestani,
A.; Hrovat, D.; Lovell, S.; Kaminski, W.; Mayer, J. M. J. Am. Chem.
Soc. 2001, 123, 1059-1071.
13
TpOsCl3 is stable and is itself inert to aniline at ambient
10.1021/ic001177x CCC: $20.00 © 2001 American Chemical Society
Published on Web 03/16/2001

TpOs(NHPh)Cl2
Inorganic Chemistry, Vol. 40, No. 8, 2001 1889
temperatures (eq 1). After the mixture is heated for 7 d at 80
°C, a small conversion of 1 to TpOsCl3 (∼12%) is observed
with HCl/Et2O in CDCl3, although no reaction is observed with
HCl (g) in CDCl3 or any of the other reagents listed above.
Under these conditions, TpOsCl3 plus aniline gives a very small
amount of 1 (<3%).14
Treatment of 1 with triflic acid (HOTf ) HOSO2CF3),
however, results in an immediate darkening of the solution and
changes in the NMR spectrum. The reactions are similar in
1
CDCl3 and in CD3CN. With 1 equiv of HOTf, the H NMR
signals for 1 broaden and new peaks are observed. Quantitative
conversion to the aniline complex [TpOs(NH2Ph)Cl2]OTf (2)
requires at least 2 equiv of triflic acid (eq 2). The observation
Figure 1. 1H NMR spectra of TpOs(15NHPh)Cl2 (1-15N) (top) and
[TpOs(15NH2Ph)Cl2]OTf (2-15N) (bottom) in CDCl3. The expanded
regions show the 15NH and 15NH2 doublets.
number of the chemical shifts are not in their normal (diamag-
netic) positions. For instance, the pyrazole peaks for 2 in CDCl3
range from 4.4 to -23.7 ppm and the three phenyl resonances
appear between 10.1 and 8.0 ppm. The pattern of Tp resonances
indicates that the Cs symmetry of 1 is retained in 2. Remarkably,
addition of 3 equiv of HOTf to 1 in CDCl3 causes the NH
resonance to shift from δ 5.5 ppm to δ ∼93 ppm (with a
concomitant increase in its integral). The chemical shifts for 2
in CDCl3 or CD3CN are dependent on triflic acid concentration.
Adding seven more equivalents of HOTf shifts δNH2 to ∼85
ppm, and smaller shifts are observed for the Tp and Ph
resonances. These shifts are probably due to changes in the
tight ion pairing between the osmium cation and OTf- or
[OTf-(HOTf)n] in CDCl3 solutions.18,19
that eq 2 is an equilibrium implies that the pKa of 2 in CH3CN
is roughly 3, the pKa of HOTf in this solvent.15 Isolation of 2
has not been feasible because of its highly acidic nature. Air-
free addition of water or stoichiometric PPN+Cl- in CDCl3
results in immediate deprotonation of 2 to give 1 (eq 3). The
The location of the added proton on the amido nitrogen has
been confirmed by NMR spectroscopy of isotopically labeled
analogues. In 15N-labeled 1, the amido proton appears as a
doublet with 1JNH ) 70.5 Hz (CDCl3), close to the NH coupling
constant reported for aniline, 78.6 Hz.20,21 When formed with
3 equiv of HOTf, 2-15N shows the NH2 peak at δ ∼90 ppm
split into a doublet with a coupling constant of 71.2 Hz (Figure
1). The observation of NH coupling shows that the NH protons
are not exchanging on the NMR time scale, either with other
NH protons or with the triflic acid present.
deprotonation of 2 by chloride is consistent with the inability
of HCl to protonate 1. Broadening and shifting of the 1H NMR
signals for 1 in CDCl3 are also observed on addition of 3 equiv
of trifluoroacetic acid, but conversion to 2 has not been
observed. Addition of chloride to this reaction mixture regener-
ates 1.
