Chemistry of coordinated nitroxyl. Reagent-specific protonations of trans-Re(CO)2(NO)(PR3)2 (R = Ph, Cy) that give the neutral nitroxyl complexes cis,trans-ReCl(CO)2(NH=O)(PR3)2 or the cationic hydride complex [trans,trans-ReH(CO)2(NO)(PPh3)2 +][SO3CF3-]

Article abstract of DOI:10.1021/ic010669m

The reactions of hydrochloric and triflic acids with the five-coordinate nitrosyl complexes trans-Re(CO)₂(NO)(PR₃)₂ (2a, R = Ph; 2b, R = Cy) have been investigated. Reaction of anhydrous HCl with 2 results in a formal protonation of the nitrosyl ligand and addition of chloride to the metal, giving the neutral nitroxyl complex cis,trans-ReCl(CO)₂(NH=O)(PR₃)₂ (3a, R= Ph; 3b, R = Cy). Reaction of Bronsted bases with 3a or 3b results in clean conversion of 3 to 2 when the base is appropriately strong (pK_b ? 7). Addition of HOSO₂CF₃ to solutions of 2a results in protonation at the metal and formation of the cationic rhenium hydride [trans,trans-ReH(CO)₂(NO)(PPh₃)₂ ⁺][SO₃CF₃^-] (4) in 74% yield; the deuteride [trans, trans-Re(²H)(CO)₂(NO)(Pph₃)₂ ⁺][SO₃CF₃^-] (4-d) was analogously prepared from ²HOSO₂CF₃. 4 crystallized from CH₂Cl₂/Et₂O solution in the orthorhombic space group Pnma, with a = 17.2201(2) A, b = 23.6119(3) A, c = 9.2380(2) A, and Z = 4. The least-squares refinement converged to R(F) = 0.039 and R(wF²) = 0.063 for the 4330 unique data with I > 2σ(I). The structure of 4 shows that the hydride (Re - H = 1.74 A) occupies the position trans to the linear nitrosyl ligand (Re - N - O = 178.1(4)°) in the pseudooctahedral complex cation. Complex 4 does not react with chloride to give 3a. DFT calculations carried out on free nitroxyl and its model complexes [Re(CO)₅(NH=O)⁺] (5), [mer, trans-Re(CO)₃(NH=O)(PH₃)2⁺] (6), and cis,trans-ReCl(CO)₂(NH=O)(PH₃)₂ (7) indicate that coordinated nitroxyl acts as both a σ-donor and π-acceptor ligand, consistent with the observed trend for ν(NO) in free HN=O (1563 cm^-1), [mer,trans-Re(CO)₃(NH=O)(PPh₃)₂⁺] (1, 1391 cm^-1), 3a (1376 cm^-1), and 3b (1335 cm^-1).

Full text of DOI:10.1021/ic010669m

Inorg. Chem. 2001, 40, 6039-6046
6039
Chemistry of Coordinated Nitroxyl. Reagent-Specific Protonations of
trans-Re(CO)2(NO)(PR3)2 (R ) Ph, Cy) That Give the Neutral Nitroxyl Complexes
cis,trans-ReCl(CO)2(NHdO)(PR3)2 or the Cationic Hydride Complex
+
-
[trans,trans-ReH(CO)2(NO)(PPh3)2 ][SO3CF3 ]
Joel S. Southern, Michael T. Green, and Gregory L. Hillhouse*
Searle Chemistry Laboratory, Department of Chemistry, The University of Chicago,
Chicago, Illinois 60637
Ilia A. Guzei and Arnold L. Rheingold*
Department of Chemistry, University of Delaware, Newark, Delaware 19716
ReceiVed June 22, 2001
The reactions of hydrochloric and triflic acids with the five-coordinate nitrosyl complexes trans-Re(CO)2(NO)-
(PR3)2 (2a, R ) Ph; 2b, R ) Cy) have been investigated. Reaction of anhydrous HCl with 2 results in a formal
protonation of the nitrosyl ligand and addition of chloride to the metal, giving the neutral nitroxyl complex cis,-
trans-ReCl(CO)2(NHdO)(PR3)2 (3a, R ) Ph; 3b, R ) Cy). Reaction of Brønsted bases with 3a or 3b results in
clean conversion of 3 to 2 when the base is appropriately strong (pKb ≈ 7). Addition of HOSO2CF3 to solutions
of 2a results in protonation at the metal and formation of the cationic rhenium hydride [trans,trans-ReH(CO)2-
(NO)(PPh3)2+][SO3CF3-] (4) in 74% yield; the deuteride [trans,trans-Re(2H)(CO)2(NO)(PPh3)2+][SO3CF3
]
-
(4-d) was analogously prepared from 2HOSO2CF3. 4 crystallized from CH2Cl2/Et2O solution in the orthorhombic
space group Pnma, with a ) 17.2201(2) Å, b ) 23.6119(3) Å, c ) 9.2380(2) Å, and Z ) 4. The least-squares
refinement converged to R(F) ) 0.039 and R(wF2) ) 0.063 for the 4330 unique data with I > 2σ(I). The structure
of 4 shows that the hydride (Re-H ) 1.74 Å) occupies the position trans to the linear nitrosyl ligand
(Re-N-O ) 178.1(4)°) in the pseudooctahedral complex cation. Complex 4 does not react with chloride to give
3a. DFT calculations carried out on free nitroxyl and its model complexes [Re(CO)5(NHdO)+] (5), [mer,trans-
Re(CO)3(NHdO)(PH3)2+] (6), and cis,trans-ReCl(CO)2(NHdO)(PH3)2 (7) indicate that coordinated nitroxyl acts
as both a σ-donor and π-acceptor ligand, consistent with the observed trend for ν(NO) in free HNdO
(1563 cm-1), [mer,trans-Re(CO)3(NHdO)(PPh3)2+] (1, 1391 cm-1), 3a (1376 cm-1), and 3b (1335 cm-1).
Introduction
HNdO) remains elusive.4 HNdO has been typically prepared
by the reaction of hydrogen atoms with nitric oxide in the gas
Nitroxyl (HNdO) is a highly reactive, unstable molecule
attracting current interest primarily because of its relationship
to nitric oxide (NO). Nitric oxide plays numerous physiological
roles, mainly functioning as a cell-signaling agent, and much
recent work has been devoted to elucidating its biochemistry.1
The one-electron redox relatives of NO, the nitrosonium cation
(NO+), and the nitroside anion (NO-), as well as the conjugate
acid of NO-, nitroxyl, have been suggested to be responsible
for some of the myriad functions attributed to nitric oxide in
mammalian biochemistry.1a,b Interest in nitroxyl also stems from
its postulated intermediacy in photochemical and free-radical
reactions and the role its formation and decomposition may play
in mechanisms for the combustion of nitrogen-containing fuels
and the oxidation of atmospheric nitrogen.2,3
phase and in frozen Ar matrixes,2 and an interesting recent report
describes the trapping of an isolated NO- ion in a “molecular
oxide bowl” along with the structural characterization of the
complex salt.5 Importantly, like the isoelectronic 1,2-diazene,6-8
nitroxyl represents a classic example of an unstable molecule
whose stability can be greatly enhanced by coordination to a
transition metal. Roper’s discovery that the addition of anhy-
drous HCl to the nitrosylosmium(0) complex Os(Cl)(CO)(NO)-
(PPh3)2 affords the nitroxyl complex cis,trans-Os(Cl)2(CO)-
(NHdO)(PPh3)2 marked the first example of the stabilization
of nitroxyl by coordination to a transition metal.9,10 Sellman
and co-workers have recently demonstrated that an HNdO
(4) Smith, P. A. S.; Hein, G. E. J. Am. Chem. Soc. 1960, 82, 5731.
