A new route to coordination complexes of nitroxyl (HN=O) via insertion reactions of nitrosonium triflate with transition-metal hydrides

Article abstract of DOI:10.1021/ja0277475

Nitrosonium triflate reacts with cold methylene chloride solutions of mer,trans-ReH(CO)₃(PPh₃)₂ (1) with 1,1-insertion of NO⁺ into the Re-H bond to give the orange nitroxyl complex [mer,trans-Re(NH=O)(CO)₃(PPh₃)₂][SO₃CF₃] (3) in 86% isolated yield. Use of [NO][PF₆] or [NO][BF₄] gives analogous insertion products at low temperature, which decompose on warning to ambient temperature to the fluoride complex mer,trans-ReF(CO)₃(PPh₃)₂ (4). A related 1,1-insertion is observed in the reaction of 1 with [PhN₂][PF₆] in acetone that affords the yellow-orange phenyldiazene salt [mer,trans-Re(NH=NPh)(CO)₃(PPh₃)₂][PF₆] (2), which has been characterized by X-ray crystallographic methods. The methyl derivative mer,trans-Re(CH₃)(CO)₃(PPh₃)₂ (5) also undergoes a 1,1-insertion reaction with [NO][SO₃CF₃] to give the nitrosomethane adduct [mer,trans-Re{N(CH₃)=O}(CO)₃(PPh₃)₂][SO₃CF₃] (6) as red crystals in 75% yield. The nitroxyl complex [cis,trans-OsBr(NH=O)(CO)₂(PPh₃)₂][SO₃CF₃] (8) can be similarly prepared as orange crystals in 52% yield by reaction of cis,trans-OsHBr(CO)₂(PPh₃)₂ (7) with [NO][SO₃CF₃] in cold methylene chloride solution. Copyright

Full text of DOI:10.1021/ja0277475

Published on Web 09/19/2002
A New Route to Coordination Complexes of Nitroxyl (HNdO) via Insertion
Reactions of Nitrosonium Triflate with Transition-Metal Hydrides
Rory Melenkivitz,† Joel S. Southern,† Gregory L. Hillhouse,*,† Thomas E. Concolino,‡
Louise M. Liable-Sands,‡ and Arnold L. Rheingold*,‡
Searle Chemistry Laboratory, Department of Chemistry, The UniVersity of Chicago, Chicago, Illinois 60637,
and Department of Chemistry, UniVersity of Delaware, Newark, Delaware 19716
Received July 17, 2002
Nitroxyl (HNdO), a simple yet highly reactive molecule, is
attracting current research interest primarily because of its relation-
ship to nitric oxide. It has relevance in biochemistry1 and has been
invoked as an intermediate in a variety of photochemical and free-
radical reactions,2 detected in interstellar clouds,3 and implicated
as an intermediate in the combustion of nitrogen-containing fuels
and the oxidation of atmospheric nitrogen.4 Several examples of
transition-metal complexes containing the HNdO ligand have been
reported, and three distinct preparative routes to nitroxyl derivatives
have been described: (i) protonation of nitrosyl ligands in
complexes of iridium, iron, osmium, and rhenium,5-8 (ii) nucleo-
philic addition of hydride to nitrosyl ligands in ruthenium com-
plexes,9,10 and (iii) selective oxidation (using Pb(OAc)4) of a
rhenium hydroxylamine complex.11 Diazene (HNdNH) is isoelec-
tronic with nitroxyl, and because 1,1-insertion of aryldiazonium
Figure 1. A perspective view of the molecular structure of the complex
cation of 2‚(0.5Et2O). H-atoms, except that on N(1), have been omitted.
ions into metal-hydride bonds has been known for decades to
provide an effective synthesis of aryldiazene complexes,12 we were
intrigued by the possibility that analogous 1,1-insertion reactions
of nitrosonium salts with metal-hydrides might afford a facile route
to nitroxyl species. This work describes our comparative examina-
tion of the reactions of aryldiazonium and nitrosonium salts with
several d6 transition-metal hydrido and methyl complexes, including
examples of nitrosonium insertion into M-H and M-R bonds to
give coordination complexes containing nitroxyl and nitrosoalkane
(RNdO) ligands.