The identification of 2 as the aniline complex [TpOs(NH2-
Ph)Cl2]OTf is derived from its reactivity (eqs 2 and 3) and its
NMR spectra. Octahedral d4 complexes of the 5d transition
metals, such as the Os(IV) complexes 1 and 2, typically have
sharp-shifted 1H NMR spectra due to their temperature-
independent paramagnetism.16,17 Most of the NMR resonances
are not significantly broadened by the paramagnetism, but a
A reaction of 1 with 3 equiv of triflic acid-d (DOTf) afforded
two peaks in the 1H NMR at δ 92.8 and δ 92.5 ppm
corresponding to NH2 and NHD isotopomers. This is larger than
the typical separation of XH2 and XHD resonances presumably
(14) Based on related work [Bennett, B. K.; Soper, J. D.; Lovell, S.;
Kaminski, W.; Mayer, J. M. (work in progress)], this net substitution
may proceed by reduction of TpOsCl3 and substitution of chloride at
Os(III) followed by reoxidation.
(15) Izutsu, K. Acid-Base Dissociation Constants in Dipolar Aprotic
SolVents; Blackwell Scientific Publications: Boston, 1990.
(16) Randall, E. W.; Shaw, D. J. Chem. Soc. A 1969, 2867-2872.
(17) Chatt, J.; Leigh, G. J.; Mingos, D. M. P. J. Chem. Soc. A 1966, 1674-
1680.
(18) Bullock, R. M.; Song, J.; Szalda, D. J. Organometallics 1996, 15,
2504-2516.
(19) Spaltenstein, E.; Erikson, T. K. G.; Critchlow, S. C.; Mayer, J. M. J.
Am. Chem. Soc. 1989, 111, 617-623.
(20) Randall, E. W.; Zuckerman, J. J. J. Am. Chem. Soc. 1968, 90, 3167-
3172.
(21) Axenrod, T.; Pregosin, P. S.; Wieder, M. J.; Milne, G. W. A. J. Am.
Chem. Soc. 1969, 91, 3681-3682.

1890 Inorganic Chemistry, Vol. 40, No. 8, 2001
Soper et al.
Table 1. Time Course of NH2 and NHD Integrals for the Reaction
TpOs(NHPh)Cl2 + 3DOTf
time
(NHD + NH2) peak areas
initial, without C4H proton exchange
10 min
0.50 predicted
0.54
45 min
105 min
0.58
0.66
165 min
225 min
0.70
0.73
26 h
1.01
1.14 predicted
final, statistical C4H incorporation
because of the paramagnetic shifting of this resonance. H/D
exchange is rapid on the chemical time scale because integration
of the NH2, NHD, and HOTf signals shows a roughly statistical
distribution. Thus, the solution of 1 + 3 DOTf showed NHD
and NH2 integrals of ca. 0.38 and 0.16, consistent with the
statistical predictions of 0.375 and 0.125.22 Over time, however,
the total NHD and NH2 peak areas increase (Table 1), and the
intensities of the pyrazole triplets (C4H) at δ 2.42 and δ 1.54
ppm decrease (eq 4). After 26 h the integrals approach the
Figure 2. ORTEP diagram of TpOs(NHPh)(OTf)2 (3).
Table 2. Selected Crystallographic Data for TpOs(NHPh)(OTf)2 (3)
empirical formula
fw
C17H16BF6N7O6OsS2
793.5
cryst syst
cryst size (mm)
space group
a (Å)
b (Å)
c (Å)
triclinic
0.12 × 0.09 × 0.04
P1h (No. 2)
9.491(1)
11.604(2)
13.231(2)
73.580(8)
90.034(8)
70.000(7)
1305.8(3)
2
2.020
5.139
Mo KR
0.710 70
161(2)
R (deg)
â (deg)
statistical value expected for complete equilibration of the three
C4H protons with the original NH and the three DOTf, with
1.01 hydrogens in the NH2 + NHD resonances, vs the predicted
statistical 1.14 (4/7 enrichment at each of the two NH2
hydrogens). Deuterium NMR spectra confirm the H/D exchange
reactions, with initial spectra showing separate peaks for NHD
and ND2 at δ ∼90 ppm followed by slow growth of two upfield
peaks (δ ∼2 ppm) in the same positions as the pyrazole triplets
γ (deg)
vol (Å3)
Z
calcd density (g/cm3)
abs coeff (mm-1
radiation
)
wavelength (Å)
temp (K)
