Acylated cyanoimido-complexes trans-[Mo(NCN){NCNC(O)R}(dppe)2]Cl and their reactions with electrophiles: Chemical, electrochemical and theoretical study

Article abstract of DOI:10.1039/c2dt30867c

Treatment of a dichloromethane solution of trans-[Mo(NCN){NCNC(O)R}(dppe) ₂]Cl [R = Me (1a), Et (1b)] (dppe = Ph₂PCH ₂CH₂PPh₂) with HBF₄, [Et ₃O][BF₄] or EtC(O)Cl gives trans-[Mo(NCN)Cl(dppe) ₂]X [X = BF₄ (2a) or Cl (2b)] and the corresponding acylcyanamides NCN(R′)C(O)Et (R′ = H, Et or C(O)Et). X-ray diffraction analysis of 2a (X = BF₄) reveals a multiple-bond coordination of the cyanoimide ligand. Compounds 1 convert to the bis(cyanoimide) trans-[Mo(NCN)₂(dppe)₂] complex upon reaction with an excess of NaOMe (with formation of the respective ester). In an aprotic medium and at a Pt electrode, compounds 1 (R = Me, Et or Ph) undergo a cathodically induced isomerization. Full quantitative kinetic analysis of the voltammetric behaviour is presented and allows the determination of the first-order rate constants and the equilibrium constant of the trans to cis isomerization reaction. The mechanisms of electrophilic addition (protonation) to complexes 1 and the precursor trans-[Mo(NCN)₂(dppe)₂], as well as the electronic structures, nature of the coordination bonds and electrochemical behaviour of these species are investigated in detail by theoretical methods which indicate that the most probable sites of the proton attack are the oxygen atom of the acyl group and the terminal nitrogen atom, respectively.

Full text of DOI:10.1039/c2dt30867c

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PAPER
Acylated cyanoimido-complexes trans-[Mo(NCN){NCNC(O)R}(dppe)2]Cl and
their reactions with electrophiles: chemical, electrochemical and theoretical
study†
Elisabete C. B. A. Alegria,a,b M. Fátima C. Guedes da Silva,a,c Maxim L. Kuznetsov,a
Sónia M. P. R. M. Cunha,a Luísa M. D. R. S. Martinsa,b and Armando J. L. Pombeiro*a
Received 20th April 2012, Accepted 23rd May 2012
DOI: 10.1039/c2dt30867c
Treatment of a dichloromethane solution of trans-[Mo(NCN){NCNC(O)R}(dppe)2]Cl [R = Me (1a),
Et (1b)] (dppe = Ph2PCH2CH2PPh2) with HBF4, [Et3O][BF4] or EtC(O)Cl gives trans-[Mo(NCN)Cl-
(dppe)2]X [X = BF4 (2a) or Cl (2b)] and the corresponding acylcyanamides NCN(R′)C(O)Et (R′ = H, Et
or C(O)Et). X-ray diffraction analysis of 2a (X = BF4) reveals a multiple-bond coordination of the
cyanoimide ligand. Compounds 1 convert to the bis(cyanoimide) trans-[Mo(NCN)2(dppe)2] complex
upon reaction with an excess of NaOMe (with formation of the respective ester). In an aprotic medium
and at a Pt electrode, compounds 1 (R = Me, Et or Ph) undergo a cathodically induced isomerization.
Full quantitative kinetic analysis of the voltammetric behaviour is presented and allows the determination
of the first-order rate constants and the equilibrium constant of the trans to cis isomerization reaction.
The mechanisms of electrophilic addition (protonation) to complexes 1 and the precursor trans-
[Mo(NCN)2(dppe)2], as well as the electronic structures, nature of the coordination bonds and
electrochemical behaviour of these species are investigated in detail by theoretical methods which
indicate that the most probable sites of the proton attack are the oxygen atom of the acyl group and
the terminal nitrogen atom, respectively.
number of complexes of cyanamide, diorgano derivatives
Introduction
(NuCNR2) or cyanoguanidine, i.e., dimeric form, NCNC
(NH2)2, with Mo,1–4 Re,27–29 Fe30 or Pt.31–33
Cyanamide (NuC–NH2) has been little explored in coordination
chemistry,1–14 in spite of the interest of this species as a substrate
Further reactions of the organocyanamide ligands have been
of both Mo- and V-nitrogenases15 and of cyanamide hydratase,16
reported only scantly and they include insertion into a metal–
a reagent in chemical synthesis,17 a fertiliser,18 a drug for treat-
ment of alcoholism,19 an amino acid precursor, a prebiotic20
and an interstellar space21 compound. Organocyanamides, e.g.
phenylcyanamides,22 have also been little applied as ligands.
In pursuit of our interest on the activation of small unsaturated
N-molecules with biological, environmental or synthetic
significance,23–26 we have already achieved the syntheses of a
carbon multiple bond,34 C–C coupling with a μ-C4Me4 ligand
derived from MeCuCMe,35 metathesis with a metal–metal
triple bond,36 nucleophilic addition of an alcohol (to
cyanoguanidine)32 or of an oxime,31 nucleophilic attack by an
adjacent hydroxide ligand,36 protonation at the unsaturated
C atom to form amidoazavinylidene species,3 and deprotonation
by a base to form the hydrogen-cyanamide (NCNH−) ligand.28
Treatment of the bis(dinitrogen) complex trans-[Mo(N2)2-
(dppe)2] (dppe = Ph2PCH2CH2PPh2) with cyanamide affords the
cyanoimide complex trans-[Mo(NCN)2(dppe)2],2,37 the electron-
rich molybdenum(0) centre {Mo(dppe)2} behaving as a dehydro-
genating and reducing agent of the cyanamide. The cyanoimide
ligand has been also obtained by different synthetic pathways,
such as NCN abstraction from the bicyclic cyanoamine 2,3:5,6-
dibenzo-7-azabicyclo[2.1.1]hepta-2,5-diene (NCdbabh),38 salt
metathesis reactions of dipotassium cyanamide, K2(NCN),39 or
cyanation of nitride complexes.40
aCentro de Química Estrutural, Complexo I, Instituto Superior Técnico,
Technical University of Lisbon, Av. Rovisco Pais, 1049-001 Lisboa,
Portugal. E-mail: pombeiro@ist.utl.pt
bÁrea Departamental de Engenharia Química, Instituto Superior de
Engenharia de Lisboa, Rua Conselheiro Emídio Navarro, 1059-007
Lisboa, Portugal
cUniversidade Lusófona de Humanidades e Tecnologias, ULHT Lisbon,
Campo Grande 376, 1749-024 Lisboa, Portugal
†Electronic supplementary information (ESI) available: Discussion of
the X-ray structure of 2a, electronic structure of NCN2−, electrostatic
potential distribution and electrochemical behaviour of trans-1a+′,
Tables S1–S4 with X-ray data, absolute and relative energies, HSAB
properties and atomic coordinates. CCDC 862356. For ESI and crys-
tallographic data in CIF or other electronic format see DOI:
10.1039/c2dt30867c
The introduction of the cyanoimide ligand (NCN2−) into a
metal coordination sphere was achieved via Na(NCNH)41 to give
the di- and tetrairidium complexes [Cp*Ir(μ2-NCN-N,N)]2,
[Cp*Ir(μ3-NCN-N,N,N)]4 and [Cp*Ir(μ3-NCN-N,N,N)]4 (Cp* =
13876 | Dalton Trans., 2012, 41, 13876–13890
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η5-C5Me5), or via Na2NCN to afford the anionic bis(cyan-
amido)-capped triruthenium complex [(Cp*Ru)3(μ3-NCN)2]−.42
The ligated cyanoimido can behave as an oxido analogue, as
PPh3 or alkenes add to it in OsIV complexes to afford cyanoimino-
phosphorane and cyanoaziridine complexes, respectively.40 It can
also act as a Lewis base towards Lewis acid metal species, thus
behaving as a linker of transition-metal units in μ-NCN com-
plexes.1,38,39 For example, heterometallic complexes of the types
trans-[Mo(NCN)(dppe)2(μ-NCN)M] and [Mo(dppe)2{(μ-NCN)M}2]
(M = VIII, WIV, ReV, ReI, FeII or PtII centre) were prepared by
reactions of trans-[Mo(NCN)2(dppe)2] with the corresponding
transition-metal Lewis acid (M) precursors.1
At trans-[Mo(NCN)2(dppe)2], the cyanoimido is also suscep-
tible to electrophilic attack by various electrophiles E+ (proton
or organocation) to give the corresponding mononuclear adducts
trans-[Mo(NCN)(NCNE)(dppe)2]+ which, in a few cases, can be
further protonated to give the cyanamide trans-[Mo(NCN)-
(NCNHE)(dppe)2]2+ products.2
We now extend further the study of the nucleophilicity of
the acylcyanoimide complexes trans-[Mo(NCN){NCNC(O)R}-
(dppe)2]Cl (R = Me, Et) towards various electrophiles (E+),
namely HBF4, [Et3O][BF4] or EtC(O)Cl, leading, upon a second
electrophilic attack, to the formation of acylcyanamides. More-
over, these complexes display interesting electrochemical
behaviour, which is also studied theoretically, appearing to
provide an example of an ET-induced isomerisation which is
also discussed. Theoretical methods are also applied to the estab-
lishment of the mechanisms of the electrophilic additions, as
well as the electronic structures and nature of the coordination
bonds.