(5) Kawanami, N.; Ozeki, T.; Yagasaki, A. J. Am. Chem. Soc. 2000, 122,
1239.
(6) Smith, M. R., III; Cheng, T.-Y.; Hillhouse, G. L. J. Am. Chem. Soc.
1993, 115, 8638.
(7) Cheng, T.-Y.; Peters, J. C.; Hillhouse, G. L. J. Am. Chem. Soc. 1994,
116, 204.
(8) Cheng, T.-Y.; Ponce, A.; Rheingold, A. L.; Hillhouse, G. L. Angew.
Chem., Int. Ed. Engl. 1994, 33, 657.
In contrast to its cousins NO and NO+, which are com-
mercially available reagents, a facile synthesis for NO- (or
(1) (a) Butler, A. R.; Flitney, F. W.; Williams, D. L. H. Trends Pharmacol.
Sci. 1995, 16 (1), 18, and references therein. (b) Stamler, J. S.; Singel,
D. J.; Loscalzo, J. Science 1992, 258, 1898, and references therein.
(c) Culotta, E.; Koshland, D. E., Jr. Science 1992, 258, 1862.
(2) (a) Dalby, F. W. Can. J. Phys. 1958, 36, 1336. (b) Jacox, M. E.;
Milligan, D. E. J. Mol. Spectrosc. 1973, 48, 536.
(9) Grundy, K. R.; Reed, C. A.; Roper, W. R. J. Chem. Soc., Chem.
Commun. 1970, 1501.
(10) Wilson, R. D.; Ibers, J. A. Inorg. Chem. 1979, 18, 336.
(3) Guadagnini, R.; Schatz, G. C.; Walch, S. P. J. Chem. Phys. 1995,
102, 774, and references therein.
10.1021/ic010669m CCC: $20.00 © 2001 American Chemical Society
Published on Web 09/29/2001

6040 Inorganic Chemistry, Vol. 40, No. 23, 2001
Southern et al.
was added to the flask, whose contents were at room temperature. The
flask was then evacuated at -78 °C. At room temperature, 60 mL of
diethyl ether was transferred to the flask via a cannula under a CO
atmosphere. The contents of the flask were stirred under a CO
atmosphere for 3 days, after which the flask was attached to a
preevacuated swivel-frit assembly with a Schlenk collection flask under
argon. The solution was filtered through Celite, and the Schlenk flask
containing the brown-red filtrate was fitted with a septum under argon.
The contents of the flask were evaporated to dryness, and the brown-
red residue was treated with 30 mL of N2-purged absolute ethanol.
The flask was attached to another preevacuated swivel-frit assembly
under argon, and the insoluble brown-red solid was filtered, washed
with N2-purged absolute ethanol, and dried under vacuum to afford
0.197 g (19.2% yield) of air-sensitive 2b. Anal. Calcd for C38H66O3-
NP2Re: C, 54.79; H, 7.98; N, 1.68. Found: C, 54.84; H, 8.14; N, 1.43.
ligand can be prepared by the action of hydride on a coordinated
nitrosyl, along with the structural characterization of the resulting
complex.11 In an important study with clear biological relevance,
Farmer and co-workers have reported the synthesis, isolation,
and characterization of an HNdO adduct of myoglobin.12 Our
group has described the synthesis of a cationic, pseudooctahedral
nitroxyl complex of rhenium, [mer,trans-Re(CO)3(NHdO)-
(PPh3)2+][SO3CF3-] (1), produced by the selective oxidation
of the hydroxylamine ligand of [mer,trans-Re(CO)3(NH2OH)-
(PPh3)2+][SO3CF3-] using lead tetraacetate (eq 1).13 By analogy
to transition-metal complexes of 1,2-diazene (NHdNH), which
react with bromide to provide clean sources of the free NHd
NH molecule (eq 2),6 it is hoped that these nitroxyl complexes
can ultimately serve as HNdO synthons.
IR: ν(CO) 1916 (m), 1827 (s) cm-1; ν(NO) 1582 (s) cm-1
.
31P{1H}
NMR (162.0 MHz, CD2Cl2, 20 °C): δ 23.5 (s).
cis,trans-Re(Cl)(CO)2(NHdO)(PPh3)2 (3a). This procedure is a
modification of the literature procedure.14 A two-necked round-bottomed
flask attached to a swivel-frit assembly and containing a septum was
placed on a vacuum line, 20 mL of diethyl ether was vacuum transferred
into the flask at -78 °C, and the flask was filled with argon. The
apparatus was removed from the line and purged with anhydrous HCl
in a hood for 1 min, then was again placed on a vacuum line in a
hood. Under an argon counterflow, 0.101 g (0.127 mmol) of 2a and
0.050 g (0.191 mmol) of PPh3 were added, and the solution was stirred
at room temperature for 20 min. The dark red-brown solid was insoluble
in diethyl ether and turned green in color during the reaction. This
green solid was filtered, washed with diethyl ether, and dried under
vacuum to yield 0.084 g (79.2% yield) of 3a (85% of product mixture);
IR and 31P{1H} NMR spectroscopies also show the presence of mer,-
trans-Re(Cl)(CO)3(PPh3)2 (8%) and cis,trans-Re(Cl)2(CO)(NO)(PPh3)2
(7%).14,16 Anal. Calcd for C38.01H30.85O2.93N0.92Cl1.07P2Re: C, 54.77; H,
3.73; N, 1.55. Found: C, 55.57; H, 3.55; N, 1.58. 3a: IR: ν(NH) 3057
(w) cm-1; ν(CO) 1975 (vs), 1878 (vs) cm-1; ν(NO) 1376 (s) cm-1. 1H
To better understand the factors that give rise to stable nitroxyl
complexes and to compare the properties of charge-neutral
derivatives with cationic derivatives such as 1, the reactions of
the five-coordinate rhenium nitrosyl complexes trans-Re(CO)2-
(NO)(PR3)2 (2a, R ) Ph; 2b, R ) Cy) with the Brønsted acids
HCl and HOSO2CF3 have been investigated. In 1974, LaMonica
et al. reported that 2a and HCl react in alcohol or ether to give
a mixture of products, one component of which exhibits a low-
energy ν(NO) suggestive of ligated HNdO.14 We have found
the products of these reactions to be reagent specific, depending
on the nature of the counterion, and these results are described
herein.
