Synthesis and characterisation of the first examples of four-membered ring ruthenalactam complexes

Article abstract of DOI:10.1016/j.inoche.2014.04.002

Reaction of the complexes [RuCl₂(PPh₃)L] (L = η⁶-p-cymene or η⁶-hexamethylbenzene) with N-cyanoacetylurethane [NCCH₂C(O)NHCO₂Et] and tertiary amine base in refluxing methanol gives the first examples of mononuclear complexes containing the four-membered ruthenalactam ring, Ru-C-C(O)-N. The complexes were characterised by ¹H and ³¹P{¹H} NMR spectroscopy and ESI mass spectrometry. A single-crystal X-ray diffraction study on the p-cymene complex showed the four-membered ruthenalactam ring to be planar.

Full text of DOI:10.1016/j.inoche.2014.04.002

Inorganic Chemistry Communications 45 (2014) 51–54
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis and characterisation of the first examples of four-membered
ring ruthenalactam complexes
⁎
Ashwin Gopalan Nair, William Henderson , Brian K. Nicholson
Department of Chemistry, University of Waikato, Private Bag 3105, Hamilton 3240, New Zealand
a r t i c l e i n f o
a b s t r a c t
=
η6-p-cymene or η6-hexamethylbenzene) with N-
Article history:
Reaction of the complexes [RuCl2(PPh3)L] (L
Received 14 March 2014
Accepted 2 April 2014
Available online 13 April 2014
cyanoacetylurethane [NCCH2C(O)NHCO2Et] and tertiary amine base in refluxing methanol gives the first examples
of mononuclear complexes containing the four-membered ruthenalactam ring, Ru–C–C(O)–N. The complexes were
characterised by 1H and 31P{1H} NMR spectroscopy and ESI mass spectrometry. A single-crystal X-ray diffraction
study on the p-cymene complex showed the four-membered ruthenalactam ring to be planar.
© 2014 Elsevier B.V. All rights reserved.
Keywords:
Ruthenium
Metallalactam
Ruthenalactam
Electrospray ionisation mass spectrometry
Crystal structure
The four-membered metallalactam ring system 1 is well-known for
metal centres such as platinum(II) and palladium(II) [1–5] together
with gold(III) [6]. We have developed facile synthetic routes to such
complexes, involving the reaction of a metal dichloride complex with a
suitable ligand precursor, which are reacted in the presence of base, ei-
ther a tertiary amine in methanol, or silver(I) oxide in dichloromethane.
N-Cyanoacetylurethane, NCCH2C(O)NHCO2Et, is one such precursor that
has been used to synthesise metallalactam complexes of platinum(II) [1]
and palladium(II) [2], giving the substituted ring system 2. The presence
of relatively acidic C–H and N–H protons in N-cyanoacetylurethane al-
lows synthesis of such metallalactam complexes under mild reaction
conditions, with the platinum complex being formed through an inter-
mediate monodentate N-bonded ligand, as shown by the isolation and
characterisation of the complex cis-[PtCl{N(CO2Et)C(O)CH2CN}(PPh3)2]
3 [7]. In the case of gold(III) derivatives of N-cyanoacetylurethane, the
initial four-membered auralactam AuCH(CN)C(O)N(CO2Et) ring was
found to undergo an interesting dimerisation process, giving an eight-
membered Au{CH(CN)C(O)N(CO2Et)}2Au ring in complex 4 [6]. We
therefore wished to investigate the coordination chemistry of N-
cyanoacetylurethane towards other transition metal centres. However,
simple four-membered metallalactam ring systems formed by the
other platinum group metals (Rh, Ir, Ru, Os) have not been isolated to
date, though the related diruthenium complex 5 has been structurally
characterised [8], and some ruthenalactam species have been proposed
as intermediates in some ruthenium-catalysed cycloaddition reactions
of isocyanates [9,10]. In this paper the first isolated examples of
ruthenalactam complexes, containing π-arene ancillary ligands, are re-
ported; this metal–ligand system has been previously used to synthesise
a range of ruthenacyclic complexes [11].