Xe atoms. Nominal molecular masses were calculated using the
most abundant isotopes, i.e. 98Mo (24.13%), and the expected
natural abundance isotope cluster patterns were observed for the
various ion clusters. However, further complexity due to addition
(from the matrix) or loss of hydrogen was usually not taken into
account. Mass calibration for data system acquisition was
achieved using CsI. The calculation of the theoretical isotopic
patterns was performed by using a computer program taking into
account the natural abundance of the various isotopes. The C, H,
N elemental analyses were carried out by the Microanalytical
Service of the Instituto Superior Técnico.
The electrochemical experiments were performed on an
EG&G PAR 273A potentiostat/galvanostat connected to a
personal computer through a GPIB interface.
Cyclic voltammograms were obtained in 0.2 M [nBu4N][BF4]
in CH2Cl2, at a platinum disc working electrode (d = 0.5 mm)
whose potential was controlled vs. a Luggin capillary connected
to a silver wire pseudo-reference electrode; a Pt auxiliary elec-
trode was employed.
Controlled-potential electrolyses (CPE) were carried out in
electrolyte solutions with the above-mentioned composition, in a
three-electrode H-type cell. The compartments were separated by
a sintered glass frit and equipped with platinum gauze working
and counter electrodes. A Luggin capillary connected to a silver
wire pseudo-reference electrode was used to control the working
electrode potential. The CPE experiments were monitored regu-
larly by cyclic voltammetry (CV), thus ensuring no significant
potential drift occurred during the electrolyses. The redox poten-
tials of the complexes were measured by CV (see above) in the
presence of ferrocene as the internal standard, and the redox
potential values are quoted relative to the SCE by using the
ox
[Fe(η5-C5H5)2]0/+ (E1/2 = 0.55 V vs. SCE)43 redox couple in
0.2 M [nBu4N][BF4]/CH2Cl2. The use, as reference, of an elec-
trode in aqueous medium was avoided due to the sensitivity of
the complexes to water.
Experimental section
Materials and physical measurements
All manipulations and reactions were performed under an atmo-
sphere of dinitrogen using standard vacuum and inert-gas flow
techniques. Solvents were purified by standard procedures and
freshly distilled immediately prior to use. The complexes trans-
[Mo(NCN)2(dppe)2]2,37 and trans-[Mo(NCN){NCNC(O)R}-
(dppe)2]Cl (R = Et 1b or Ph 1c)2 were prepared by literature
methods. The cyanamides were used as purchased from Aldrich.
Infrared spectra (4000–400 cm−1) were recorded on a Perkin-
Elmer 683 spectrophotometer or on a Bio-Rad FTS 3000MX
instrument in KBr pellets; extension to the far infrared region
(400–200 cm−1) was achieved on a Vertex 70 spectrophotometer,
in polyethylene pellets; abbreviations: vs = very strong, s =
strong, m = medium, w = weak, br = broad. 1H, 31P and
13C NMR spectra were recorded on a Varian Unity 300 spectro-
meter at ambient temperature; δ values are in ppm relative to
SiMe4 (1H or 13C) or H3PO4 (31P). In the 13C NMR data, assign-
ments and coupling constants common to the 13C–{1H} NMR
spectra are not repeated. Coupling constants are in Hz; abbrevia-
tions: s = singlet, d = doublet, t = triplet, qt = quartet, qnt =
quintet, m = complex multiplet, br = broad, dd = doublet of
doublets, dt = doublet of triplets, tm = triplet of complex multi-
plets. Positive-ion FAB mass spectra were obtained on a Trio
2000 spectrometer by bombarding 3-nitrobenzyl alcohol (NBA)
matrices of the samples with 8 keV (ca. 1.28 × 1015 J)
The mechanism of the electroinduced isomeric interconver-
sion was investigated by digital simulation (program ESP)44 of
the voltammograms at different scan rates (in the 0.1–60 V s−1
range). The E0 values were taken as the average of Epa and Epc
for both isomers; values for khet (khet1 = 0.025; khet2 = 0.005)
and α (α = 0.5) for both electron transfers were chosen to allow
a close correspondence between simulated and experimental
voltammograms.
Computational details
The full geometry optimization of all structures has been carried
out at the DFT/HF hybrid level of theory using Becke’s three-
parameter hybrid exchange functional in combination with the
gradient-corrected correlation functional of Lee, Yang and Parr
(B3LYP)45 with the help of the Gaussian-9846 program package.
The restricted approximations for the structures with closed elec-
tron shells and the unrestricted methods for the structures with
open electron shells have been employed. Symmetry operations
were not applied for all structures. The geometry optimization
was carried out using a quasi-relativistic Stuttgart pseudopoten-
tial that described 28 core electrons and the appropriate con-
tracted basis set (8s7p6d)/[6s5p3d]47 for the molybdenum atom
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 13876–13890 | 13877

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and the 6-31G(d) basis set for other atoms. Then, single-point
calculations were performed on the basis of the found equili-
brium geometries using the 6-311+G(d,p) basis set for non-metal
atoms. The Hessian matrix was calculated analytically for the
optimized structures at the B3LYP/6-31G(d) level in order to
prove the location of correct minima (no imaginary frequencies),
and to estimate the thermodynamic parameters, the latter being
calculated at 25 °C.
calculated as52
S ꢂ 1=ðI ꢀ AÞ
where I and A are the vertical ionization potential and electron
affinity, respectively. Fukui functions of the atom x in a molecule
with N electrons, fx and fx (the former for nucleophilic attack
and the latter for electrophilic attack), were defined in a finite-
difference approximation using the equations53
+
−
The structure of the complex trans-[Mo(NCN){NCNC(O)Me}-
(dpe)2]Cl (dpe = PH2CH2CH2PH2) with a chloride counterion
was constructed on the basis of the equilibrium geometry of the
cationic species trans-[Mo(NCN){NCNC(O)Me}(dpe)2]+. Then
Cl− was added in the plane formed by the Mo and P atoms (see
ESI† for atomic coordinates) without the following geometry
optimization.