1
NMR (400 MHz, CD2Cl2, 20 °C): δ 20.66 (s, 1 H, NHdO, | JNH| )
66.2 Hz), 7.75-7.55 (m, 12 H, Ph), 7.50-7.35 (m, 18 H, Ph). 31P{1H}
NMR (162.0 MHz, CD2Cl2, 20 °C): δ 15.4 (s).
cis,trans-Re(Cl)(CO)2(NHdO)(PCy3)2 (3b). A 0.189 g (0.227
mmol) sample of 2b was added to a two-necked round-bottomed flask
equipped with a septum and attached to a swivel-frit assembly, and
the apparatus was evacuated on a vacuum line. At room temperature
under argon, 10 mL (20 mmol) of a 2.0 M solution of hydrogen chloride
in diethyl ether (Aldrich) was added to the flask via syringe. The red-
brown solid, which is insoluble in Et2O, immediately turned blue-green
in color. This heterogeneous solution was stirred at room temperature
for 30 min and was then filtered. The blue-green solid was recrystallized
from benzene/diethyl ether to yield 0.057 g (29% yield) of deep green
3b‚1/2C6H6 as a benzene solvate. The solid shows at least short-term
(∼12 h) air stability. Anal. Calcd for C41H70O3NClP2Re: C, 54.20; H,
7.77; N, 1.54. Found: C, 54.48; H, 8.37; N, 1.52. IR: ν(CO) 1968
Experimental Section
General Considerations. Reactions were carried out using standard
high-vacuum and Schlenk techniques using dry, air-free solvents.
Elemental analyses were performed by Desert Analytics (Tucson, AZ).
NMR spectra were recorded using a Bruker DRX400 spectrometer.
1H NMR spectra were obtained at 400 MHz and were referenced to
residual proton peaks of the solvent (CD2Cl2, δ 5.32). 31P{1H} NMR
spectra were recorded at 162.0 MHz and referenced to external 85%
phosphoric acid (δ 0). Infrared spectra were recorded on a Nicolet
20SXB spectrometer in a Fluorolube-S30 mull with CaF2 plates. Re-
(CO)2(NO)(PPh3)2 (2a) was prepared according to the literature
procedure.14
(vs), 1872 (vs) cm-1; ν(NO) 1335 (s) cm-1 1H NMR (400 MHz,
.
CD2Cl2, 20 °C): δ 21.35 (s, 1 H, NHdO), 7.35 (s, 1 H, 1/2 C6H6),
2.33 (br m, 7 H, Cy), 1.95 (d, 6 H, Cy, J ) 12.0 Hz), 1.83 (m, 19 H,
Cy), 1.68 (br s, 5 H, Cy), 1.45 (m, 10 H, Cy), 1.25 (m, 19 H, Cy).
31P{1H} NMR (162.0 MHz, CD2Cl2, 20 °C): δ 16.4 (s).
Re(CO)2(NO)(PCy3)2 (2b). This preparation is a modification of a
literature procedure for Re(CO)2(NO)(PMe3)2.15 A 500 mL Schlenk
flask was evacuated on a vacuum line. Under argon, 10 mL of mercury
were added to the flask, followed by 1.0 g of sodium metal in small
portions (the reaction between mercury and sodium is very exothermic)
with stirring, to produce the sodium amalgam for reaction. Under an
Reaction of 3 with Brønsted Bases. In a typical reaction, 1.2-1.7
equiv of the base was added to a solution of 3 in CH2Cl2 (preparatory-
scale reactions) or CD2Cl2 (NMR-scale reactions). Preparatory-scale
reactions were run under argon by warming from -78 °C to room
temperature and monitored by IR. NMR-scale reactions were run at
room temperature, and the potential disappearance of the reactant
nitroxyl complex (δ 15.4 (3a), δ 16.4 (3b)) and the formation of the
product nitrosyl complex (δ 19.8 (2a), δ 23.5 (2b)) was monitored by
31P{1H} NMR (162.0 MHz). DBU, piperidine, triethylamine, ammonia,
4-(N,N-dimethylamino)pyridine, 2-aminomethylpyridine, 2,4,6-collidine,
imidazole, N,N-diethylaniline, and pyridine were used as bases. Results
of these reactions appear in Table 3.
15
argon counterflow, Re(Cl)2(CO)(NO)(PCy3)2 (1.078 g, 1.23 mmol)
(11) Sellmann, D.; Gottschalk-Gaudig, T.; Ha¨ussinger, D.; Heinemann, F.
W.; Hess, B. A. Chem. Eur. J. 2001, 7, 2099.
(12) Lin, R.; Farmer, P. J. J. Am. Chem. Soc. 2000, 122, 2393.
(13) Southern, J. S.; Hillhouse, G. L.; Rheingold, A. L. J. Am. Chem. Soc.
1997, 119, 12406.
(14) LaMonica, G.; Freni, M.; Cenini, S. J. Organomet. Chem. 1974, 71,
57.
(15) Hund, H.-U.; Ruppli, U.; Berke, H. HelV. Chim. Acta 1993, 76, 963.
(16) Cameron, T. S.; Grundy, K. R.; Robertson, K. N. Inorg. Chem. 1982,
21, 4149.

Chemistry of Coordinated Nitroxyl
Inorganic Chemistry, Vol. 40, No. 23, 2001 6041
Table 1. Spectroscopic Data for HNdO Ligands of 3a and 3b
δ(NHdO),
1
b
compound
ν(NO)a ν(NH)a
| JNH
|
Re(Cl)(CO)2(NHdO)(PPh3)2 (3a)
Re(Cl)(CO)2(NHdO)(PCy3)2 (3b)
1376
1335
3059
20.66, 66.2
21.35, c
c
a Fluorolube-S30 mull; cm-1
.
b 1H NMR (CD2Cl2, 400 MHz); J in
Hz. c Not observed or determined.
Table 2. Relevant Bond Distances (Å) and Bond Angles (deg)
for 4
Re-H
1.74
Re-N
1.829(5)
2.036(6)
2.039(6)
2.4685(11)
N-O(3)
1.185(6) Re-C(19)
1.129(7) Re-C(20)
1.129(7) Re-P
178.1(4) Re-C(19)-O(1) 175.9(5)
172.6(5) P′-Re-P
C(19)-O(1)
C(20)-O(2)
Re-N-O(3)
Re-C(20)-O(2)
C(19)-Re-C(20) 163.8(2) N-Re-C(19)
N-Re-C(20) 98.5(2) N-Re-P′
161.46(4)
97.6(2)
99.26(2)
Table 3. Results of Reaction of 3a,b with Bases at Room
Temperature
a
base
pKb
3a f 2a
3b f 2b
Figure 1. Amplification of the nitroxyl proton in the 1H NMR spectrum
of 3a (CD2Cl2, 400 MHz, 20 °C). The central resonance is due to
DBUb
piperidine
triethylamine
(N,N-dimethylamino)pyridine
ammonia
2-aminomethylpyridine
2,4,6-collidine
imidazole
N,N-diethylaniline
pyridine
2.5c
2.8
3.2
4.3
4.8
5.4
6.5
6.9
7.4
8.8
yes
yes
yes
yes
1
14NHdO (δ 20.66) and the satellites (| JNH| ) 66.2 Hz) are due to
natural-abundance15NHdO (shifted 9 ppb upfield).
yes
yes
yes
yes
yes
yes
no
yes
no
no
no
a Measured in H2O; from Perrin, D. D. Dissociation Constants of
Organic Bases in Aqueous Solution; Butterworths: London, 1965
(except where noted). b 1,8-Diaza[5.4.0]bicycloundec-7-ene. c Yamana-
ka, H.; Yokoyama, M.; Sakamoto, T.; Shiraishi, T.; Sagi, M.; Mizugaki,
M. Heterocycles 1983, 20, 1541.
[trans,trans-Re(H)(CO)2(NO)(PPh3)2+][SO3CF3-] (4). A 0.287 g
(0.361 mmol) sample of 2a was placed in a two-necked round-bottomed
flask attached to a swivel-frit assembly and containing a septum. The
apparatus was placed on a vacuum line, 20 mL of diethyl ether was
vacuum transferred into the flask at -78 °C (solid is insoluble in diethyl
ether), and the flask was filled with argon. At room temperature, triflic
acid (0.11 mL, 1.24 mmol) was added via syringe. The solution was
allowed to stir for 60 min, during which time the insoluble solid changed
from a dark red-brown to a pale yellow color. This yellow solid was
filtered, washed with diethyl ether, and dried under vacuum to afford
0.252 g (73.9% yield) of 4. Anal. Calcd for C39H31O6NSF3P2Re: C,
49.47; H, 3.30; N, 1.48. Found: C, 49.56; H, 3.18; N, 1.46. IR: ν(CO)
2103 (w), 2039 (s) cm-1; ν(NO) 1766 (s) cm-1 [coupled to ν(ReH)
Figure 2. Perspective view of the complex cation of 4 showing the
atom-numbering scheme. The triflate anion and the phenyl hydrogen
atoms are omitted for clarity.
suggested the centrosymmetric space group Pnma, which yielded
chemically reasonable and computationally stable results of refinement.