reflns collected
34 227
5489
362
5.61, 12.58
0.874
1
in the H NMR of 2. Such H/D exchange at the pyrazole C4
independent reflns
no. parameters
final R, Rw (%)
GOF
position has been previously reported and likely occurs by
protonation/deprotonation at this position (eq 4).23,24
Measurement of molar magnetic susceptibilities for 1 and 2
1
by H NMR using the Evans method gives, after diamagnetic
CH3Cl, converting 1 to the amidobis(triflate) complex TpOs-
(NHPh)(OTf)2 (3) (eq 5). CH3Cl is observed in the reaction
corrections, 3.2 × 10-4 emu mol-1 (1) and 1.1 × 10-3 emu
mol-1 (2). Conversion of these values to magnetic moments
(which would be 0.9 and 1.6 µB, respectively) is not appropriate
because these compounds are believed to be temperature-
independent paramagnets rather than standard Curie (unpaired
spin) paramagnets.16,25
Reaction with Methyl Triflate. No reaction occurs between
1 and excess methyl triflate (CH3OTf) in CDCl3 at ambient
temperatures, but heating at 80 °C for 6 days results in
conversion to a new product in 80% yield. Rather than attack
at the nitrogen, CH3OTf removes the chloride ligands as
1
mixture (δ 3.0 ppm in the H NMR), and its identity was
confirmed by independent synthesis from PPN+Cl- and CH3-
OTf in CDCl3. There is no evidence for attack at the amido
nitrogen by CH3OTf or by other electrophiles such as CH3I and
acetyl chloride (except for H+).
(22) Statistical exchange between the one proton (from OsNHPh) and three
deuterons (from three DOTf) gives 25% H and 75% D at each site.
The relative concentrations are given by [NH2] ) (0.25)(0.25) times
(its integral of 2) ) 0.125 and [NHD] ) (0.25)(0.75) times (statistical
factor of 2) ) 0.375.
1H NMR spectra of 3 are sharp-shifted, indicating the Os-
(IV) formulation, and show a phenyl group and a 2:1 ratio of
pyrazole resonances. Its X-ray structure (Figure 2, Tables 2 and
3) contains isolated molecules with pseudo-octahedral coordina-
tion. The triflate ligands are covalently bound to the osmium
with Os-O distances of 2.114(8) and 2.100(7) Å. The coordina-
tion geometry and bond lengths are similar to those of 1. The
Os-N(amide) bond length of 1.939(8) Å for 3 is within 3σ of
the Os-N(amide) distance in 1 of 1.919(6) Å. The amide ligand
(23) Clementi, S.; Forsythe, P. P.; Johnson, C. D.; Katritzky, A. R. J. Chem.
Soc., Perkin Trans. 2 1973, 1675-1680.
(24) Protasiewicz, J. D.; Theopold, K. H. J. Am. Chem. Soc. 1993, 115,
5559-5569.
(25) O’Connor, C. J. Progress in Inorganic Chemistry; Lippard, S. J., Ed.;
Wiley: New York, 1982; Vol. 29, p 212.
(26) West, D. X.; Castineiras, A.; Bermejo, E. J. Mol. Struct. 2000, 520,
103-106.
(27) Lawrance, G. C. Chem. ReV. 1986, 86, 17-33.
(28) Soper, J. D.; Kaminsky, W.; Mayer, J. M. Work in progress.

TpOs(NHPh)Cl2
Inorganic Chemistry, Vol. 40, No. 8, 2001 1891
Table 3. Selected Bond Lengths (Å) and Angles (deg) for
TpOs(NHPh)(OTf)2 (3)
Scheme 2. Comparison of pKa Values for Aniline and
TpOs(NHPh)Cl2 (1)15,30
Os-N1 2.061(8) N7-C10 1.324(13) O1-Os-O4
Os-N3 2.021(7) C10-C11 1.415(12) O1-Os-N3
Os-N5 2.051(9) C11-C12 1.364(15) O4-Os-N5
Os-N7 1.939(8) C12-C13 1.409(16) N3-Os-N5
86.0(4)
87.9(3)
96.2(3)
89.1(3)
Os-O1 2.114(8) C13-C14 1.383(14) Os-N7-C10 134.8(7)
Os-O4 2.100(7) C14-C15 1.371(16) N1-Os-N7 176.8(3)
C15-C10 1.433(15)
in 3 has an Os-N(7)-C(10) angle of 134.8(7)°, with the phenyl
ring interleaved between the pyrazole rings of the Tp ligand.