Total energies corrected for solvent effects (Es) were estimated
at the single-point calculations on the basis of gas-phase geome-
tries at the CPCM-B3LYP/6-311+G(d,p)//gas-B3LYP/6-31G(d)
level of theory using the polarizable continuum model in the
CPCM version48 with CH2Cl2 as solvent. The entropic term
in CH2Cl2 solution (Ss) was calculated according to the pro-
cedure described by Wertz49 and Cooper and Ziegler50 using
eqn (1)–(4):
þ
f x ¼ ½qxðN þ 1Þ ꢀ qxðNÞꢁ
ꢀ
f x ¼ ½qxðNÞ ꢀ qxðN ꢀ 1Þꢁ
where qx is the NBO electronic population of atom x in a mole-
cule. The local atomic softnesses sx+ and sx− were calculated as
þ
ꢀ
sx ¼ f xþS and sx ¼ f xꢀS
Syntheses
[Mo(NCN){NCNC(O)Me}(dppe)2]Cl (1a). To a suspension of
trans-[Mo(NCN)2(dppe)2] (0.131 g, 0.135 mmol) in CH2Cl2
(40 mL) was added a 1 : 11 CH2Cl2 diluted solution of acetyl
chloride MeC(O)Cl (0.10 mL, 0.135 mmol). The mixture was
stirred, at room temperature, during ca. 24 h and then the suspen-
sion was filtered through celite. Concentration in vacuo followed
by addition of Et2O led to the precipitation of 1a as a green solid
compound which was filtered off, washed with Et2O and dried
in vacuo (0.072 g, 51% yield). 1a is soluble in chloroform
and dichloromethane. IR (KBr pellets): 2145 (sh) and 2116
[s, νas(NvCvN)], 1620 [s, ν(CvO)], 1280 [s, sharp,
νs(NvCvN) and/or ν(MoN)], 285 [s, ν(Mo–Cl)]. NMR:
ΔS1 ¼ RlnVs =Vm;gas
ð1Þ
ð2Þ
m;liq
ΔS2 ¼ R ln Vom=Vs
m;liq
So;sliq ꢀ ðSo;sgas þ R ln Vsm;liq=Vm;gas
Þ
α ¼
ð3Þ
ðSo;sgas þ R ln Vsm;liq=Vm;gas
Þ
1H (CD2Cl2), δ 7.50 [t, 4H, JHH = 7.4, Hp (dppe)], 7.42 [t, 4H,
3
Ss ¼ Sg þ ΔSsol ¼ Sg þ ½ΔS1 þ αðSg þ ΔS1Þ þ ΔS2ꢁ
3JHH = 7.4, Hp′ (dppe)], 7.32 [t, 8H, JHH = 7.5, Hm (dppe)],
3
¼ Sg þ ½ðꢀ11:8 cal molꢀ1 Kꢀ1
Þ
3
7.18 [t, 8H, JHH = 7.5, Hm′ (dppe)], 7.09 [m, 16H, Ho + Ho′
ꢀ 0:21ðSg ꢀ 11:8 cal molꢀ1 Kꢀ1Þ þ 5:45 cal molꢀ1 Kꢀ1
ꢁ
(dppe)], 3.05 [m, 4H, CH2 (dppe)], 2.98 [m, 4H, CH2 (dppe)],
2.03 [s, 3H, NCNC(O)CH3]. 31P–{1H}(CD2Cl2), δ 37.4 s.
13C–{1H}(CD2Cl2), δ 172.69 [s, NCNC(O)CH3], 132.92 [qnt,
virtual JCP = 2.6, Co (dppe)], 132.21 [qnt, virtual JCP = 2.8, Co′
(dppe)], 131.60 [s, Cp (dppe)], 131.45 [qnt, virtual JCP = 11.6,
ð4Þ
where Sg = gas-phase entropy of solute, ΔSsol = solvation
entropy, So,sliq, So,sgas, and Vsm,liq = standard entropies and molar
volume of the solvent in liquid or gas phases (174.5 and
270.28 J mol−1 K−1 and 64.15 mL mol−1, respectively, for
CH2Cl2), Vm,gas = molar volume of the ideal gas at 25 °C
(24 450 mL mol−1), and Vom = molar volume of the solution cor-
responding to the standard conditions (1000 mL mol−1). The
enthalpies and Gibbs free energies in solution (Hs and Gs) were
estimated using the expressions (5) and (6):
Ci (dppe)], 131.27 [s, Cp′ (dppe)], 130.36 [qnt, virtual JCP
=
11.6, Ci′ (dppe)], 129.30 [qnt, virtual JCP = 2.3, Cm (dppe)],
128.80 [qnt, virtual JCP = 2.4, Cm′ (dppe)], 128.54 [s, NCN],
121.45 [s, NCNC(O)CH3], 26.97 [qnt, virtual JCP 9.4,
CH2 (dppe)], 22.3 [s, NCNC(O)CH3]. 13C (CD2Cl2) δ 172.69,
132.92 [dm, JCH = 161.7], 132.21 [dm, JCH = 161.3],
2
131.60 (dt, JCH = 163.7, JCH = 7.5), 131.27 (dt, JCH = 162.6,
2JCH = 7.5), 129.30 (dm, JCH = 169.3), 128.80 (dm, JCH 161.9),
128.54, 121.45, 26.97 (tm, JCH = 132.1), 22.30 (q, JCH = 135.8).
FAB+-MS: m/z 1017 ([M]+), 969 ([M − NCNC(O)Me + Cl]+).
FAB−-MS m/z 83 [NCNC(O)Me−], 35 (counterion Cl−).
MoC56H51N4P4OCl·3/2CH2Cl2 requires C, 58.6; H, 4.6;
N, 4.8%. Found: C, 59.0; H, 5.0; N, 4.1%.
Hs ¼ Esð6-311 þ Gðd; pÞÞ þ Hgð6-31GðdÞÞ
ð5Þ
ꢀ Egð6-31GðdÞÞ
Gs ¼ Hs ꢀ TSs
ð6Þ
where Es, Eg and Hg are the total energies in solution and in gas
phase and gas-phase enthalpy calculated at the corresponding
level.
−
trans-[MoCl(NCN)(dppe)2]X [X = BF4 (2a), Cl− (2b)]. A
The natural bond orbital (NBO) partitioning scheme51 was
used for the analysis of the bonding nature in some compounds
and for calculations of the atomic charges and Wiberg bond
indices. For the HSAB consideration, the global softness (S) was
dark green solution of trans-[Mo(NCN){NCNC(O)Et}(dppe)2]-
Cl 1b (0.067 g, 0.063 mmol) in CH2Cl2 (20 mL) was treated,
slowly and dropwise, with a 1 : 11 Et2O diluted solution of 54%
[Et2OH][BF4] (0.19 mL, 0.126 mmol). After ca. 5 h the solution
13878 | Dalton Trans., 2012, 41, 13876–13890
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was concentrated in vacuo, whereupon Et2O was added leading
to a light green solid (2a) that was separated by filtration,
washed with Et2O and dried in vacuo (0.048 g, 71% yield).
Compound 2a can also be obtained (in a similar way and with a
comparable yield – 76%) by treatment of a CH2Cl2 (20 mL)
solution of 1b (0.063 g, 0.059 mmol) with [Et3O][BF4]
(0.013 g, 0.118 mmol).
methyl propionate ester EtCOOMe (white precipitate isolated
after concentration of the solution) and to complex 0 (by concen-
tration of the resulting solution). IR (KBr pellets): 1738
[s, ν(CvO)]. NMR (298 K): 1H (CD2Cl2), δ 3.39 [s, 3H,
Et2COOCH3], 2.06 [qt, 4H, (CH3CH2)2COOMe], 1.24 [t, 6H,
(CH3CH2)2COOMe].
Compound 2b was prepared by treatment of a CH2Cl2
(20 mL) solution of 1b (0.090 g, 0.085 mmol) with a 1 : 11
CH2Cl2 diluted solution of acetyl chloride EtC(O)Cl (0.081 mL,
0.085 mmol), stirring for ca. 24 h and precipitation with Et2O
(0.057 g, 67% yield). 2a and 2b are soluble in chloroform and
dichloromethane.