The structure was solved using direct methods, completed by subsequent
difference Fourier synthesis, and refined by full-matrix least-squares
procedures. The least-squares refinement converged to R(F) ) 0.039
and R(wF2) ) 0.063 for the 4330 unique data with I > 2σ(I). The
cation is located on a mirror plane and atoms S, F(3), and O(4) of the
triflate anion are disordered over a mirror plane. All non-hydrogen
atoms were refined with anisotropic displacement coefficients. The
hydrogen atom on the rhenium atom was located from the difference
map, and its displacement parameter was allowed to refine. All other
hydrogen atoms were treated as idealized contributions. All software
and sources of the scattering factors are contained in the SHELXTL
(version 5.03) program library (G. Sheldrick, Siemens XRD, Madison,
WI). Table 2 contains a listing of relevant bond distances and bond
angles. A perspective view of the complex, along with the atom-
numbering scheme, is shown in Figure 2.
1
(not observed)]. H NMR (400 MHz, CD2Cl2, 20 °C): δ 7.70-7.55
(m, 18 H, Ph), 7.50-7.40 (m, 12 H, Ph), -0.68 (t, 1 H, Re-H, 2JPH
21.9 Hz). 31P{1H} NMR (162.0 MHz, CD2Cl2, 20 °C): δ 9.4 (s).
)
[trans,trans-Re(2H)(CO)2(NO)(PPh3)2+][SO3CF3-] (4-d) was pro-
duced in an analogous manner by reaction with triflic acid-d (2HOSO2-
CF3, Aldrich Chemical Co., 98 atom % D). IR: ν(CO) 2103 (w), 2039
(s) cm-1; ν(NO) 1781 (s) cm-1
20 °C): δ 9.4 (s).
.
31P{1H} NMR (162.0 MHz, CD2Cl2,
Crystal Structure Determination of 4. Crystal data: C39H31O6-
NSF3P2Re, M ) 946.85, orthorhombic, space group Pnma, a )
17.2201(2) Å, b ) 23.6119(3) Å, c ) 9.2380(2) Å, V ) 3756.16(9)
Å3, Z ) 4, Dx ) 1.674 g‚cm-3, F(000) ) 1872, µ ) 34.39 cm-1, λ )
0.71073 Å, R(F) ) 0.039 R(wF2) ) 0.063 for the 4330 unique data
with I > 2σ(I).
Crystals of excellent quality of 4 were obtained by vapor diffusion
of diethyl ether into a solution of the complex in CH2Cl2 at 0 °C. A
suitable crystal was selected and mounted in a thin-walled glass capillary
under an inert atmosphere. The systematic absences in the diffraction
data were consistent for space groups Pna21 and Pnma. E-statistics
Results and Discussion
Reaction of trans-Re(CO)2(NO)(PR3)2 with HCl. The
addition of solid trans-Re(CO)2(NO)(PPh3)2 (2a) to a diethyl

6042 Inorganic Chemistry, Vol. 40, No. 23, 2001
Southern et al.
ether solution saturated with anhydrous HCl results in an
immediate color change of the ether-insoluble solid from red-
brown to deep green. Formal protonation of the nitrosyl ligand
and addition of chloride to the metal result, yielding the charge-
neutral nitroxyl complex cis,trans-Re(Cl)(CO)2(NHdO)(PPh3)2
(3a) as the major metal-containing product in excellent yield
(eq 3). Additionally, two side products, mer,trans-Re(Cl)(CO)3-
(PPh3)2 and cis,trans-Re(Cl)2(CO)(NO)(PPh3)2,14 are formed in
the reaction. The addition of 1.5 equiv of triphenylphosphine
to the reaction mixture before addition of the nitrosyl complex
allowed for the minimization of these side products (3a, 85%
of the product mixture; Re(Cl)(CO)3(PPh3)2, 8%; Re(Cl)2(CO)-
(NO)(PPh3)2, 7%), but we were unable to separate these three
compounds, as they were produced under all reaction conditions
we investigated and cocrystallized regardless of the solvent
combinations we used for recrystallization. For example,
beautiful crystals with well-formed faces of the green solid were
grown from toluene/Et2O, but they were not suitable for X-ray
analysis because of side-product contamination in the individual
crystals. (Note that these products are the same as reported by
LaMonica14 but optimized for 3a.)
and 31P{1H} NMR) and by elemental analysis. The infrared
spectrum of 2b shows ν(CO) (1916 (m), 1827 (s) cm-1) and
ν(NO) (1582 (s) cm-1) vibrations. As expected, the ν(CO) and
ν(NO) vibrations occur at lower energy for 2b relative to 2a14
due to the increased donating ability of PCy3 relative to PPh3.
When excess anhydrous HCl is added to a suspension of 2b
in diethyl ether, the insoluble compound changes from red-
brown to blue-green in color. Recrystallization of the solid
product from benzene/Et2O affords the charge-neutral nitroxyl
complex cis,trans-Re(Cl)(CO)2(NHdO)(PCy3)2 (3b‚1/2 C6H6)
as a deep green benzene solvate in modest yield, uncontaminated
by byproducts (eq 5). No additional tricyclohexylphosphine was
added to the reaction mixture. Complex 3b has been character-
ized by spectroscopic methods (IR, 1H and 31P{1H} NMR), by
elemental analysis, and by its reaction chemistry. Both 3a and
3b show short-term air stability and are moderately stable in
CD2Cl2 solution. The presence of a pure compound and the good
stability of 3b in solution allowed for the growth of single
crystals of 3b from CH2Cl2/Et2O solution. Unfortunately, the
resultant X-ray structure exhibited unresolvable site disorder
of the Cl, CO, and HNdO ligands in the equatorial plane.
Complex 3a has been characterized by spectroscopic methods
(IR, 1H and 31P{1H} NMR) and by its reaction chemistry. The
spectroscopic data for 3a are summarized in Table 1 and are
similar to those observed for 1 as well as for other reported
nitroxyl complexes.9-14 A diagnostic singlet is observed at δ
20.66 for the Re(NHdO) proton in the 1H NMR spectrum (CD2-
Cl2, 20 °C) of 3a. Upon amplification of this resonance, 15N
(0.37% natural abundance) satellites are observable (Figure 1).