The amide NH appears to form weak hydrogen bonds26 to O(6)
and O(3) of the triflate ligands, with N(7)-O(6) and N(7)-
O(3) distances of 3.098(11) and 3.134(12) Å, respectively. With
the amide hydrogen placed 0.967 Å from N(7) and bisecting
the Os-N-C angle, the H-O distances and N-H-O angles
are 2.52 Å and 118° for O(6) and 2.39 Å and 133° for O(3).
of methyl triflate or methyl iodide to a benzene solution of 4.
As noted above, acetyl chloride and acetic anhydride protonate
4 to give 1 rather than acylating the reactive nitrogen. With
benzoyl chloride, a non-enolizable acylating agent, decomposi-
tion of 4 is observed rather than protonation or simple acylation.
The lack of nucleophilicity of 4 is likely the result of an
oligomeric structure in benzene solution, as indicated by its
significant solubility. The formation of a cluster with bridging
Mg2+ cations would provide steric protection for the nitrogen.
The lack of reactivity of 4 with electrophiles contrasts with
related tungsten and rhenium “deprotonated amide” complexes,
TpMe2(CO)2W[N(Li)R] and CpRe(NO)(PPh3)[N(Li)CH3], which
alkylate readily at nitrogen.31,32
The IR spectrum of 3 shows a strong absorbance at 1337 cm-1
,
indicating monodentate-bound triflate.27
Protonation of complex 3 has not been observed under
conditions similar to those required for protonation of 1. No
1
reaction is observed by H NMR on treating a solution of 3 in
CDCl3 with >6 equiv of HOTf. Similarly, there is no reaction
between 3 and excess HCl/Et2O in CDCl3 even at 80 °C.
Complex 3 can be quantitatively converted to 1 by being heated
in a solution of PPN+Cl- in CDCl3 for 4 days 80 °C. The forcing
conditions required indicate the lack of lability of the triflate
ligands.
Acidity of 1. Reactions of 1 with methyllithium, phenyl
Grignard, or potassium tert-butoxide do not result in deproto-
nation but rather appear to proceed by reduction of the osmium.
Reactions with nitrogen bases take a different course, as will
be discussed elsewhere.28 However, some measure of the acidity
of 1 can be gained from the reactivity of a deprotonated material,
TpOs[NPh(MgBr)]Cl2 (4). Complex 4 is formed in the reaction
of TpOs(N)Cl2 and PhMgBr in the synthesis of 1 (Scheme 1
above) and can be isolated as a mixture with TpOs(N)Cl2 by
air-free workup at low temperatures.13 Exposure of 4 to air or
water results in instant and quantitative hydrolysis to 1. Benzene
solutions of 4 are also protonated by stoichiometric addition of
CHCl3, CH2Cl2, CH3CN, DMSO, acetone, acetic anhydride, or
acetyl chloride (eq 6; DMSO ) (CH3)2SO). When acetonitrile-
Discussion
The osmium(IV) amido complex TpOs(NHPh)Cl2 (1) has,
to our knowledge, the lowest basicity and nucleophilicity of
any transition metal amido complex. Its lack of reaction with
HCl and the deprotonation of [TpOs(NH2Ph)Cl2]+ (2) by
chloride are remarkable. Complex 2, which has a pKa of roughly
3 in CH3CN, appears to be significantly more acidic than other
osmium(IV) amine complexes. [Os(en)(en-H)2]2+ and cis-[Os-
(NH3)4Cl2]2+ have aqueous pKa values of 5.8 and ∼1,33,34 much
weaker than triflic acid in aqueous media (en-H ) N-
deprotonated ethylenediamine). The phenyl substituent likely
+
contributes to the high acidity of 2 because PhNH3 is more
acidic than NH4+ (pKa’s 11 and 16 in CH3CN).15 But the phenyl
group does not appear to make 1 any more acidic, as revealed
in the very high basicity of TpOs[NPh(MgBr)]Cl2 (4), pKa >
35. The pKa of aniline is 31 in DMSO.30 Complex 1 is
simultaneously less basic and less acidic than aniline (Scheme
+
2). Note that 1 is not protonated by PhNH3 in CDCl3.
The low basicity of the anilido complex 1 indicates that the
lone pair on nitrogen is not very accessible. This can be
attributed both to Os-N π bonding and to inductive effects.