Results and discussion
Synthesis and characterization of trans-[Mo(NCN){NCNC(O)Me}-
(dppe)2]Cl (1a) and trans-[Mo(NCN)Cl(dppe)2]X [X = BF4 (2a),
Cl (2b)]
2a: IR (KBr pellets): 2118 [s, sharp, νas(NvCvN)], 1283
[s, sharp, νs(NvCvN) and/or ν(MoN)], 1100–1000 [vs, br,
ν(BF)]. NMR (298 K): H (CD2Cl2), δ 7.49 [t, 4H, JHH = 7.4,
Hp (dppe)], 7.41 [t, 4H, JHH = 7.4, H′p (dppe)], 7.31 [t, 8H,
JHH = 7.5, Hm (dppe)], 7.18 [t, 8H, JHH = 7.5, H′m (dppe)],
7.08 [m, 16H, Ho and H′o (dppe)], 3.06 [m, 4H, CH2 (dppe)],
2.96 [m, 4H, CH2 (dppe)]. 31P–{1H}(CD2Cl2), δ 36.9 s. 13C–{1H}-
(CD2Cl2) δ 133.33 [qnt, JCP = 2.6, Co (dppe)], 132.61 [qnt,
JCP = 2.8, Co′ (dppe)], 132.06 [s, Cp (dppe)], 131.71 [ s, Co′
trans-[Mo(NCN){NCNC(O)Me}(dppe)2]Cl 1a (dppe = Ph2PCH2-
CH2PPh2) is obtained (Scheme 1, reaction 1), as a green solid,
via a similar synthetic route to that of the related acylcyanoimide
compounds trans-[Mo(NCN){NCNC(O)R}(dppe)2]+ [R = Et (1b),
Ph (1c)],2 upon electrophilic attack by RC(O)Cl (R = Me for 1a)
at a cyanoimide ligand in trans-[Mo(NCN)2(dppe)2] (0), added
(in a dichloromethane solution and in a stoichiometric amount)
to a dichloromethane suspension of the starting complex.
Complex 0 was prepared by the known37 reaction of cyanamide
(NCNH2) with the bis(dinitrogen) complex trans-[Mo(N2)2-
(dppe)2] (Scheme 1, reaction 0).
Treatment of a dichloromethane solution of [Mo(NCN)-
{NCNC(O)Et}(dppe)2]Cl 1b with the electrophiles HBF4,
[Et3O][BF4] or EtC(O)Cl gives the chloro-cyanoimido com-
pound trans-[Mo(NCN)Cl(dppe)2]X [X = BF4 (2a) or Cl (2b)],
isolated in good yields, as a pale green product, and the corre-
sponding acylcyanamide NCN(R′)C(O)Et [R′ = H, Et, C(O)Et]
(Scheme 1, reactions 2–4). Moreover, 1b reacts with an excess
of NaOMe in methanol to fully regenerate trans-[Mo(NCN)2-
(dppe)2] (0), with formation of methyl propionate ester
EtCOOMe (Scheme 1, reaction 5). The complexes were charac-
terized by elemental analysis, IR and NMR spectroscopies,
FAB-MS spectrometry, electrochemistry (CV and CPE) and, in
the case of 2a, also by X-ray diffraction analysis, although the
crystals were of very poor quality. The table with crystal data
and structure refinement details as well as an illustrative Ortep
diagram for 2a are shown in a ESI file (Table S1 and Fig. S1†).
The IR spectra (KBr pellets) of 1a, 2a and 2b exhibit strong
bands at 2116–2118 cm−1, assigned to νas(NvCvN) of the
axial cyanoimide ligand (NCN2−), which is shifted to higher
wavenumbers relative to that found in (0) (νas(NvCvN) at
2043 cm−1);2 the νs(NvCvN) and/or ν(MoN) bands are
observed at ca. 1280 cm−1. Moreover, 1a exhibits a strong CvO
band at 1620 cm−1. For 2a and 2b, the medium intensity ν(Mo–
Cl) band at 285 cm−1 confirms the presence of the chloride
ligand; for the former, the broad and intense band at
1100–1000 cm−1 is typical of the BF4− counterion.
1
(dppe)], 129.79 [qnt, JCP = 2.3, Cm (dppe)], 129.28 [qnt, JCP
=
2.5, Cm′ (dppe)], 128.31 (s, NCN), 27.30 [qnt, JCP 9.6, CH2
(dppe)]. 13C (CD2Cl2) δ 133.33 (dm, JCH = 159.9), 132.61 (dm,
2
JCH = 161.2), 132.06 (dt, JCH = 162.3, JCH = 7.2), 131.71 (dt,
2
JCH = 162.1, JCH = 7.2), 129.79 (dm, JCH = 164.8), 129.28
(dm, JCH = 163.8), 128.31, 27.30 (tm, JCH = 132.1). 2b: FAB+-
MS: m/z 969 ([M]+), 570 ([M − dppe ]+). FAB−-MS m/z 35
(counterion Cl−). MoC53H48N2P4Cl2·2CH2Cl2 requires C, 56.3;
H, 4.4; N, 2.4%. Found: C, 56.1; H, 4.4; N, 2.4%.
NCN(R′)C(O)R (R = Me, Et, R′ = H, Et, COEt). Treatment of
a CH2Cl2 solution of 1b with a 1 : 11 Et2O diluted solution of
54% [Et2OH][BF4] led to 2a and the corresponding acylcyan-
amide NCN(H)C(O)Et (0.004 g, 54%), obtained by concen-
tration of the resulting solution after isolation of 2a. NCN(H)-
C(O)Et: IR (KBr pellets): 2252 [s, ν(NuC)], 1690 [s, ν(CvO)].
1H NMR (298 K, CD2Cl2): δ 2.10 [qt, 2H, JHH = 7.2, NCN(H)-
C(O)CH2CH3], 1.15 [t, 3H, JHH = 7.5, NCN(H)C(O)CH2CH3].
FAB−-MS: m/z 97 (NCNC(O)Et−). Moreover, treatment of a
CH2Cl2 solution of 1b with [Et3O][BF4] led to 2a and NCN(Et)-
C(O)Et (0.005 g, 65%), obtained by concentration of the result-
ing solution after isolation of 2a. NCN(Et)C(O)Et: IR (KBr
pellets): 2235 [s, ν(NuC)], 1671 [s, ν(CvO)]. 1H NMR
(298 K, CD2Cl2): δ 4.20 [qt, 2H, JHH = 7.4, NCN(CH2CH3)-
C(O)Et], 2.32 [t, 3H, JHH = 7.4, NCN(CH2CH3)C(O)Et], 2.12
[qt, 2H, JHH = 7.2, NCN(Et)C(O)CH2CH3], 1.21 [t, 3H, JHH
7.5, NCN(Et)C(O)CH2CH3].
=
NCN{C(O)Et}2. This acylcyanamide is formed upon reaction
of a CH2Cl2 solution of 1b with a 1 : 11 CH2Cl2 diluted solution
of acetyl chloride EtC(O)Cl and was isolated (0.008 g, 59%)
by concentration of the resulting solution after isolation of 2b.
IR (KBr pellets): 2242 [s, ν(NuC)], 1719 [s, ν(CvO)]. 1H
NMR (298 K, CD2Cl2): δ 2.21 [qt, 4H, JHH = 7.3, NCNC{(O)
CH2CH3}2], 1.26 [t, 6H, JHH = 7.4, NCN(Et){C(O)-CH2CH3}2].
The equivalence of the two diphosphanes in 1a and 2 is indi-
cated by the singlet at δ ca. 37 in the 31P–{1H} NMR spectra.