The spectroscopic data for 3b are summarized in Table 1
and are very typical for a metal-nitroxyl complex.9-14 The 1H
NMR spectrum (CD2Cl2, 20 °C) of 3b shows a low-field singlet
at δ 21.35 for the nitroxyl proton. The infrared spectrum of 3b
shows a low-energy ν(NO) of medium intensity at 1335 cm-1
,
but the ν(NH) vibration was not identifiable. Two strong
vibrations at 1968 and 1872 cm-1 indicate a cis-orientation of
the CO ligands. The tricyclohexylphosphine groups resonate as
a singlet at δ 16.4 in the 31P{1H} NMR spectrum (CD2Cl2,
20 °C).
1
The observed | JNH| of 66.2 Hz is a typical value for an N-bound
nitroxyl ligand.10-13 As in the cationic complex 1, there is an
isotopic perturbation of the chemical shift in 3a, with the doublet
resonance for Re(15NHdO) shifting 9 ppb upfield of the Re-
(14NHdO) singlet resonance. The infrared spectrum of 3a shows
ν(NO) (1376 (m) cm-1) and ν(NH) (3059 (w) cm-1) vibrations
for the nitroxyl ligand. The cis-(CO)2 geometry is confirmed
Reaction of trans-Re(CO)2(NO)(PPh3)2 with HOSO2CF3.
The reaction of trifluoromethanesulfonic acid with 2a proceeds
differently from that of hydrochloric acid. Addition of HOSO2-
CF3 to a suspension of 2a in diethyl ether causes the insoluble
red-brown solid to turn yellow in color. In contrast to the
reaction of 2a with HCl (eq 3), which results in formal
protonation of the nitrosyl ligand and production of a nitroxyl
complex, the addition of triflic acid results in protonation at
the metal center, affording the cationic hydride complex [trans,-
trans-Re(H)(CO)2(NO)(PPh3)2+][SO3CF3-] (4) as a pale yellow,
analytically pure compound in 74% isolated yield (eq 6).
Reagent-specific protonations of this type have been observed
for other low-valent metal-nitrosyl complexes. For example,
the reaction of Ir(NO)(PPh3)3 with excess HCl results in multiple
protonations of the nitrosyl ligand, affording IrCl3(NH2OH)-
(PPh3)2,9 while reaction with HClO4 results in protonation at
the metal center, yielding [Ir(H)(NO)(PPh3)3+][ClO4-].17
by two intense vibrations for ν(CO) at 1975 and 1878 cm-1
.
The chemically equivalent triphenylphosphine groups resonate
as a singlet at δ 15.4 in the 31P{1H} NMR spectrum (CD2Cl2,
20 °C).
The tricyclohexylphosphine analogue of 2a, trans-Re(CO)2-
(NO)(PCy3)2 (2b), was prepared in order to study the effect of
varying the phosphine ligands on the HCl-addition reaction.
Complex 2b was prepared by the route Berke and co-workers
have reported for the preparation of Re(CO)2(NO)(PMe3)2 that
utilizes the reduction of dichlororhenium(I) complexes.15 Reduc-
tion of Re(Cl)2(CO)(NO)(PCy3)215 with sodium amalgam under
a CO atmosphere (Et2O solution, 3 days) yields the desired
nitrosyl complex 2b in low (19%) yield (eq 4). Complex 2b
was isolated as an air-sensitive, analytically pure red-brown solid
(17) (a) Reed, C. A.; Roper, W. R. J. Chem. Soc., Chem. Commun. 1969,
155. (b) Mingos, D. M. P.; Ibers, J. A. Inorg. Chem. 1971, 10, 1479.
1
and characterized by standard spectroscopic methods (IR, H

Chemistry of Coordinated Nitroxyl
Inorganic Chemistry, Vol. 40, No. 23, 2001 6043
The stereochemistry of 4 is that expected for the protonation
of 2a at Re under both thermodynamic and kinetic conditions.
The trans triphenylphosphine ligands of 2a remain trans in the
product, minimizing steric interactions. Due to the instability
of metal complexes containing trans-disposed nitrosyl and
carbonyl ligands,24 a trans arrangement of the carbonyl ligands
is preferred. Additionally, the trans relationship between the
nitrosyl ligand, the most proficient π-acceptor ligand in the
complex, and the strong σ-donor hydride ligand also contributes
to the thermodynamic stability of 4. Kinetically, the orbital most
Complex 4 has been characterized by standard spectroscopic
methods (IR, 1H and 31P{1H} NMR), elemental analysis, and a
single-crystal X-ray diffraction study. The H NMR spectrum
1
(CD2Cl2, 20 °C) of 4 exhibits a high-field 1:2:1 triplet at δ -0.68
indicative of a rhenium-hydride moiety coupled to two
magnetically equivalent phosphine ligands (2JPH ) 21.9 Hz).
The IR spectrum of 4 shows ν(CO) vibrations (2103 (w), 2039
(s) cm-1) typical of trans-disposed carbonyl ligands, and ν(NO)
at 1766 (s) cm-1. The ν(ReH) vibration is not easily identifiable
in the expected 1700-2200 cm-1 range, even when compared
to the spectrum of the deuteride, [trans,trans-Re(2H)(CO)2(NO)-
(PPh3)2+][SO3CF3-] (4-d). Complex 4-d is prepared in a manner
analogous to 4 by reacting 2a with triflic acid-d (98 atom %
2H). The ν(NO) of 4-d (1781 cm-1) shifts significantly relative
to that of protio 4 (1766 cm-1, ∆ ) 15 cm-1), indicative of a
resonance interaction between the trans ν(NO) and (unobserved)
ν(ReH) modes in 4.18,19 The trans relationship between these
two ligands has been confirmed in the X-ray diffraction analysis.
Figure 2 shows the geometry of the complex cation of 4.
Relevant bond distances and bond angles appear in Table 2.
Complete lists of bond distances and angles and crystal and
structural refinement data appear in the Supporting Information.
The metal-bound hydrogen atom was located from the difference
map, and its displacement parameter was allowed to refine. The
complex cation is approximately pseudooctahedral in geometry
with trans carbonyl ligands, trans triphenylphosphine ligands,
and a linear nitrosyl ligand ( Re-N-O(3) ) 178.1(4)°) trans
to the hydride ligand. The atoms Re, C(19), O(1), C(20), O(2),
N, O(3), and H lie in a crystallographic mirror plane. The Re-H
bond distance (1.74 Å) compares favorably with those of neutral
rhenium-hydride complexes of the type Re(H)(CO)3-x(PR3)2+x
(x ) 0, R ) OiPr; x ) 1, 2, R ) Me) calculated by Berke and
2
available for protonation in 2a is mainly metal dz in character,
facilitating protonation trans to the nitrosyl ligand.25
Mechanism of HCl Addition to trans-Re(CO)2(NO)-
(PPh3)2. The mechanism of HCl addition to 2 deserves
comment, since the linear, formally electrophilic nitrosyl ligand
(NO+) of 2 might not be expected to be susceptible to
protonation at nitrogen. Furthermore, the independent observa-
tion that the strong acid HOSO2CF3 protonates at the metal and
not at nitrogen leads one to conclude that either (i) an event
occurs prior to protonation (at N) to render the nitrosyl ligand
nucleophilic, presumably by bending, or (ii) the transformation
of the Re-NO ligand to the Re(NHdO) ligand does not involve
classical “protonation” of a nitrogen lone-pair.