Complex 1 can be described as a mixture of singly and doubly
bonded resonance structures (Chart 1). Resonance form B
represents an 18-electron complex. Complex 1 therefore differs
from most later-transition metal amides, which have an 18-
electron count in the absence of metal-nitrogen π bonding. In
d3 or acetone-d6 are added to 4 in benzene-d6, the Tp and phenyl
peaks indicative of 1 appear as expected in the 1H NMR
spectrum but no NH peak is apparent at δ 2.2 ppm.29 Thus, 4
is protonated by the deuterated acetonitrile or acetone and not
by trace water in the solvent. On the basis of its immediate
protonation in DMSO, the pKa of 1 is estimated to be >35 in
that solvent.30
(31) Powell, K. R.; Perez, P. J.; Luan, l.; Feng, S. G.; White, P. S.;
Brookhart, M.; Templeton, J. L. Organometallics 1994, 13, 1851-
1864.
Despite its high basicity, 4 does not act as a nucleophile. No
1
N-methylated product is observed by H NMR upon addition
(32) Dewey, M. A.; Stark, G. A.; Gladysz, J. A. Organometallics 1996,
15, 4798-4807.
1
(29) The NH resonance in the H NMR spectrum of 1 is highly solvent-
(33) Buhr, J. D.; Winkler, J. R.; Taube, H. Inorg. Chem. 1980, 19, 2416-
dependent, appearing at δ 2.2 ppm in benzene-d6 vs δ 5.5 ppm in
CDCl3.
(30) Bordwell, F. G. Acc. Chem. Res. 1988, 21, 456.
2425.
(34) Lay, P. A.; Sargeson, A. M.; Skelton, B. W.; White, A. H. J. Am.
Chem. Soc. 1982, 104, 6161-6164.

1892 Inorganic Chemistry, Vol. 40, No. 8, 2001
Soper et al.
Chart 1
with E1/2 ) +0.35 V vs Cp2Fe+/0 in CH3CN,39 and 2 should be
as well. Complex 1 has a much lower reduction potential, -1.05
V vs Cp2Fe+/0, presumably because the Os-N π bonding
stabilizes Os(IV) much more than it does Os(III). The electron-
withdrawing character of the osmium, both σ and π, is even
stronger in the bistriflate complex 3. There is no evidence of
lone pair reactivity at nitrogen in this complex. It is not
protonated with excess HOTf or attacked by excess CH3OTf at
elevated temperatures.
Table 4. Comparison of Os-N Bond Lengths (Å) in
TpOs(NHAr)X2 Compounds
compound
Os-NHR
Os-N(pz)cis
Os-N(pz)trans
It is interesting that the interactions that make 2 a strong acid
do not similarly facilitate deprotonation of 1. Complex 1 is a
very weak acid because its conjugate base deprotonates aceto-
nitrile and DMSO. Thus, the [TpOsIVCl2+] fragment and the
anilido ligand NHPh- are well matched, giving a complex that
is neither significantly acidic nor basic. The stability of 1 and
3 is further indicated by the inertness of the ancillary chloride
or triflate ligands. The unusual kinetic inertness of σ-bonded
ligands in TpOs(IV) complexes will be discussed in a future
publication.14
a
TpOs(NHPh)Cl2
1.919(6) 2.039(6), 2.053(6)
2.097(5)
2.061(8)
2.103(7)
2.056(8)
TpOs(NHPh)(OTf)2 1.939(8) 2.021(7), 2.051(9)
b
TpOs(NHAr)Cl2
TpOs(NH2Et)Cl2
1.945(7) 2.053(6), 2.056(7)
2.129(8) 2.051(5)
c
a Reference 12. b Ar ) p-C6H4N(c-C5H10); ref 28. c Reference 13.
such complexes, the nitrogen lone pair has a π-antibonding
interaction with filled metal d orbitals (which may or may not
be important11,35-37). Such π-antibonding interactions have been
used to explain the high basicity of amide complexes, for
instance, the remarkable ability of trans-Ru(dmpe)2(H)(NH2)
to deprotonate toluene.37 Some osmium-nitrogen multiple bond
character is supported by the Os-N(amide) bond lengths in 1,
3, and a related anilido complex, 1.934 ( 0.015 Å (Table 4).