However, in their 1H NMR spectra, the two broad multiplets
in the δ 3.1–2.9 range are due to non-equivalent sets of
dppe methylene protons and result from the presence of two
different ligands in the axial positions. This behaviour is observed
in other Mo(IV) bis(dppe) complexes such as trans-[Mo(NCN)-
(NCNH2)(dppe)2][BF4]2 or [MoH2(NCR)2(dppe)2][BF4] {R =
NH2, NMe2, NEt2 or NC(NH2)2}.4 In the 13C NMR spectra of
EtCOOMe. Treatment of a CH2Cl2 solution of 1b with a
1 : 10 excess of a methanolic solution of NaOMe led to the
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Scheme 1
1a and 2, the NCN resonance appears as a singlet at δ ca. 128, a
value comparable to that quoted for the parent complexes 1b and
trans-[Mo(NCN){NCNC(O)Ph}-(dppe)2]Cl 1c.2
The FAB-MS spectra clearly show the positive (for the com-
plexes) and negative (for the counterions) molecular ions, with
the expected isotopic patterns.2,4,54 The fragmentation pathways
occur either by metal–ligand bond cleavage or bond rupture in
the ligands, and the loss of the monodentate moieties appears to
be favoured relative to that of the chelating diphosphane. The
decomposition of the coordinated dppe is similar to that of the
related complex 0,2 and follows a pathway different from that of
the free diphosphane,55 showing the influence of coordination in
the process.
The acylcyanamides NCN(R′)C(O)Et [R′ = H, Et or C(O)Et],
obtained upon reaction of 1b with the above electrophiles,
present the expected characteristic IR and NMR spectroscopic
data. Moreover, for the EtCOOMe ester formed by reaction of
1b with an excess of NaOMe, the corresponding data are identi-
cal to those of the commercially available compound.
Fig. 1 Cyclic voltammogram, initiated by the anodic sweep, of trans-
[Mo(NCN){NCNC(O)Me}(dppe)2]Cl 1a (0.84 mM, in 0.2 M [nBu4N]-
[BF4]/CH2Cl2, at a Pt disc electrode). Potential in V vs. SCE, scan rate =
0.2 V s−1. *Oxidation of the Cl− counterion.
Electrochemical studies of [Mo(NCN){NCNC(O)R}(dppe)2]Cl
[R = Me (1a), Et (1b), Ph (1c)] and [Mo(NCN)Cl(dppe)2]X
[X = BF4 (2a), Cl (2b)]
observed for the parent neutral bis(cyanoimide) complex 0, in
accord with the expected56,57 weaker electron-donor character of
the acylcyanoimide (in 1) or chloride (in 2) ligand as compared
to that of the dianionic cyanoimide(2−) ligand (in 0). For those
compounds with Cl− as the counterion, an irreversible oxidation
wave at ca. 1.1 V vs. SCE is also observed, due to the oxidation
of this ion. Two distinct single-electron reduction waves, Ired (at
a potential in the (−0.54)–(−0.59) V range) and IIred (at a poten-
tial in the (−1.3)–(−1.7) V range) (Fig. 1, Table 1), are also
At a platinum electrode, at 25 °C in 0.2 M [nBu4N][BF4]/
CH2Cl2 solution, the five cationic complexes 1a–1c and 2a–2b
exhibit a first single-electron irreversible (or partially reversible)
anodic wave (Iox) (Fig. 1 and Table 1) at a potential of ca. 1.6 V
vs. SCE, assigned to the MoIV to MoV oxidation process which
occurs at a much higher potential than that (0.79 V vs. SCE)2
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detected and assigned to the Mo(IV)/Mo(III) and Mo(III)/Mo(II)
reduction processes, respectively.
the experiment. Additional evidence for this fact is that CPE at
the first cathodic process (Ired) did not produce complex B.
No significant variation of the above behaviour was detected
upon changing the concentration of the complex in the solution,
suggesting the involvement of first-order chemical steps.
Moreover, interesting behaviour is observed for the acylated
(1a and 1b) and aroylated (1c) complexes (herein denoted by
A+) at the first cathodic wave (Ired). Upon scan reversal follow-
ing this wave, one new oxidation wave (IIox) is observed (Fig. 1,
Table 1) at a higher potential than that corresponding to the
anodic counterpart of this cathodic wave, corresponding to the
oxidation of a new species (B0) which is generated at the catho-
dic process of A+.
These observations can be interpreted by considering an elec-
trochemical–chemical (ECEC) square-type process (Scheme 2),
in which the cathodic reduction of A+ to A0 induces its conver-
sion into another reduced species B0 which is oxidized at a
higher potential, with involvement of the chemical equilibria A0
⇄ B0 and A+ ⇄ B+, between interconvertible species. This be-
haviour has a close resemblance to those we39,57–61 and others62
have observed for ET-induced geometrical isomerizations of
various octahedral phosphinic complexes with nitrile, isocyanide
or carbonyl ligands and thus we propose that, in the current study,
A+ undergoes a cathodically-induced trans-to-cis isomerization,
B being the cis isomer (Scheme 2). This proposal is somehow
corroborated by theoretical calculations discussed below.
This behaviour is dependent on the scan rate and the ratio of
the peak currents of the waves IIox (B0/+) and Ired (A+/0), i.e. ρ =
ip(B0/+)/ip(A+/0), shows a bell-shaped dependence on log ν (ν =
scan rate) (Fig. 2 and 3), with a maximum at a scan rate (ν) in
the 1–2 V s−1 range.
The formation of B (detected by its oxidation at IIox) occurs
at the expense of the reduced A+ (i.e. A which is detected by the
anodic counterpart of the cathodic wave of A+), i.e. the reversi-
bility of Ired, ip(A0/+)/ip(A+/0), varies inversely with ρ, and its
minimum value occurs for the maximum ρ.
Hence, the mechanism of Scheme 2 was investigated in detail
by digital simulation of the cyclic voltammograms at several
scan rates (in the 0.1–60 V s−1 range) using eqn (7)–(12):
For a sufficiently high scan rate, we observe an almost revers-
ible reduction process since there is no time for the occurrence
of appreciable conversion of A0 in B0. For a sufficiently low
scan rate, no B0/+ oxidation wave appears (Fig. 2a) due to the
instability of B0 that decomposes during the extended time of
transþ þ eꢀ Ð trans0 khet1
ð7Þ
Table 1 Cyclic voltammetric dataa for trans-[Mo(NCN){NCNC(O)R}-
(dppe)2]Cl [R = Me (1a), Et (1b), Ph (1c)] and trans-[Mo(NCN)Cl
(dppe)2]X [X = BF4 (2a), Cl (2b)]
IEp
Complex (Iox
IEpred (E1/2
(Ired
)
IIEp
(IIred
IIEp
(IIox b
ox
red
red
ox
)
)
)
)
1a
1b
1c
2a
2b
1.62
1.66
1.63
1.61
(1.56)
(−0.54)
(−0.55)
(−0.59)
−0.54
−1.65
−1.66
−1.27
−1.65
−1.68
−0.30
−0.29
−0.32
—
−0.56
—
a Values in V 0.02 relative to SCE (see Experimental); scan rate of 0.2
V s−1. Values for reversible waves are given in parentheses. For
compounds with Cl− as the counterion, an irreversible oxidation wave is
observed at ca. 1.1 V, due to oxidation of this ion. b Oxidation upon scan
reversal following the first reduction (Ired).
Fig. 3 Experimental (symbols) and theoretical (line) variation of ρ =
ip(B0/+)/ip(A+/0) as a function of the logarithm of scan rate (V s−1) for
trans-[Mo(NCN){NCNC(O)Et}(dppe)2]Cl 1b, according to the mechan-
ism proposed in Scheme 2 and using the rate constant values of Table 3.
Fig. 2 Cyclic voltammograms of trans-[Mo(NCN){NCNC(O)Et}(dppe)2]Cl 1b (2.05 mM, in 0.2 M [nNBu4][BF4]/CH2Cl2, at a Pt disc electrode).
Potential in V vs. SCE; A and B denote the trans and cis isomers, respectively (see text). Scan rate: 0.02 (a), 0.4 (b) and 60 V s−1 (c).
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cycle of Scheme 2, in which K1 = [cis0]/[trans0] = k1/k−1 and
K2 = [cis+]/[trans+] = k2/k−2
.
ꢀ
ꢁ
h
i
K2
K1
F
¼ exp
E0ðtrans0=þÞ ꢀ E0ðcis0=þÞ
ð13Þ
RT
The ratio K2/K1 corresponds to the equilibrium constant of the
cross redox reaction (eqn (14)), and the values of all the equili-
brium constants are given in Table 3.