Enemark and Feltham have reported that addition of Br- to
dicationic, trigonal bipyramidal Co(diars)2(NO)2+ ( Co-N-O
) 178(2)°) results in bending of the NO ligand in the
six-coordinate product Co(diars)2(Br)(NO)+ ( Co-N-O )
132(1)° in the analogous NCS- complex).26,27 In light of the
differing reactivities of hydrochloric and triflic acids toward 2
-
and considering that the anions Cl- and SO3CF3 have
significantly different coordinating characteristics (triflate being
relatively more weakly coordinating),28 we have looked for
evidence of interactions between nucleophiles and 2a that might
lead to coordination and concomitant nitrosyl bending. No
spectroscopic change (monitored by IR and 31P{1H} NMR) is
observed upon the addition of 20 equiv of [n-Bu4N+][Cl-] to a
CD2Cl2 solution of the neutral complex 2a, and while negative
evidence of this type is by its nature inconclusive, we see no
evidence of even a small equilibrium concentration of a bent-
nitrosyl species (e.g., Re(Cl)(CO)2(NO)(PPh3)2-) in this system.
1
co-workers (1.69-1.77 Å) from H NMR T1 relaxation mea-
surements.20 There is substantial bending of the triphenylphos-
phine and carbonyl ligands toward the hydride site in 4 ( P′-
Re-P ) 161.46(4)°, C(19)-Re-C(20) ) 163.8(2)°). Similar
octahedral distortions are observed in the related neutral group
6 hydrides trans,trans-W(H)(CO)2(NO)(PPh3)2 ( P-W-P )
170.9(1)°, C-W-C ) 159.7(2)°)21 and trans,trans-Cr(H)-
(CO)2(NO)(PPh3)2 ( P-Cr-P ) 165.5(1)°, C-Cr-C )
153.7(2)°).22 While these distortions are expected on steric
grounds, electronic factors also favor the bending of the carbonyl
and triphenylphosphine ligands toward the purely σ-donating
hydride ligand and away from the nitrosyl ligand, a strong
π-acceptor.17b,21-23
A second mechanistic possibility for the production of 3 from
2 involves initial (kinetic) protonation at the metal followed by
H-migration from rhenium to nitrogen with chloride ligation.
This mechanistic postulate is testable since we have prepared
the putative hydride intermediate independently. We have shown
that the cationic hydride complex 4 does not react with added
chloride to yield 3a; hence it is probably not an intermediate in
the formation of 3a from 2a. The observation that 4 is the kinetic
product of protonation of 2 also argues against initial protonation
at the nitrosyl oxygen followed by H-migration, although it
(23) Elian, M.; Hoffman, R. Inorg. Chem. 1975, 14, 1058.
(24) Shi, Q.-Z.; Richmond, T. G.; Trogler, W. C.; Basolo, F. Inorg. Chem.
1984, 23, 957, and references therein.
(25) Enemark, J. H.; Feltham, R. D. Proc. Natl. Acad. Sci. U.S.A. 1972,
69, 3534.
(26) (a) Enemark, J. H.; Feltham, R. D.; Riker-Nappier, J.; Bizot, K. F.
Inorg. Chem. 1975, 14, 624. (b) Enemark, J. H.; Feltham, R. D. Coord.
Chem. ReV. 1974, 13, 339, and references therein.
(27) A significant decrease in ν(NO) is reported for the transformation of
Co(diars)2(NO)2+ (1852 cm-1) to Co(diars)2(Br)(NO)+ (1565, 1550
cm-1).26 The authors report that the bent nitrosyl ligand of Co(diars)2-
(Br)(NO)+ is protonated upon reaction with HClO4, forming Co(diars)2-
(Br)(NHdO)2+, but the reported spectroscopic values of the product
are inconsistent with those of ligated nitroxyl.9-14
(18) No ν(Re2H) is observed for 4-d, indicating that this band has very
low intrinsic intensity. Analogous results were obtained for the charge-
neutral group 6 analogues trans,trans-M(H)(CO)2(NO)(PPh3)2 (M )
Mo, W), which show strong resonance coupling of the trans-disposed
nitrosyl and hydride ligands (for M ) Mo19a and W19b).
(19) (a) Smith, M. R., III; Cheng, T.-Y.; Hillhouse, G. L. Inorg. Chem.
1992, 31, 1535. (b) Hillhouse, G. L.; Haymore, B. L. Inorg. Chem.
1987, 26, 1876.
(20) Gusev, D. G.; Nietlispach, D.; Vymenits, A. B.; Bakhmutov, V. I.;
Berke, H. Inorg. Chem. 1993, 32, 3270.
(21) van der Zeijden, A. A. H.; Sontag, C.; Bosch, H. W.; Shklover, V.;
Berke, H.; Nanz, D.; von Philipsborn, W. HelV. Chim. Acta 1991, 74,
1194, and references therein.
(22) Peters, J. C.; Hillhouse, G. L.; Rheingold, A. L. Polyhedron 1994,
13, 1741.
(28) Lupinetti, A. J.; Strauss, S. H. Chemtracts-Inorg. Chem. 1972, 11,
565.

6044 Inorganic Chemistry, Vol. 40, No. 23, 2001
Southern et al.
Table 4. Calculated Bond Distances and Observed Vibrational Frequencies
a Vibration contains significant HNO bending character (ref 2a). b Experimental infrared values for 1 (ref 13). c Experimental infrared values for
3a. d Experimental infrared values for 3b.
should be noted that the metastable NOH isomer has been
spectroscopically characterized in an Ar matrix.29
the reaction of 3a with 2,4,6-collidine, which requires >30 min
for complete conversion to 2a, probably due to the steric
properties of the base. The reaction shown in eq 8 is irreversible;
no reaction is observed between 2a and Et3N‚HCl or pyridine‚
HCl (∼3 equiv). In an analogous manner, 3b eliminates HCl
in the presence of sufficiently strong Brønsted bases. By
monitoring the reactions by 31P{1H} NMR (CD2Cl2, 20 °C), a
clean conversion of 3b (δ 16.4) to 2b (δ 23.5) is observed when
an appropriately strong base is used (Table 3). For both 3a,b,
imidazole (pKb ) 6.9) is strong enough to effect dehydrochlo-
rination to give 2a,b, but the nitroxyl compounds fail to react
with N,N-diethylaniline (pKb ) 7.4) or weaker bases. In
comparison to 3a, a qualitatively slower rate is observed for
the reactions of 3b with the bases, with complete conversion
by suitably strong bases requiring >30 min in all cases, possibly
a consequence of greater steric hindrance by the bulkier PCy3
ligands.
An alternative scenario that merits discussion is one involving
direct 1,2-addition of HCl across the Re-N multiple bond of
the nitrosyl, as depicted in eq 7. The ν(NO) exhibited by 2a
(1622 cm-1) is a low value for a linearly coordinated nitrosyl
ligand. This value is indicative of strong dπ(Re)fp*(NO) back-
bonding and a relatively large amount of electron density at
nitrogen. A similar mechanism has been proposed for the
intermolecular exchange of nitrosyl and chloride ligands in other
transition metal systems30 and is consistent with the observed
stereochemistry for 3a having Cl cis to the HNdO ligand. This
same orientation is found in the structure of Os(Cl)2(CO)(NHd
O)(PPh3)2.10
For purposes of reference, the pKa of free HNdO in water
has been measured as 4.7 in pulse radiolysis experiments,31
although in our experiments we are not directly measuring the
pKa of bound nitroxyl since we are both removing a proton from
nitrogen and breaking a Re-Cl bond. Nonetheless, these data
show that it is qualitatively more difficult to remove a proton
from bound HNdO in 3a,b than from the free molecule, a point
addressed in the following section.