Reaction of [RuCl2(PPh3)(η6-C6Me6)] [12] with N-cyanoacetylurethane
in refluxing methanol with triethylamine base, followed by precipita-
tion of the product by addition of water gave a yellow precipitate of
the ruthenalactam complex [Ru{CH(CN)C(O)N(CO2Et)}(PPh3)(η6-
C6Me6)] 6,[13] which was found to be spectroscopically pure and gave
satisfactory microanalytical data. However, reaction of the related com-
plex [RuCl2(PPh3)(η6-p-cymene)] [14] (p-cymene = p-MeC6H4Pri)
with N-cyanoacetylurethane was less straightforward, despite several
attempts using different reaction conditions. Refluxing of the reaction
mixture resulted in a green colour; recovery of the product by precipi-
tation with water, followed by recrystallisation from dichloromethane
and petroleum spirits gave a small number of yellow crystals of 7 to-
gether with some dark green material [15].
Complexes 6 and 7 give strong [M + H]+ ions in their positive ion
ESI mass spectra using gentle ionisation conditions (capillary exit volt-
age of 100 V) giving ions for 6 at m/z 681.291 (calculated 681.183)
and for 7 at m/z 653.039 (calculated m/z 653.151). The [M + Na]+
ions were also observed, but were weak. The fragmentation behaviour
of 6 was also investigated; at increased voltages (160 V), fragmentation
by loss of PPh3 occurred giving ions such as [M + Na-PPh3]+ (m/z
441.150, calculated 441.073). The ions showed the expected isotope
patterns that arise due to the presence of polyisotopic Ru. 31P{1H}
NMR spectra of the complexes showed a single peak at similar chemical
shifts (6 δ 49.2, 7 δ 49.8) that are shifted relative to the starting complex
[RuCl2(PPh3)(η6-p-cymene)] (δ 24.2). The IR spectrum of 6 showed the
expected bands, with a C`N stretch at 2350 cm−1 and two CO stretches
⁎
Corresponding author at: Department of Chemistry, University of Waikato, Private
Bag 3105, Hamilton 3240, New Zealand.
E-mail address: w.henderson@waikato.ac.nz (W. Henderson).
at 1701 and 1644 cm−1
.
http://dx.doi.org/10.1016/j.inoche.2014.04.002
1387-7003/© 2014 Elsevier B.V. All rights reserved.

52
A. Gopalan Nair et al. / Inorganic Chemistry Communications 45 (2014) 51–54
The 1H NMR spectrum of the hexamethylbenzene complex 6 was
relatively straightforward, showing a triplet and quartet from the
cyanoacetyl ethyl group, a singlet from the C6Me6 ligand (δ 1.89), a
set of multiplets for the PPh3 protons, together with a doublet at δ
2.12 for the ruthenalactam CH proton, showing coupling to phosphorus
[3J(PH) 8.7 Hz]. This resonance is significantly shifted upfield,
CO Et
2
R
N
CO Et
2
N
O
M
O
N
L M
O
Ph P
3
n
Pt
CH
2
Ph P
3
Cl
CN
CN
H
1
3
2, M=Pt, Pd
O
OMe
O
H
EtO C
2
CN
Me
N
N
2
N
Au
Au
Ru
Ru
CO
N
Me
2
N
CO
NC
CO Et
2
H
OMe
O
5
4
CO Et
2
CO Et
2
Ru
N
Ru
N
Ph P
3
Ph P
O
3
H
CN
O
H
CN
6
7
Ha
Hb
δ/ppm
4.00
3.95
3.90
3.85
Fig. 1. Part of the 1H NMR spectrum (CDCl3 solution) of [Ru{CH(CN)C(O)N(CO2Et)}(PPh3)(η6-p-cymene)] 7, showing the presence of two doublets of quartets for the inequivalent CH2
protons Ha and Hb of the ethyl ester substituent.

A. Gopalan Nair et al. / Inorganic Chemistry Communications 45 (2014) 51–54
53
relative to the CH2 protons in N-cyanoacetylurethane, which appear
at δ 4.06.