K2 ½trans0ꢁ ½cisþꢁ
¼
ð14Þ
K1 ½transþꢁ ½cis0ꢁ
Since E0 (cis0/+) > E0 (trans0/+), one concludes that K1 > K2.
In fact, for all three cases K2/K1 is in the range of 5.4 × 10−5 to
6.3 × 10−5, i.e. the equilibrium of the cross reaction lies towards
the trans+ and the cis0 species rather than the trans0 and cis+
isomers. Although at the starting oxidation state, i.e., for
[Mo(NCN){NCNC(O)R}(dppe)2]+ (1a–1c), the trans isomer is
thermodynamically more favourable than the cis one (K2 ≪ 1),
at the reduced level, i.e. for [Mo(NCN){NCNC(O)R}(dppe)2],
there occurs a shift of the relative stability towards the cis isomer
which becomes the dominant one (K1 > 1).
Scheme 2
cis0 ꢀ eꢀ Ð cisþ khet1
ð8Þ
Theoretical studies
trans0 Ð cis0 k1; k
transþ Ð cisþ k2; k
ð9Þ
ð10Þ
ð11Þ
ꢀ1
ꢀ2
ꢀ3
To interpret the experimental results described above and to
understand details of the mechanisms of electrophilic additions,
theoretical calculations at the B3LYP hybrid level of theory
have been performed for model complexes bearing the
PH2CH2CH2PH2 (dpe) ligands instead of dppe. In the next two
sections, the chemical bond nature in the starting dicyanamide
complex [Mo(NCN)2(dpe)2] (0′) and the electrophilic addition to
this species are discussed.
transþ þ cis0 Ð trans0 þ cisþ k3; k
cis0 Ð decomposition k4; kꢀ4ðk ¼ 0Þ
ꢀ4
ð12Þ
A good approximation to the experimental behaviour was
obtained by applying the above six equations and using the opti-
mized rate constant values indicated in Table 2 for the complexes
1a–1c. This is illustrated for complex 1b by the close agreement
of the simulated and the experimental data shown in Fig. 3 for
the current ratio ρ = ip(B0/+)/ip(A+/0) as a function of logarithm
of scan rate, and in Fig. 4 for the cyclic voltammograms at
different scan rates (the simulation was applied successfully at
regular scan rate intervals over the 0.1–60 V s−1 range).
The isomerization equilibrium constants and the redox poten-
tials are related by eqn (13) derived from the relationship
between the Gibbs free energy change (ΔG0) and the equilibrium
constant (K) or the standard potential, i.e. ΔG0 = −RT ln K =
−nFE0, and considering that ΔG0 = 0 for the thermochemical
1. Equilibrium and electronic structures of complexes trans-/
cis-[Mo(NCN)2(dpe)2] (trans-/cis-0′). Comparison of the calcu-
lated total and Gibbs free energies (Table S2, ESI†) indicates
that (i) the ground state of 0′ is the singlet one and (ii) the trans-
0′ isomer is significantly, by 20.8 kcal mol−1, more stable than
the cis-0′ one, in accord with the experimental results.2 The main
calculated bond lengths in trans-0′ are in good agreement with
the X-ray structural data for complex trans-[Mo(NCN)2-
(dppe)2].37 The maximum deviations are 0.03 Å and 0.02 Å for
the Mo–N and C(2)N(3) bonds, whereas those for the Mo–P and
C(2)N(1) bonds do not exceed 0.01 Å. The NCNMoNCN frag-
ment is linear.
For the free cyanamide ion NCN2−, three resonance structures
a–c of equal weights may be drawn (Fig. 5, see also ESI† for
Table 2 Kinetic data (rate constants) obtained by digital simulation for the electron-transfer-induced isomerization of the acylated trans-[Mo(NCN)-
{NCNC(O)R}(dppe)2]Cl [R = Me (1a), Et (1b), Ph (1c)]a
Compound
k1/s−1
k−1/s−1
k2/s−1
k−2/s−1
k3/M−1 −1
s
k−3/M−1 −1
s
k4/s−1
khet1/cm s−1
khet2/cm s−1
1a
1b
1c
50
40
15
10
5.5
12
0.95
1
0.45
3.5 × 103
2.5 × 103
5.5 × 103
9.5
5.5
5.4
1.8 × 105
1 × 105
5 × 10−4
5 × 10−4
5 × 10−4
2.5 × 10−2
2.5 × 10−2
2.5 × 10−2
5 × 10−3
5 × 10−3
1.5 × 10−2
8.3 × 104
a For the meaning of the rate constants, see Scheme 2 and eqn (1)–(6). k−4 = 0 s−1, α = 0.5.
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Fig. 4 Experimental (right) and simulated (left) cyclic voltammograms (according to the mechanism in Scheme 2, and using the rate constant values
of Table 2) for a solution of trans-[Mo(NCN){NCNC(O)Et}(dppe)2]Cl 1b at different scan rates: 0.4 (a), 4 (b), 50 (c) V s−1
.
details). The coordination of NCN2− to Mo via N(1) destabilizes
the structure c and stabilizes the structure a due to the distri-
bution of the excess of electron density from the N(1) atom to
the metal and the establishment of an additional π-bonding
between Mo and N(1). Indeed, (i) the calculated bond orders
(Wiberg bond indices) for the N(1)C(2) and C(2)N(3) bonds in
trans-0′ are 1.39 and 2.56, respectively (Fig. 6), (ii) the NBO
atomic charges on the N(1) and N(3) atoms are −0.64 e and
−0.43 e, correspondingly, and (iii) the N(1)C(2) bond elongates
(by 0.03 Å) while the C(2)N(3) bond shortens (by 0.07 Å) upon
Table 3 Equilibrium constants for the electron-transfer induced
reactionsa for trans-[Mo(NCN){NCNC(O)R}(dppe)2]Cl [R = Me (1a),
Et (1b), Ph (1c)]
Complex
K1 (k1/k−1
)
K2 (k2/k−2
)
K2/K1
1a
1b
1c
5
7.3
1.3
2.7 × 10−4
4.0 × 10−4
8.2 × 10−5
5.4 × 10−5
5.5 × 10−5
6.3 × 10−5
a For the meaning of the constants, see Scheme 2 and eqn (1)–(7).
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coordination. The NBO analysis shows that among the resonance
structures characterizing the trans-NCN–Mo–NCN hypervalent
bond (Fig. 7), the structures d–g with the triple C(2)N(3) bonds
give a predominant contribution (with the non-Lewis occu-
pancies (non-LO) of 2.21%), followed by h–j (2.44%) and k–l
(2.68%). This is consistent with the IR band of trans-[Mo-
(NCN)2(dppe)2] at 2040 cm−1, assigned to ν(CuN) of the
NCN2− ligand.37 The contributions of structures m–n, o–q, and
r–s with double C(2)N(3) bonds are lower (the non-LO of 2.78,
2.98, and 3.20%, respectively). Thus, the NCN2− ligands in
trans-0′ should be considered as cyanoimido, justifying this
earlier proposed designation.37
2. Electrophilic addition to trans-0′
(i) Charge control considerations. The reactivity of complex
trans-0′ towards electrophilic additions can be interpreted in
terms of charge control arguments. The effective NBO charges
on the Mo and N atoms are negative and decrease, in absolute
value, along the row N(1) (−0.64 e) > N(3) (−0.43 e) > Mo
(−0.22 e) while the charge on the C(2) atoms is positive
(0.39 e). However, the analysis of the electrostatic potential
(ESP) distribution indicates that the most extended region with
the highest negative ESP (up to −0.11 au) is located near the
terminal N(3) atoms (Fig. 8A). There is also a less extended
region of the negative ESP near the N(1) atoms (Fig. 8B) but
with much lower potential values (up to −0.03 au). Moreover, in
Fig. 5 Resonance structures of NCN2−
.
Fig. 6 NBO atomic charges and bond orders (Wiberg bond indices) in
trans-0′.