DFT Calculations on Nitroxyl and Model Nitroxyl Com-
plexes. To obtain a better understanding of the stability that
metal coordination confers to the nitroxyl molecule, DFT
calculations were used to examine the bonding in nitroxyl and
the model nitroxyl complexes Re(CO)5(NHdO)+ (5), mer,trans-
Re(CO)3(NHdO)(PH3)2+ (6), and cis,trans-Re(Cl)(CO)2(NHd
O)(PH3)2 (7), the structures of which are shown in Table 4.
Complexes 6 and 7 were chosen as models for the known
nitroxyl complexes 1,13 and 3a,b, respectively. The substitution
of PH3 in the model complexes for the PR3 (R ) Ph, Cy) ligands
in 1 and 3a,b simplifies the calculations by reducing the number
of atoms present in the complexes (electronic consequences in
making this simplification are addressed below). Calculations
were performed on complex 5 in order to probe the electronic
effect of increasing the number of strong π-acceptor ligands in
this system. The B3LYP functional (GAUSSIAN94),32 which
has been shown to yield reliable results for many transition-
Reaction of Re(Cl)(CO)2(NHdO)(PR3)2 with Brønsted
Bases. The reactions of 3a and 3b with proton-accepting
nitrogen bases have been investigated. In a typical reaction for
3a, 1.5 equiv of the nitrogen base was added to a CH2Cl2 or
CD2Cl2 solution of the complex, and the reaction was monitored
by solution IR and/or 31P{1H} NMR spectroscopy. The 31P-
{1H} NMR spectra show clean conversion of 3a (δ 15.4) to 2a
(δ 19.8) (eq 8) when an appropriately strong base is used
(summarized in Table 3). Importantly, both 3a and 3b are stable
in solution with respect to HCl loss in the absence of added
base.
The successful reactions for 3a shown in Table 3 are rapid,
with complete conversion requiring ca. 5 min. An exception is
(29) Maier, G.; Reisenauer, H. P.; De Marco, M. Angew. Chem., Int. Ed.
Engl. 1999, 38, 108.
(30) Unbermann, C. B.; Caulton, K. G. J. Am. Chem. Soc. 1976, 98, 3862.
(31) Gra¨tzel, M.; Taniguchi, S.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem.
1970, 74, 1003.

Chemistry of Coordinated Nitroxyl
Inorganic Chemistry, Vol. 40, No. 23, 2001 6045
nitroxyl complexes 1, 3a, and 3b, ν(NO) decreases relative to
the value for free HNdO (Table 4), indicating that nitroxyl acts
as a π-acceptor ligand. It is well appreciated that NO+ is a strong
π-acceptor ligand35 and HNdO functions in a similar fashion.
A significant difference between these two ligands is that NO+,
the stronger π-acceptor, contains two perpendicular π* orbitals
for accepting back-donation from the metal (the nitrogen is sp-
hybridized), while HNdO contains only one (the nitrogen is
sp2-hybridized).
Figure 3. Calculated structural parameters for HNdO.
Further illustrating the π-acceptor nature of nitroxyl, the HNd
O ligand in each of the model complexes is oriented in the plane
containing the largest number of competing π-acceptor ligands.
This allows HNdO to minimize the competition with other
ligands for π-donation from the metal center and to maximize
its own π-interactions. This phenomenon is also observed
structurally. In the X-ray crystal structures of cis,trans-Os(Cl)2-
(CO)(NHdO)(PPh3)210 and [cis,trans-Os(Br)(CO)2(NHdNH)-
(PPh3)2+][SO3CF3-],8 the HNdX ligand is oriented in the plane
containing the greatest number of competing π-acceptor ligands.
While this orientational preference could be interpreted as a
minimization of steric effects, it is notable that in the structure
of the related nitrosoalkane complex W(CO)5(N(tBu)dO) the
tBuNdO ligand is aligned in a plane containing three CO
ligands, even though more sterically favorable orientations are
possible.36 That nitroxyl is a π-acceptor is clearly illustrated in
the results shown in Table 4: the calculated N-O bond length
increases (with a concomitant decrease in the Re-N bond
length) in complexes 5-7 as the number of competing π-ac-
ceptor ligands decreases. Further experimental evidence for the
π-accepting nature of HNdO is provided by inspection of the
carbonyl stretching frequencies of 1 (ν(CO) ) 2082(w), 2007-
(s), 1977(s) cm-1) and its hydroxylamine precursor [mer,trans-
Re(CO)3(NH2OH)(PPh3)2 ][SO3CF3 ] (ν(CO) ) 2061(w), 1966-
(s), 1926(s) cm-1; see eq 1).13 The large increase (∆ ≈ 21-51
cm-1) in the values of ν(CO) when the purely σ-donating NH2-
OH ligand is converted into the HNdO ligand in an otherwise
identical coordination environment reflects decreased back-
bonding to the CO ligands of 1 relative to [Re(CO)3(NH2-
OH)(PPh3)2+][SO3CF3-] due to competition for metal π-electron
density from the nitroxyl ligand. This is apparently also
manifested in the excited state. The electronic spectrum (CH2-
Cl2) of 3a shows an intense band in the visible region at
λmax ) 418 nm (ꢀ ≈ 3 × 103 M-1 cm-1), assigned as a metal-
to-ligand (M(dπ)fHNdO(π*)) charge-transfer (MLCT) transi-
tion. MLCT bands of similar energy and intensity have been
observed in related nitrosoarene and nitrosoalkane complexes,
which like 1, 3a, and 3b are also are intensely colored.36,37
It is appreciated that these calculations using PH3 (instead of
PPh3 or PCy3) introduce perturbation in the electronic environ-
ment at the metal. However, it should be noted that the lone
pair of PH3 resides in an orbital that is almost purely s in
character,38 making PH3 a poor σ-donor. The calculations on
complexes 5-7, which incorporate PH3 ligands, thus underes-
timate the lengthening of the nitroxyl N-O bond and the
shortening of the Re-N bond that would occur in 1 and 3a,
which contain the better σ-donor ligand PPh3, and especially
in 3b, which contains the relatively strong σ-donor ligand PCy3.
Figure 4. Pictorial representation of the calculated HOMO of HNd
O, consisting of N-H σ-bonding and N-O π-antibonding (in the HNO
plane) interactions.
metal systems, was used to examine these compounds. The
species examined were optimized using the 6-311G(d) basis on
all atoms except Re, which was described using the LANL2DZ
basis set and effective core potentials.33
Calculations on free nitroxyl yielded the following structural
parameters: an N-O bond length of 1.199 Å, an N-H bond
length of 1.065 Å, and an H-N-O bond angle of 108.75° (see
Figure 3 and Table 4). These values are in good agreement with
those determined by Dalby from analysis of the electronic
absorption spectrum of HNdO (d(N-O) 1.211(6) Å, d(N-H)
1.062(8) Å, H-N-O 108.5(8)°)2a and recent theoretical
treatments.34 A view of the HOMO of HNdO, which consists
of N-O π-antibonding (in the plane of the hydrogen) and N-H
σ-bonding interactions, is pictured in Figure 4. The LUMO of
HNdO consists of an N-O π-antibonding interaction in the
plane perpendicular to the molecular plane.