The 1H NMR spectrum of 7 was more complex. All four aromatic CH
protons from the p-cymene ligand are inequivalent. Each appears as a
doublet due to 3J(HH) coupling, with three of the signals showing addi-
tional resolved long-range coupling, presumably cross-ring 4J ‘W’-
coupling [16]. Two of the protons clearly form an AB type pattern, with
more intense inner lines. The methyl groups of the cymene iso-propyl
substituent are inequivalent and appear as two doublets at δ 1.08 and
1.23, with coupling to the iso-propyl CH proton. The protons of the ethyl
group also show a more complex pattern in 7 compared to 6. The CH3 pro-
tons appear as a triplet, but the CH2 protons (Fig. 1) are diastereotopic and
each appears as a resonance approximating to a doublet of quartets due to
2J(HH) geminal coupling, together with 3J(HH) coupling to the CH3 pro-
tons in an ABX3 spin system. The ruthenalactam proton appears at δ 2.2
as a doublet due to phosphorus coupling [3J(PH) 8.6 Hz], while the
CH(CH3)2 proton from the p-cymene ligand gives a quintet at δ 2.63.
In order to unequivocally characterise the ruthenalactam ring system,
an X-ray structure determination was carried out on the p-cymene com-
plex 7, which gave a small number of crystals suitable for study [17]. The
molecular structure is shown in Fig. 2 together with selected bond lengths
and angles. The complex contains the typical ‘piano-stool’ arrangement of
an η6 p-cymene ligand, the chelating N,C-bonded ligand (confirming the
formation of the ruthenalactam ring), and a triphenylphosphine. The
ruthenalactam ring system pays a strong resemblance to the correspond-
ing platinalactam ring in [Pt{CH(CN)C(O)N(CO2Et)}(cod)] [1], (cod =
cyclo-octa-1,5-diene) which is the only other structurally characterised
four-membered ring metallalactam derived from N-cyanoacetylurethane.
The Ru–C–C–N ring is highly planar, with a fold angle between the
N(1)–Ru(1)–C(1) and C(1)–C(2)–N(1) planes of only 1.84°. The torsion
angles Ru(1)–C(1)–C(2)–O(1) and Ru(1)–N(1)–C(2)–O(1) [both
178.8(2)°] corroborate this planarity. The N(1)–Ru(1)–C(1) bite angle of
the metallacyclic ligand is 64.49(7)°, which is comparable to the angle
of 67.0(3)° in the platinum complex [Pt{CH(CN)C(O)N(CO2Et)}(cod)]
[1]. The CO2 group of the ester substituent is also reasonably coplanar
Ru
Pt
Fig. 3. A comparison of the metallalactam rings of [Ru{CH(CN)C(O)N(CO2Et)}(PPh3)(η6-p-
cymene)] 7 (left) and [Pt{CH(CN)C(O)N(CO2Et)}(cod)] (right, cod = cyclo-octa-1,5-
diene), showing the different arrangements of the CO2Et substituent with respect to the
metallalactam ring.
with the ruthenalactam ring, with C(2)–N(1)–C(4)–O(2) and Ru(1)–
N(1)–C(4)–O(3) torsion angles of 169.6(2) and 172.1(1)° respectively.
The Ru–N and Ru–CH(CN) bond distances of 7 are in the expected
range, for example the Ru–C(1) bond distance of 2.1654(19) Å is identical
to that of 2.169(4) Å in the cyanoalkyl complex [Ru{CH(CN)SO2Ph}(η5-
C5H5)(CO)(PPh3)] [18]. The Ru–C and Ru–N bonds of 7 are larger by
0.0788 and 0.0964 Å respectively, when compared to [Pt{CH(CN)C(O)
N(CO2Et)}(cod)], consistent with the 0.1 Å greater covalent radius of ru-
thenium. The C_O bond distances of 7 [C(2)–O(1) 1.209(3) and C(4)–
O(2) 1.222(3) Å] are also comparable to the corresponding distances in
[Pt{CH(CN)C(O)N(CO2Et)}(cod)] [1.196(9) and 1.215(9) Å]. The cyano
group, C(3)–N(2), is directed towards the p-cymene ring, and the
ruthenalactam hydrogen points H(1) towards the PPh3 ligand [with a
H(1)⋯H(22) non-bonded distance of 2.4537(1) Å]. This arrangement pre-
sumably arises in order to minimise steric interactions between the CN
group and the PPh3 ligand.