Fig. 7 Resonance structures of trans-0′ with non-Lewis occupancies indicated.
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the case of the real systems bearing the dppe ligands with 8
phenyl groups, the attack of an electrophile at the N(1) and Mo
atoms is additionally hampered by significant steric hindrance
from the diphosphanes. Thus, a simple examination of the ESP
distribution suggests that the easiest way for the electrophilic
addition is at the N(3) atoms.
the chemical behaviour of the molecule as a whole, and the two
latter describe the reactivity of a particular site of the molecule.
As was shown,63 the ratios s+/s− and s−/s+ (“relative electrophili-
city” and “relative nucleophilicity”, respectively) reproduce the
experimental trend better than the individual s+ and s− values.
The site of a molecule with the highest s+/s− value is the most
probable for a nucleophilic attack and the site with the highest
s−/s+ value is the most susceptible to an electrophilic attack. The
calculated s−/s+ value in trans-0′ is the highest one for the metal
(2.273 au, Table S3†), followed by the N(3) and N(1) atoms
(0.750 and 0.254 au, respectively), and, hence, from the HSAB
viewpoint, the probability of electrophilic attack decreases along
this row.
(ii) Orbital control considerations. Another driving force for
the reaction concerns the interaction of the frontier molecular
orbitals (FMOs) of the reactants. From the FMO theory view-
point, the electrophilic addition is determined by the compo-
sition and energy of the HOMO of the substrate. The two
highest occupied MOs of trans-0′ are degenerate and centred
mostly at the N(1) and N(3) atoms whereas the third highest-
lying occupied MO is composed of a d orbital of the metal
(Fig. 9). Hence, the N(1) and N(3) atoms are favourable for the
electrophilic attack in terms of frontier-orbital arguments.
(iv) Thermodynamic stability. To estimate the relative thermo-
dynamic stability of the products of the electrophilic additions to
trans-0′, the equilibrium structures of the protonated complexes
trans-0Ha+′–trans-0Hd+′ (Scheme 3) have been calculated
(H+ serves as an electrophile). Complex trans-[Mo(NCN)-
{NCN(H)}(dpe)2]+ (trans-0Ha+′) with the protonated N(3) atom
appears to be the most stable one and corresponds to the
(iii) HSAB considerations. One of the useful tools for the
analysis of the most plausible site of the electrophilic attack is
the “hard–soft acid–base” (HSAB) principle49 based on the reac-
tivity descriptors, such as hardness (η), global (S) and local (s)
softnesses, and Fukui functions (f). The two former determine
Scheme 3 Protonation of trans-0′ (the relative stability is indicated in
Fig. 8 Distribution of negative electrostatic potential for trans-0′ with
contour values of 0.06 au (A) and 0.023 au (B).
kcal mol−1).
Fig. 9 Plots of frontier MOs of trans-0′.
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experimentally observed2 reaction product, i.e. trans-[Mo(NCN)-
{NCN(H)}(dppe)2][BF4] obtained upon protonation of trans-
[Mo(NCN)2(dppe)2] by HBF4 (reaction 6, Scheme 1).
The second most stable structure is that with the protonated
N(1) atom trans-[Mo(NCN){N(H)CN}(dpe)2]+ (trans-0Hb+′,
less stable by 9.5 kcal mol−1). For the hydride complex trans-
[Mo(NCN)2(H)(dpe)2]+, two structures, with pentagonal bipyra-
mid (trans-0Hc+′) and capped octahedron (trans-0Hd+′) coordi-
nation polyhedra, have been found, and they are less stable than
trans-0Ha+′ by 32.9 and 61.2 kcal mol−1, respectively.
In summary, the results presented above, considered
altogether, suggest that the N(3) atoms of 0′ are the most prob-
able sites of the electrophilic attack because (i) N(3) has a
significant negative charge and is surrounded by an extended
region with the most negative ESP; (ii) the electrophile approach
is not hampered by steric hindrance; (iii) there is a significant
contribution of the p-orbitals of N(3) in the HOMOs of trans-0;
(iv) the relative nucleophilicity of N(3) is higher than that of
N(1) (although lower than that of Mo); (v) the products of the
electrophilic addition to N(3) have the highest thermodynamic
stability. Thus, the reaction of protonation of trans-0′ experi-
mentally studied earlier2 should occur in one step. However, in
the case of more complex electrophiles like RC(O)Cl, a stepwise
mechanism of the reaction (reaction 1, Scheme 1) is not surpris-
ing because, apart from the addition, the reagent decomposition
(Cl− elimination) takes place.
In the next two sections, the bonding situation and electro-
chemical properties of the model product of electrophilic addition,
[Mo(NCN){NCNC(O)Me}(dpe)2]+ (1a+′), are discussed.
Fig. 10 NBO atomic charges and bond orders (Wiberg bond indices)
in trans-1a+′.
Fig. 11 Resonance structures of trans-1a+′ with non-Lewis occupancies indicated.
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3. Equilibrium and electronic structures of complexes trans-/
cis-[Mo(NCN){NCNC(O)Me}(dpe)2]+ (trans-/cis-1a+′). Similarly
to 0′, the ground state of 1a+′ is the singlet one. The electrophilic
addition of the acetyl group to 0′ results in a great elongation
of the MoN(1) and C(2)N(3) bonds of the reacting ligand (by
0.183–0.198 and 0.060–0.067 Å, respectively) and in a shorten-
ing of the N(1)C(2) bond (by 0.085–0.086 Å). The calculated
orders of the N(1)C(2) and C(2)N(3) bonds in 1a+′ are 2.07–2.12
and 1.64–1.69, respectively (Fig. 10), indicating a significant
redistribution of electron density compared to 0′. The NBO
analysis demonstrates that the most stable resonance structure of
trans-1a+′ is m (a non-LO of 2.15%) (Fig. 11) in which the
NCNC(O)Me ligand exhibits the nitrile character. However,
structure h with the NCNC(O)Me ligand of the imine type has
only a slightly higher non-LO (2.26%). Moreover, the non-LO
values of resonance structures a, e, f, i, k, l, and n–p are also
similar and do not exceed 2.6%. Noticeable contributions
coming from f, i, and c provide some bending of the MoN(1)C-
(2) fragment (angle of 164°). It is interesting that in the similar
alkylated complex trans-[Mo(NCN)(NCNEt)(dppe)2][BF4] this
fragment is almost linear.2,64 Thus, the nature of the electrophile
affects significantly the electron density distribution in the reac-
tion product.
pair), and this trend correlates with the hypothesis of the trans-
to-cis isomerization. However, the trans-isomer of the reduced
species trans-1a0′ still should be more stable than the cis-isomer
cis-1a0′ in CH2Cl2 solution (by 7.2 kcal mol−1). Thus, the calcu-
lations cannot fully establish the proposed electrochemically
induced trans-to-cis isomerization. Nevertheless, it should be
noted that the presence of a great amount of the electrolyte
[nBu4N][BF4] used for the electrochemical studies may signifi-
cantly alter the relative stability of the trans- and cis-isomers
compared to the pure CH2Cl2 solution and, thus, prevent a direct
comparison of the theoretical and experimental data. In addition,
all the other hypotheses of structural transformations of trans-
1a0′ induced cathodically (see ESI† for their discussion) are less
favourable than the proposed trans-to-cis isomerization.
According to the experimental data, the acylated complexes 1
undergo further electrophilic attack and following decomposition
to give the acylcyanamides NCN(R′)C(O)R and the chloro-
cyanoimido complex 2 (reactions 2–4, Scheme 1). Theoretical
results describing details of this reaction are discussed in the
next sections.