+
-
Nitroxyl interacts with a metal center in a σ-donor, π-acceptor
fashion through its HOMO and LUMO orbitals. For a d6 metal
system, the nitroxyl HOMO interacts in a σ-fashion with an
2
empty metal dz orbital, and the filled metal dxz or dyz orbital
donates electron density into the HNdO π-system via its overlap
with the nitroxyl LUMO. Our calculations on complexes 5-7
show that, relative to free nitroxyl, the metal-coordinated species
display a lengthened N-O bond. These calculations are
consistent with experimentally determined infrared data. For
(32) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Frisch, M. J.;
Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb,
M. A.; Cheeseman, J. R.; Keith, T.; Peterson, G. A.; Montgomery, J.
A.; Raghavachari, K.; Al-Latham, M. A.; Zakrzewski, V. G.; Ortiz,
J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara,
A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong,
M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.;
Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.;
Head-Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian94, Revision
E.2; Gaussian, Inc.: Pittsburgh, PA, 1995.
(33) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (b) Wadt,
W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (c) Hay, P. J.; Wadt,
W. R. J. Chem. Phys. 1985, 82, 299.
(34) (a) Mordaunt, D. H.; Flo¨thmann, H.; Stumpf, M.; Keller, H.-M.; Beck,
C.; Schinke, R.; Yamashita, K. J. Chem. Phys. 1997, 107, 6603. (b)
Tschumper, G. D.; Schaeffer, H. F., III. J. Chem. Phys. 1997, 107,
2529.
(35) Clarkson, S. G.; Basolo, F. Inorg. Chem. 1973, 12, 1528.
(36) Pilato, R. S.; McGettigan, C.; Geoffroy, G. L.; Rheingold, A. L.; Geib,
S. J. Organometallics 1990, 9, 312.
(37) Bowden, W. L.; Little, W. F.; Meyer, T. J. J. Am. Chem. Soc. 1974,
96, 5605.
(38) Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry:
Principles of Structure and ReactiVity, 4th ed.; Harper Collins: New
York, 1993; p 226.

6046 Inorganic Chemistry, Vol. 40, No. 23, 2001
Table 5. Calculated N-H Bond Dissociation Energies
Southern et al.
N-H bond dissociation
complex
energy (kcal/mol)
HNdO
48.5a
62.1b
64.8b
mer,trans-Re(CO)3(NHdO)(PH3)2+ (6)
cis,trans-Re(Cl)(CO)2(NHdO)(PH3)2 (7)
a Eq 9. b Eq 10.
Our calculations show that the N-H bond of nitroxyl, in
contrast, is shortened upon ligation and is not strongly affected
by varying ancillary ligands. This result is also consistent with
experimentally determined infrared data, which show an increase
in ν(NH) for nitroxyl complexes 1 and 3a relative to the value
for free HNdO (Table 4, ν(NH) was not observed for 3b).
Energies of the N-H bonds (Table 5) were calculated using eq
9 and eq 10 for HNdO and its model complexes, respectively.
The N-H bond energy for nitroxyl was calculated as the
difference in energy between HNdO and the sum of the energies
of its separate H and NO components (eq 9). Similarly, the
nitroxyl N-H bond energy in model complexes 6 and 7 was
calculated as the difference in energy between the complex (Ln-
Re(NHdO)) and the sum of the energies of the H atom and the
optimized structure of LnRe(NO) (eq 10). The bond dissociation
energy of the weak N-H bond in free HNdO was calculated
to be 48.5 kcal/mol, which is in good agreement with the 50.1
kcal/mol value reported using high-level CASSCF/ICCI calcula-
tions and other reported values obtained by ab initio methods.3,34a
As can be seen in Table 5, metal coordination strengthens this
bond by 13.6 kcal/mol in cationic 6 and by 16.3 kcal/mol in
charge-neutral 7.
Figure 5. MO diagram of the metal-NO interaction. The NO π*
orbital available for bonding with hydrogen is lowered in energy due
2
to σ-interaction with the metal dz orbital.
amounts of two side products. Complexes 3a,b exhibit spec-
troscopic properties typical of nitroxyl complexes, with diag-
1
nostic H NMR resonances (δ 20.66 for 3a, δ 21.35 for 3b)
and IR stretches (ν(NO) ) 1376 cm-1 for 3a, 1335 cm-1 for
3b) for the HNdO ligand. While further studies are necessary
in order to more fully understand the mechanism of the nitrosyl
ligand “protonations” in these and related complexes, the
evidence currently in hand favors direct 1,2-addition of HCl
across the Re-N multiple bond of the nitrosyl, and not simple
protonation at nitrogen or the metal. Both 3a,b cleanly eliminate
HCl in the presence of sufficiently strong Brønsted bases to
re-form the parent nitrosyl complexes.
The protonation reactions are acid-dependent. Reaction of 2a
with triflic acid (HOSO2CF3) results in metal protonation,
yielding the cationic hydridorhenium(I) complex [trans,trans-
Re(H)(CO)2(NO)(PPh3)2+][SO3CF3-] (4), which has been char-
acterized by X-ray crystallography.
The driving force for strengthening of the N-H bond upon
coordination in the d6 pseudooctahedral metal systems employed
here can be seen in the molecular orbital diagram presented in
Figure 5. One of the π* orbitals of NO interacts with a metal
Finally, we have carried out DFT calculations on free HNdO
and several of its model complexes. The calculations show
nitroxyl interacting with these d6 pseudooctahedral metal centers
in a σ-donor, π-acceptor fashion through its HOMO and LUMO
orbitals. Moreover, metal coordination strengthens the N-H
bond in HNdO bond by ca. 14-16 kcal/mol relative to the
weak N-H bond (48.5 kcal/mol) in the free molecule.
t
2g orbital in a π-fashion, while the other π* orbital is positioned
2
for a σ-interaction with the dz orbital of the metal eg set. This
σ-interaction stabilizes the π* level of the coordinated NO unit
and places it in closer (relative to the free NO molecule)
energetic proximity to the 1s orbital of hydrogen. Since the
strength of interaction between two orbitals depends inversely
upon their energetic difference, this results in a stronger N-H
bond for the metal-coordinated nitroxyl species.
Acknowledgment. We are grateful to the National Science
Foundation (CHE-9816341) and the donors of the Petroleum
Research Fund, administered by the American Chemical Society
(Grant 29176-AC3), for financial support of this research
through grants to G.L.H., and to the Department of Education
for GAANN Fellowships to J.S.S. and M.T.G.
Conclusions
The reaction of trigonal bipyramidal metal-nitrosyl com-
plexes with hydrogen chloride has proved to be a fruitful method
for the synthesis of charge-neutral, d6 pseudooctahedral metal-
nitroxyl complexes. The nitroxyl complexes cis,trans-Re(Cl)-
(CO)2(NHdO)(PPh3)2 (3a) and cis,trans-Re(Cl)(CO)2(NHdO)-
(PCy3)2 (3b) are produced by the addition of anhydrous HCl to
diethyl ether suspensions of Re(CO)2(NO)(PPh3)2 (2a) and Re-
(CO)2(NO)(PCy3)2 (2b), respectively. Complex 3b is produced
cleanly, after recrystallization, while 3a is contaminated by small
Supporting Information Available: Listings of crystallographic
details, atomic coordinates, bond angles and distances, anisotropic
thermal parameters, and hydrogen atom coordinates. This material is
available free of charge via the Internet at http://pubs.acs.org.
IC010669M