Selected metrical parameters: Re(1)-N(1) ) 2.149(5), N(1)-N(2) ) 1.245-
(7) Å; Re(1)-N(1)-N(2) ) 126.4(4), N(1)-N(2)-C(9) ) 118.3(5)°.
We chose mer,trans-ReH(CO)3(PPh3)2 (1) for our initial studies
because the nitroxyl complex [mer,trans-Re(NHdO)(CO)3(PPh3)2]-
[SO3CF3] is known and exhibits moderate stability.11 Reaction of
acetone solutions of 1 with [PhN2][PF6] affords the phenyldiazene
complex salt [mer,trans-Re(NHdNPh)(CO)3(PPh3)2][PF6] (2) as
analytically pure, yellow-orange crystals in 71% yield (eq 1). The
parent diazene analogue, [mer,trans-Re(NHdNH)(CO)3(PPh3)2][SO3-
CF3], is also known.13c Characterization of 2 followed from its IR
and 1H and 31P{1H} NMR spectra, elemental analysis, and a single-
crystal X-ray diffraction study.14 The 1H NMR spectrum of 2 shows
a characteristic singlet for the diazenyl proton at δ 12.94 in CD2-
Cl2,13 and by utilizing an inverse-detection NMR protocol (bilinear
Treatment of a cold (-78 °C) methylene chloride solution of 1
with 1 equiv of nitrosonium triflate, [NO][SO3CF3], results in clean
1,1-insertion of nitrosonium into the Re-H bond to give [mer,trans-
Re(NHdO)(CO)3(PPh3)2][SO3CF3] (3a) as an analytically pure
crystalline salt in 86% isolated yield (eq 2).14 The characterization
of 3a followed by spectroscopic comparison to an authentic sample
prepared by the oxidation of [mer,trans-Re(NH2OH)(CO)3(PPh3)2]-
[SO3CF3] by Pb(OAc)4.11 Of particular note is the diagnostic proton
resonance for the HNdO ligand (δ 21.66, CD2Cl2). The reaction
appears to proceed by direct insertion of NO+ into the metal-
1
15
rotational decoupling), the | J NH| ) 68.4 Hz was observed, a
typical value for a proton attached to a metal-bound sp2-hybridized
nitrogen.5-9,11,13,15 A perspective view of the complex cation of 2
is shown in Figure 1; the N(1)-N(2) bond distance (1.245(7) Å)
is typical for an N-N double bond and is within the range normally
found for aryldiazene complexes (∼1.22 to ∼1.26 Å).13
hydride bond, although we cannot exclude a mechanism involving
+
initial intermolecular electron transfer to give [ReH(CO)3(PPh3)2
]
and NO• followed by insertion of nitric oxide into the cationic
Re-H intermediate; NO has recently been shown to couple with a
Ru-H moiety to give a nitroxyl adduct.10 No other metal products,
* To whom correspondence should be addressed. E-mail: g-hillhouse@
uchicago.edu.
† The University of Chicago.
‡ University of Delaware.
9
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10.1021/ja0277475 CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
such as those resulting from carbonyl displacement, have been
observed. It is noteworthy that NO+ insertion into the Re-H bond
of 1 is also observed for the more commonly encountered,
commercially available nitrosonium salts [NO][X] (X ) PF6, BF4)
investigated. We observed that nitrosonium, like the related
aryldiazonium ions, undergoes facile 1,1-insertions with d6 hydrido
and alkyl complexes to give new complexes containing nitroxyl
and nitrosoalkane ligands, the stabilities of which are highly
dependent on the nature of the nitrosonium salt used. Nitrosonium
hexafluorophosphate and tetrafluoroborate salts yield thermally
unstable complexes that decompose with ligand loss to the neutral
fluorides, whereas nitrosonium triflate reacts to give the moderately
stable salts [mer,trans-Re(NHdO)(CO)3(PPh3)2][SO3CF3] (3), [mer,-
trans-Re{N(CH3)dO}(CO)3(PPh3)2][SO3CF3] (6), and [cis,trans-
OsBr(NHdO)(CO)2(PPh3)2][SO3CF3] (8). The related phenyl-
diazene salt [mer,trans-Re(CO)3(NHdNPh)(PPh3)2][PF6] (2) has
also been prepared and crystallographically characterized.