5. Protonation of trans-1a+′
(i) Charge control considerations. The N(1), N(3) and O
atoms of the acetylcyanoimide ligand in trans-1a+′ have the most
negative NBO atomic charges (−0.68 and −0.60, Fig. 10) while
those on the N(1′) and N(3′) atoms of the cyanoimido ligand are
significantly less negative (−0.46 and −0.25). It is important that
the charge distribution in the overall neutral compound bearing
the Cl− counterion trans-[Mo(NCN){NCNC(O)Me}(dpe)2]Cl
(trans-1a′, see Computational details) is similar. The analysis of
the ESP distribution around trans-1a′65 indicates the presence of
three regions with negative ESP values (Fig. 13). The most
extended one is placed near the terminal N(3′) atom despite its
comparatively low negative charge. The second region is situated
near the O atom of the acetyl group and the smallest one is
found near the N(3) atom of the NCNC(O)Me ligand. There is
no region of negative ESP near the N(1) and N(1′) atoms (see
ESI† for additional discussion). Thus, in terms of simple electro-
static arguments, the N(3′), O, and N(3) atoms are the most prob-
able sites for protonation.
4. Electrochemical behaviour of 1a+′ and isomeric stability.
The one-electron reduction of 1a+′ leads to the formation of
neutral complexes [Mo(NCN){NCNC(O)Me}(dpe)2] (1a0′) for
which the doublet spin state was found to be the ground one. As
a result, shortening of the Mo–P, N(1′)C(2′) and N(1)C(2) bonds
and elongation of the Mo–N, C(2′)N(3′) and C(2)N(3) bonds
occur. Analysis of the LUMO composition in trans-1a+′ indi-
cates that, upon reduction, the electron should go to the MO
centred mainly on the Mo, N(1′) and N(3′) atoms (Fig. 12). The
atomic spin densities in trans-1a0′ are highest on these atoms
(0.58, 0.24, and 0.23 e, respectively).
The trans-isomer of 1a+′ was calculated to be more stable, by
13.8 kcal mol−1, than the cis-isomer, and the former is reduced
at a lower potential than the latter (the electron affinities of
trans-1a+′ and cis-1a+′ are 4.43 and 4.72 eV, respectively), all
these being in qualitative agreement with the experimental data
and proposed assignment of CV waves (see above). Moreover,
the relative stability of the trans-isomer vs. the cis-one clearly
decreases upon reduction (from 13.8 kcal mol−1 for the trans-
1a+′/cis-1a+′ pair to 7.2 kcal mol−1 for the trans-1a0′/cis-1a0′
(ii) Orbital control considerations. The HOMO of trans-1a+′
is centred on the Mo atom (Fig. 12). However, the direct proto-
nation of the metal is not favourable due to the absence of any
region with negative ESP and great steric hindrance from the
Fig. 12 Plots of frontier MOs of trans-1a+′.
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Fig. 13 Distribution of negative electrostatic potential for trans-1a′
with a contour value of 0.01 au.
dppe ligands in the real complexes. The main contribution to the
next MO, HOMO-1, comes from the O atom followed by the
N(3) and N(1) atoms. The highest-lying MO with significant
contribution from the N(1′) and N(3′) atoms (together with the
O, N(1), and N(3) atoms) is only HOMO-3, which is by 0.96 eV
lower in energy than HOMO-1. Thus, based on orbital argu-
ments, the protonation of the NCNC(O)Me ligand (via the
oxygen atom) is more favourable than that of the NCN ligand.
(iii) HSAB considerations. As in the case of trans-0′, in
trans-1a+′ the relative nucleophilicity s−/s+ is highest for the Mo
atom (1.725 au). The N(3) and O atoms have the s−/s+ values
(1.074 and 0.951 au) which are higher compared to the N(3′)
and, in particular, N(1′) atoms (0.756 and 0.309 au), and hence
the former are more susceptible to protonation than the latter.
The local softness of the N(1) atoms has a negative value that
has no physical meaning.
Scheme 4 Protonation of trans-1a+′ (relative energies are indicated in
kcal mol−1).
decomposition towards the final products, as discussed in the
next section.
6. Decomposition of trans-1aHa2+′. Electrophilic additions to
the acylcyanoimide–cyanoimido complexes 1 facilitate their
decomposition affording, in accord with experimental data (see
above), free NCN(R′)C(O)R and the chloro-complex 2 (reactions
2–4, Scheme 1). Indeed, the theoretical calculations demonstrate
that the protonation of trans-1a+′ results in a significant
elongation and weakening of the Mo–N bond for the protonated
ligand (by 0.114–0.280 Å). The other Mo–N bond with the
intact ligand shortens by 0.027–0.208 Å.
(iv) Thermodynamic stability. Geometry optimization of the
protonated complexes trans-1aH2+′ (Scheme 4) shows that struc-
ture trans-1aHa2+′ with the protonated oxygen atom is the most
stable one followed by trans-1aHb2+′ in which the proton is
localized at the N(3) atom (less stable by 4.9 kcal mol−1). The
complex trans-1aHc2+′ with the protonated terminal atom N(3′)
of the NCN ligand is even less stable, by 8.8 kcal mol−1 relative
to trans-1aHa2+′. Finally, the N(1)- and N(1′)-protonated struc-
tures trans-1aHd2+′ and trans-1aHe2+′ are the least stable ones
with the Gs energies 16.2 and 27.0 kcal mol−1 higher than that
Among the three possible organic products of decomposition
(Scheme 5), the most stable one is NCN(H)C(O)Me (acylcyan-
amide) followed by HNCNC(O)Me and NCNC(OH)Me (less
of trans-1aHa2+′
.
In summary, the oxygen atom of the acyl group of the acyl-
cyanamide–cyanoimido Mo complexes is the most favourable
site for protonation due to (i) the existence of an extended region
with negative ESP near the O atom which has a large negative
charge, (ii) a major contribution from the orbitals of this atom to
the HOMO-1, (iii) a significant value of the relative nucleophili-
city, (iv) the absence of steric hindrance upon H+ attack, and (v)
the highest stability of the O-protonated complex trans-1aHa2+′
among the other protonated species. Hence, reaction
2
(Scheme 1) is expected to proceed via initial protonation at the
O atom of the acylcyanamide ligand, followed by further
Scheme 5 Decomposition of trans-1aHa2+′
.
13888 | Dalton Trans., 2012, 41, 13876–13890
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Scheme 6
Scheme 7
stable by 5.1 and 7.8 kcal mol−1, respectively). Thus, the liber-
ation of the acylcyanoimide ligand from the coordination
sphere of the metal is accompanied by proton migration from
the O atom to the N(3) one. The decomposition of the system
{trans-1aHa2+′ + Cl− present as a counterion} to NCN(H)-
C (O)Me and trans-[Mo(NCN)Cl(dpe)2]+ (trans-2a+′) is strongly
exoergonic with the ΔGs value of −42.3 kcal mol−1. All these
results correlate well with the experimental data and allow their
interpretation.
better knowledge of the promising, but yet relatively little
explored, chemistry of cyanoimide and derived ligands.
Acknowledgements
This work has been partially supported by the Fundação para a
Ciência e a Tecnologia (FCT), Portugal, and its “Strategic pro-
gramme” PEst-OE/QUI/UI0100/2011. M.L.K. is grateful to FCT
and IST for a research contract within the Ciência 2007 scientific
programme. The authors gratefully acknowledge Dr Maria
Cândida Vaz (IST) for the direction of the elemental analysis
service and the Portuguese NMR Network (IST-UTL Centre) for
providing access to the NMR facility.
Conclusions
This work contributes to the development of the still underdeve-
loped coordination chemistry of the cyanoimide (NCN) and acyl-
cyanoimide {NCNC(O)R} species, by studying their generation
and reactivity at a Mo–phosphinic centre.
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Dorecht, The Netherlands, 1993, p. 79 and references therein.
35 M. H. Chisholm, K. Folting, J. C. Huffman and N. S. Marchant, Organo-
metallics, 1986, 5, 602–603.
64 The theoretical calculations of the model complex trans-[Mo(NCN)
(NCNMe)(dpe)2]+ reproduce the experimental data about the linearity of
the MoNC fragment.
65 A counterion was added for the ESP calculations to compensate the posi-
tive charge of the complex moiety.
13890 | Dalton Trans., 2012, 41, 13876–13890
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