These distances are much shorter than the 2.02-2.10 Å Os-
Npyrazole distances and much shorter than the Os(III)-amine
distance of 2.129(8) Å in the related TpOs(NH2Et)Cl2.13 Os-N
multiple bonding has been proposed for the osmium(IV)
alkylamide complex [Os(en)(en-H)2]Br2, which exhibits a similar
difference between Os-amide [1.896(7) Å] and Os-amine
distances [2.113(9) and 2.194(7) Å (cis and trans to the amides,
respectively)].34 In [Os(en)(en-H)2]Br2 (1) and 3, the amide
ligands show a significant trans influence ()0.04 Å for 1 and
3, Table 4). In sum, the structural data suggest an Os-amide
bond order of greater than 1. A full Os-anilide double bond
would make 1 and 3 18-electron complexes, which would seem
to be inconsistent with their paramagnetism. Protonation of 1
to 2 increases the paramagnetism of the osmium center because
it eliminates the π bonding, and 2 is clearly a 16-electron
complex. This argument is, however, complicated by the
likelihood that the complexes are not open-shell Curie para-
magnets but rather temperature-independent paramagnets.38 The
presence of Os-N π bonding should reduce the basicity of the
anilido ligand, but it should be noted that π bonding in early
transition metal amide complexes does not inhibit protonation
at nitrogen.5
Conclusions
The Os(IV) amide complex TpOs(NHPh)Cl2 (1) is remark-
ably inert toward protonation and other electrophilic attack.
Protonation requires excess triflic acid, and the protonated
species is so acidic that it is deprotonated by chloride. Methyl
triflate does not alkylate at nitrogen but rather removes the
chloride ligands, forming TpOs(NHPh)(OTf)2 (3). These reac-
tions contrast with the typical high reactivity of amide ligands
toward electrophiles, particularly in late transition metal com-
plexes. It is suggested that the low reactivity observed for 1 is
a result of both Os-N(amide) π bonding and the inductive effect
of the oxidizing (electron-poor) osmium center. These effects
do not, however, make 1 acidic. The deprotonated complex
TpOs[NPh(MgBr)]Cl2 (4) is very basic, being protonated to give
1 by such weak acids as CH3CN and acetic anhydride. Complex
1 is both a weaker base and a weaker acid than aniline.
Experimental Section
General Considerations. All reactions were performed under
anaerobic conditions using standard high-vacuum and nitrogen-filled
glovebox techniques unless otherwise noted. NMR spectra were
acquired on Bruker WM-500, DRX-499, AF-300, and AC-200 spec-
trometers at ambient temperatures. Proton NMR chemical shifts were
1
referenced to the residual H NMR signals of the deuterated solvents
and are reported vs TMS. Magnetic susceptibility measurements were
made by the Evans method at 20 °C using a TMS reference in CDCl3
solutions.40 Diamagnetic ligand corrections were calculated.25,41 IR
spectra were obtained as KBr pellets using a Perkin-Elmer 1720 infrared
Fourier transform spectrophotometer. Electrospray ionization mass
spectrometry was carried out with acetonitrile solutions using a Bruker/
HP Esquire-LC mass spectrometer. Elemental analysis was performed
by Atlantic Microlab, Inc. in Norcross, Georgia.
Another reason for the low basicity of 1 and high acidity of
2 is that the osmium(IV) center in this class of compounds is
electron-withdrawing overall, not just in a π fashion. The related
ammine complex [TpOs(NH3)Cl2]+ is a very potent oxidant,
(35) Holland, P. L.; Andersen, R. A.; Bergman, R. G. J. Am. Chem. Soc.
1996, 118, 1092-1104.
Materials. All solvents used for the syntheses were degassed and
dried according to standard procedures.42 Deuterated solvents were
purchased from Cambridge Isotope Laboratories, degassed, dried, and
vacuum-transferred prior to use. CDCl3 and CD2Cl2 were dried over
CaH2, and C6D6 was dried over sodium metal. CD3CN was dried over
CaH2 followed by P2O5. Reagents were purchased from Aldrich and
used as received unless otherwise noted. Acetyl chloride was distilled
(36) Holland, P. L.; Andersen, R. A.; Bergman, R. G. Comments Inorg.
Chem. 1999, 21, 115-129.