1
when the reaction is monitored at low temperature by H NMR.
However, the resulting hexafluorophosphate and tetrafluoroborate
complex salts (3b and 3c) are much less stable than the corre-
sponding trifluoromethanesulfonate derivative 3a, readily decom-
posing at ambient temperature to the neutral fluoride complex
mer,trans-ReF(CO)3(PPh3)2 (4; eq 2).16 This observation may
explain why related insertion reactions of nitrosonium salts with
metal hydrides have not been previously reported.
Reaction of nitrosonium triflate with CH2Cl2 solutions of
mer,trans-Re(CH3)(CO)3(PPh3)2 (5) gives the nitrosomethane com-
plex [mer,trans-Re{N(CH3)dO}(CO)3(PPh3)2][SO3CF3] (6) as red
crystals in 75% isolated yield (eq 3).14 Legzdins has reported
NO+ insertion into Cr-alkyl bonds of CpCr(NO)2R to give
[CpCr(NO)2(NRdO)][PF6],17 and Bergman and co-workers have
observed intramolecular alkyl/nitrosyl migratory insertions in (η5-
C5H5)Co(NO)(R) complexes that give stable η1-nitrosoalkane
products (η5-C5H5)Co(NRdO)(PPh3) in the presence of trapping
Acknowledgment. We are grateful to the National Science
Foundation and the Petroleum Research Fund, administered by the
ACS, for financial support of this research through grants to G.L.H.,
and to the Department of Education for GAANN Fellowships to
R.M. and J.S.S.
Supporting Information Available: Synthetic, spectroscopic, and
analytical details (PDF); crystallographic details for 2 (CIF). This
material is available free of charge via the Internet at http://pubs.acs.org.
1
PPh3.18,19 The H NMR spectrum of 6 shows a methyl resonance
at δ 3.12, and νNO is observed at 1378 cm-1 in the IR spectrum,
values similar to those observed for other coordinated nitrosoal-
kanes.18 The methyl carbon of 6 resonates as a quartet at δ 88.5 in
the 13C NMR spectrum. The trans,mer-geometry for the phosphine
and carbonyl ligands is maintained as indicated by the 31P{1H}
NMR (singlet, δ 10.4) and IR spectra (νCO ) 2078(w), 2001(s),
1967(s) cm-1). As was observed in the reactions of 1 (eq 2), reaction
of 5 with [NO][X] (X ) PF6, BF4) resulted in the isolation of the
fluoride complex 4.
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To assess whether the nitrosonium insertions were peculiar to
this Re(I) system, we have investigated the reaction of a d6 osmium
hydride, cis,trans-OsHBr(CO)2(PPh3)2 (7), with nitrosonium triflate
(eq 4). The reaction affords a new nitroxyl complex [cis,trans-
OsBr(NHdO)(CO)2(PPh3)2][SO3CF3] (4) as orange crystals in 52%
yield, which has been characterized by IR and NMR (1H, 31P)
spectroscopy and elemental analysis.14 The nitroxyl proton resonates
1
at δ 20.70 in the H NMR spectrum (CD2Cl2), and the ligand’s
νNO occurs at 1365 cm-1, both typical for M-NHdO moieties.
The 31P{1H} NMR shows a singlet for the trans-disposed phosphines
at δ 2.7, and a cis orientation for the carbonyl ligands is indicated
by two strong νCO at 2059 and 1998 cm-1 in the infrared. The IR
and 31P NMR data are similar to those found in the isoelectronic
diazene complex [cis,trans-OsBr(NHdNH)(CO)2(PPh3)2][SO3CF3]
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(νCO ) 2061(s) and 2001(s) cm-1
;
31P ) δ -2.1) which has been
crystallographically characterized.13d
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In summary, insertion reactions of phenyldiazonium and nitroso-
nium salts with several hydride and alkyl complexes have been
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