The most significant difference between [Ru{CH(CN)C(O)N
(CO2Et)}(PPh3)(η6-p-cymene)] and [Pt{CH(CN)C(O)N(CO2Et)}(cod)]
involves the orientation of the ester substituent. In [Pt{CH(CN)C(O)
N(CO2Et)}(cod)] the OEt group is proximal to the platinum atom, where-
as in [Ru{CH(CN)C(O)N(CO2Et)}(PPh3)(η6-p-cymene)] it is distal, as
shown in the comparison in Fig. 3. This is presumably due to the greater
steric bulk of the p-cymene and PPh3 ligands in the Ru complex. The p-
cymene ligand bonds slightly asymmetrically to the ruthenium, as a re-
sult of the steric effects involving the PPh3 ligand; the Ru(1)–C(13) and
Ru(1)–C(14) bonds – those closest to the PPh3 ligand – are the two lon-
gest Ru–C(cymene) bond lengths.
C(12)
C(13)
C(11)
C(10)
C(17)
C(19)
C(18)
N(2)
C(14)
C(16)
Ru(1)
C(31)
C(3)
O(1)
C(1)
C(2)
N(1)
C(41)
P(1)
O(2)
This study indicates that metallalactam complexes of another plati-
num group metal can be readily synthesised by a simple one-pot proce-
dure; investigations into metallalactam complexes of other metals are in
progress.
C(4)
O(3)
C(21)
C(5)
Acknowledgments
C(6)
We thank the University of Waikato for financial support of this
work and Dr. Tania Groutso (University of Auckland) for collection of
the X-ray data set.
Fig. 2. Molecular structure of the complex [Ru{CH(CN)C(O)N(CO2Et)}(PPh3)(η6-p-
cymene)] 7. Only the ipso carbons of the triphenylphosphine ligand are shown for clarity.
Selected bond lengths (Å) and angles (°): Ru(1)–P(1) 2.3259(5), Ru(1)–N(1) 2.0948(17),
Ru(1)–C(1) 2.1654(19), N(1)–C(2) 1.377(3), C(1)–C(2) 1.539(3), C(2)–O(1) 1.209(3),
C(1)–C(3) 1.451(3), C(3)–N(2) 1.150(3), N(1)–C(4) 1.367(3), Ru–C(cymene) range
2.217(2) to 2.277(2), mean 2.244(2), C(1)–Ru(1)–N(1) 64.49(7), Ru(1)–N(1)–C(2)
100.62(12), N(1)–C(2)–C(1) 102.36(16), Ru(1)–C(1)–C(2) 92.50(12), C(1)–C(3)–N(2)
177.5(2), P(1)–Ru(1)–N(1) 89.02(5), P(1)–Ru(1)–C(1) 85.65(6).
Appendix A. Supplementary material
CCDC 976610 contains the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The Cambridge

54
A. Gopalan Nair et al. / Inorganic Chemistry Communications 45 (2014) 51–54
After 5 min. aqueous trimethylamine solution (1 mL) was added and the mixture
refluxed for ca. 1 hour. Water (100 mL) was added, giving a murky green solution.
The mixture was evaporated to dryness on a rotary evaporator. The solid was
redissolved in dichloromethane (3 mL) and petroleum spirits (30 mL) added.
Slow evaporation of the solution gave a mixture of green solid on the bottom
of the vessel, and a small number of yellow crystals on the side. 31P{1H}
NMR (CDCl3), δ 49.8 (s). 1H NMR (CDCl3), δ 7.55–7.40 (m, 15H, PPh3), 5.90 [dd,
1H, CH of p-cymene, 3 J(HH) 6.3, 4 J(HH) 1.4], 5.48 [dd, 1H, CH of p-cymene, 3 J(HH)
5.8, 4 J(HH) 1.0], 5.38 [dd, 1H, CH of p-cymene, 3 J(HH) 5.8, 4 J(HH) 1.1], 4.51 [d, 1H,
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Supplementary data associated with this article can be found, in the on-
line version, at http://dx.doi.org/10.1016/j.inoche.2014.04.002.
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CH of p-cymene, 3 J(HH) 6.2], 3.96 [dq, 1H, CH2 J(HH) 10.7, 3 J(HH) 7.1], 3.87 [dq,
2
1H, CH2, J(HH) 10.6, 3 J(HH) 7.1], 2.62 [quintet, 1H, CH(CH3), 3 J(HH) 6.9], 2.20
2
3
[d, 1H, CHCN, J(PH) 8.6], 1.70 (s, 3H, C6H4CH3), 1.22 [d, 3H, CH(CH3) of p-
cymene, 3 J(HH) 7.0], 1.17 [t, 3H, CH2CH3, 3 J(HH) 7.1] and 1.07 [d, 3H, CH(CH3) of
p-cymene, 3 J(HH) 6.8]. ESI MS (MeOH), m/z 653.265 [M + H]+
.