(37) Fulton, J. R.; Bouwkamp, M. W.; Bergman, R. G. J. Am. Chem. Soc.
2000, 122, 8799-8800.
(38) (a) Temperature-independent paramagnetism (TIP) is a result of
quantum mechanical mixing of excited states due to an applied
magnetic field.38b-d The magnitude of TIP depends in part on the
energy gap between the states that are mixing. As a rough generaliza-
tion, therefore, an 18-electron complex should have a larger HOMO-
LUMO gap and smaller TIP versus a related 16-electron complex.
(b) Figgis, B. N.; Lewis, J. Progress in Inorganic Chemistry; Cotton,
F. A., Ed.; Wiley: New York, 1964; Vol. 6, pp 71-72. (c) Drago, R.
S. Physical Methods for Chemists, 2nd ed.; Surfside: Gainesville, FL,
1992; p 485. (d) Greenwood, N. N.; Earnshaw, A. Chemistry of the
Elements, 2nd ed.; Butterworth, Heinemann: Oxford, 1997; p 1087.
1
from a /10 volume equivalent of dimethylaniline to remove free HCl.
(39) Bennett, B. K.; Lovell, S.; Mayer, J. M. J. Am. Chem. Soc., in press.
(40) Live, D. H.; Chan, S. I. Anal. Chem. 1970, 42, 791.
(41) Mulay, L. N.; Boudreaux, E. A. Theory and Applications of Molecular
Diamagnetism; Wiley: New York, 1976.
(42) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals, 3rd ed.; Pergamon: New York, 1988.

TpOs(NHPh)Cl2
Inorganic Chemistry, Vol. 40, No. 8, 2001 1893
TpOs(N)Cl2 and TpOs(NHPh)Cl2 (1) were prepared according to
of 62 mg of a mixture of 4 and 1 as dark-orange solids. 1H NMR (C6D6)
of 4: δ 2.70 (d, 2H), 7.70 (t, 2H), 4.86 (t, 1H) (each 8.0 Hz, NPh,
ortho, meta, para); 6.17 (t), 7.43 (d), 7.18 (d) (all 1H, 2.3 Hz, pz); 5.63
(t), 5.91 (d), 5.86 (d) (all 2H, 2.3 Hz, pz′).
1
published procedures.12,13,43 Purity was checked with H NMR spec-
troscopy.
TpOs(15NHPh)Cl2 (1-15N). 1-15N was prepared according to a
published procedure using TpOs(15N)Cl2 as the starting material.43 1
H
Crystal Structure of 3. A dark-orange, plate-shaped crystal 0.12
mm × 0.09 mm × 0.04 mm was grown by slow diffusion of hexane
into a chloroform solution and mounted on a glass capillary in epoxy.
Data were collected at -112 °C with three sets of ω scans covering a
sphere of reciprocal space. Crystal-to-detector distance was 27 mm,
and exposure time was 15 s for all sets. The scan range was 1.1°. First
the data were integrated up to a θ of just 20°, but that left insufficient
information for thermal parameters. In a reintegration process data were
used up to a much larger θ. Then data were 72.6% complete to 29.33°
in θ. A total of 34 227 partial and complete reflections were collected
covering the indices h ) -11 to 12, k ) -15 to 15, l ) -15 to 17. A
total of 5489 reflections were symmetry-independent, and Rint ) 0.1005
indicated that the quality of data was fair. Indexing and unit cell
refinement based on 189 reflections indicated a triclinic lattice. The
space group was found to be Ph1 (No. 2). Solution by direct methods
(SIR 92) produced an incomplete heavy-atom phasing model. The
remaining heavy atoms were located from successive difference electron
density maps. All hydrogen atoms were placed with idealized geometry
except for H1B (boron H) and were refined with a riding model. Uiso
values were fixed such that they were 1.1Ueq of their parent atom. All
other non-hydrogen atoms were refined anisotropically by full-matrix
least squares. An empirical correction for absorption was performed
using the program SCALEPACK. This program applies a multiplicative
correction factor (S) to the observed intensities (I) and has the following
form: S ) exp{2B[sin(θ/λ)]}2/scale. S is calculated from the scale,
and the B factor is determined for each frame and is then applied to I
to give the corrected intensity (Icorr). This method does not, however,
calculate minimum and maximum transmission coefficients.
NMR (CDCl3): δ -3.4 (d, 2H), 8.68 (t, 2H), 0.06 (t, 1H) (all 8.3 Hz,
NPh, ortho, meta, para); 5.6 (d, 70 Hz, 1 H, 15NHPh); 6.06 (t, 1H),
6.20 (d, 1H), 5.39 (d, 1H) (all 1.9 Hz, pz); 6.62 (t, 2H), 6.82 (d, 2H),
4.60 (d, 2H) (all 1.9 Hz, pz′).
[TpOs(NH2Ph)Cl2]OTf (2). HOTf (2.8 µL, 32 µmol) was added to
a Teflon screw-top NMR tube (J. Young tube) containing 1 (6 mg, 10
µmol) and CDCl3 (0.6 mL). An immediate color change from blood-
red to brown indicated formation of 2. 1H NMR (CDCl3): δ 10.39 (d,
2H), 7.97 (t, 2H), 8.14 (t, 1H) (all 7.3 Hz, NPh, ortho, meta, para);
93.3 (br s, 2H, NH2Ph); 2.42 (t), 4.39 (d), -22.49 (d) (all 1H, 2.1 Hz,
pz); 1.54 (t), 0.98 (d), -23.67 (d) (all 2H, 2.1 Hz, pz′). 2-15N. 1H NMR
(CDCl3): δ 93 (d, 70 Hz, 2H, 15NH2). The chemical shift of the NH
proton is quite sensitive to conditions, particularly the amount of HOTf
present.
TpOs(NHPh)(OTf)2 (3). A glass bomb with a Teflon stopcock was
charged with 1 (50 mg, 90 µmol), CH2Cl2 (5 mL), and CH3OTf (30
µL, 270 µmol). Heating at 80 °C for 6 days resulted in a color change
from deep-red to orange, indicating formation of 3. Chromatography
1
on a ∼6 in. × /2 in. silica column and elution with 90% CH2Cl2/10%
1
acetone gave clean red-orange 3 (56 mg, 71 µmol, 80%). H NMR
(CDCl3): δ 7.70 (t, 2H), 5.22 (t, 1H) (all 7.4 Hz, NPh, meta, para,
ortho not observed); 12.7 (br s, 1H, NHPh); 6.48 (t), 7.42 (d), 6.60 (d)
(all 1H, 2.3 Hz, pz); 6.27 (t), 6.06 (d), 5.48 (d) (all 2H, 2.3 Hz, pz′).
ESI MS: 818 (M + Na+), 834 (M + K+), 646 (M+ - OTf-). IR
(KBr): 3445 (br) (νN-H); 2527 (m) (νB-H); 3120 (m), 1502 (m), 1410(s),
1309(s), 1216 (m), 1187 (s), 1119 (vs), 1074 (m), 1052 (s), 993 (s),
769 (s), 705 (m), 635 (s), 615 (m) (all Tp); 1337 (s) (νOTf); 3215 (m);
1578 (m); 1236 (vs); 1029 (s); 1010 (s); 791 (m). Anal. Calcd (found)
for C17H16BF6N7O6OsS2: C, 25.73 (25.83, 25.94); H, 2.03 (2.08, 2.01);
N, 12.35 (12.37, 12.32).
TpOs[NPh(MgBr)]Cl2 (4). Under air-free conditions, PhMgBr (35
µL of a 3.0 M solution in Et2O, 0.11 mmol) was diluted into ∼6 mL
of THF and was added over 0.5 h to a -78 °C solution of 1 (60 mg,
0.12 mmol) in ∼2 mL of THF, forming a dark-orange solution. Addition
of ∼80 mL of pentane and filtration on a swivel frit allowed isolation
Acknowledgment. We are grateful for support from the U.S.
National Science Foundation. We thank Dr. Martin Sadilek for
mass spectrometric analyses and Dr. Werner Kaminsky for help
with the X-ray structure.
Supporting Information Available: X-ray crystallographic data
for TpOs(NHPh)(OTf)2 (3). This material is available free of charge
via the Internet at http://pubs.acs.org.
(43) Crevier, T. J.; Mayer, J. M. Angew. Chem., Int. Ed. 1998, 37, 1891-
1893.
IC001177X