[16] W. Kemp, NMR in Chemistry: A Multinuclear Introduction, Macmillan Education
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[17] Crystal data for 6: C34H35N2O3PRu; Mr = 651.68; Monoclinic; space group C2/c; a =36.
0940(11) Å; b = 10.1847(3) Å; c = 16.3123(5) Å; β = 100.033(2)°; V = 5904.8(3)
Å3; Z = 8; T = 89(2) K; λ(Mo-Kα) = 0.71073 Å; μ(Mo-Kα) = 0.623 mm−1; dcalc = 1.
466 g cm−3; 104731 reflections collected; 7086 unique (Rint = 0.0455); giving R1 = 0.
0352, wR2 = 0.0906 for data with [I N 2σ(I)] and R1 = 0.0380, wR2 = 0.0924 for all
data. Residual electron density (e−Å−3) max/min: 2.893/−0.605. An arbitrary sphere
of data were collected on a dark yellow block-like crystal, having approximate dimen-
sions of 0.38 × 0.34 × 0.24 mm, on a Bruker APEX II CCD diffractometer using a com-
bination of ω- and φ-scans of 0.5°. Data were corrected for absorption and polarisation
effects [SADABS; R. H. Blessing, Acta Crystallogr. Sect. A 51 (1995) 33] and analysed for
space group determination. The structure was solved by direct methods and devel-
oped routinely. The model was refined by full-matrix least-squares analysis of F2
against all reflections. All non-hydrogen atoms were assigned anisotropic tempera-
ture factors and H atoms were included in calculated positions. On completion of re-
finement there were two significant residual peaks. One (2.7 e Å−3) appeared to
arise from orientational disorder of the iso-propyl group but was not modelled. The
other (2.9 e Å−3) was adjacent to the mid-point of the Ru–P vector, and cannot be
assigned to any chemically-sensible feature, so must be an artifact of unknown origin.
Thermal parameters for the hydrogens were tied to the isotropic thermal parameter of
the atom to which they are bonded (1.5 × for methyl, 1.2 × for all others) [G. M.
Sheldrick, Acta Crystallogr. A64 (2008) 112].
[12] M.A. Bennett, T.-N. Huang, T.W. Matheson, A.K. Smith, Inorg. Synth. 21 (1982) 74.
[13] Synthesis of [Ru{CH(CN)C(O)N(CO2Et)}(PPh3)(η6-C6Me6)] 6: [RuCl2(PPh3)(η6-
C6Me6)] (50 mg, 0.084 mmol) and N-cyanoacetylurethane (33 mg, 0.211 mmol,
Aldrich Chemical Co.) in methanol (15 mL) was heated to reflux. After 5 min. excess
Et3N was added and the mixture refluxed for an additional 25 min. giving a yellow so-
lution. Addition of water (80 mL) gave a yellow precipitate that was allowed to stand
overnight before being filtered, washed with water (25 mL) and petroleum spirits
(20 mL) and dried under vacuum to give 6 (25 mg, 44%). Found: C 62.7; H 5.9; N 3.9.
C36H39N2O3PRu requires C 63.6; H 5.8; N 4.1%. 31P{1H} NMR (CDCl3), δ 49.2 (s). 1
H
NMR (CDCl3), δ 7.4–7.6 (m, 15H, PPh3), 4.14 [q, 2H, CH2CH3, 3 J(HH) 7.1], 2.12 [d, 1H,
3
3
CHCN, J(PH) 8.7], 1.88 (s, 18H, C6Me6), 1.25 [t, CH2CH3, J(HH) 7.1]. ESI MS
(MeOH), m/z 441.150 [M + Na-PPh3]+, 681.295 [M + H]+, 703.277 [M + Na]+. IR
(KBr disk): ν(CN) 2350 cm−1, ν(CO) 1701, 1644 cm−1
.
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p-cymene)] (61 mg, 0.106 mmol) and N-cyanoacetylurethane (81 mg, 0.
519 mmol) were mixed in methanol (22 mL) and the mixture warmed to reflux.
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