2-Picolylamino(diphenylphosphinoselenoic)amide supported zinc complexes: Efficient catalyst for insertion of N–H bond into carbodiimides, isocyanates, and isothiocyanate

Article abstract of DOI:10.1080/10426507.2017.1370590

We report here the hydroamination of heterocumulenes such as carbodiimides, isocyanates, and isothiocyanates by zinc complexes supported by the ligand 2-picolylamino-(diphenylphosphinoselenoic)amide [{(Ph₂P-(?Se)}₂NCH₂(C₅H₄N)] (1). A series of zinc complexes [κ²-{(Ph₂P-(?Se)}₂NCH₂(C₅H₄N)ZnX₂] [(X?Cl (2), Br (3a), I (4)] were prepared from ligand 1 and the corresponding zinc dihalide in a 1:1 molar ratio at 60°C in a chloroform solvent. The reaction of ligand 1 and ZnBr₂ in methanol yielded another zinc complex [κ²-{(Ph₂P-(?Se)}₂NCH₂(C₅H₄N)ZnBr₂(CH₃OH)] (3b). The molecular structures of compounds 3a, 3b, and 4 were established through single-crystal X-ray diffraction analyses. The solid-state structures of all the complexes revealed a κ²- chelation through pyridine nitrogen and selenium atoms of ligand 1 to the zinc ion. Complex 2 proved to be a competent pre-catalyst for the addition of the amine N–H bond to carbodiimides, isocyanates, and isothiocyanates. The reaction scope was expanded to reactions of aliphatic and aromatic amines with phenylisocyanate and phenylisothiocyanate in toluene solvents, which proceeded rapidly at room temperature with 5 mol% catalyst loading to yield (up to 99%) the corresponding derivatives of urea and thio-urea. However, a temperature of 90°C was needed for the hydroamination of N,N′ dicyclohexylcarbodiimide. We also report the most plausible mechanism of the hydroamination reaction.

Full text of DOI:10.1080/10426507.2017.1370590

Phosphorus, Sulfur, and Silicon and the Related Elements
ISSN: 1042-6507 (Print) 1563-5325 (Online) Journal homepage: http://www.tandfonline.com/loi/gpss20
2-Picolylamino(diphenylphosphinoselenoic)amide
Supported Zinc Complexes: Efficient Catalyst
for Insertion of N–H bond into Carbodiimides,
Isocyanates, and Isothiocyanate
Adimulam Harinath, Kulsum Bano, Shakil Ahmed & Tarun K. Panda
To cite this article: Adimulam Harinath, Kulsum Bano, Shakil Ahmed & Tarun K. Panda (2017): 2-
Picolylamino(diphenylphosphinoselenoic)amide Supported Zinc Complexes: Efficient Catalyst for
Insertion of N–H bond into Carbodiimides, Isocyanates, and Isothiocyanate, Phosphorus, Sulfur,
and Silicon and the Related Elements, DOI: 10.1080/10426507.2017.1370590
To link to this article: http://dx.doi.org/10.1080/10426507.2017.1370590
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2-Picolylamino(diphenylphosphinoselenoic)amide Supported Zinc Complexes: Efficient
Catalyst for Insertion of N–H bond into Carbodiimides, Isocyanates, and Isothiocyanate
Adimulam Harinath,[a] Kulsum Bano,[a] Shakil Ahmed,[a] and Tarun K. Panda[a]*
Department of Chemistry, Indian Institute of Technology Hyderabad, Kandi, Sangareddy,
502285, Telangana, India. Fax: + 91(40) 2301 6032; Tel: + 91(40) 2301 6036; E-mail:
tpanda@iith.ac.in
CONTACT Tarun K. Panda tpanda@iith.ac.in
Abstract
We report here the hydroamination of heterocumulenes such as carbodiimides, isocyanates, and
isothiocyanates by zinc complexes supported by the ligand 2-picolylamino-
(diphenylphosphinoselenoic)amide [{(Ph2P-(=Se)}2NCH2(C5H4N)] (1). A series of zinc
complexes [2-{(Ph2P-(=Se)}2NCH2(C5H4N)ZnX2] [(X = Cl (2), Br (3a), I (4)] were prepared
from ligand 1 and the corresponding zinc dihalide in a 1:1 molar ratio at 60°C in a chloroform
solvent. The reaction of ligand 1 and ZnBr2 in methanol yielded another zinc complex [2-
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{(Ph2P-(=Se)}2NCH2(C5H4N)ZnBr2(CH3OH)] (3b). The molecular structures of compounds 3a,
3b, and 4 were established through single-crystal X-ray diffraction analyses. The solid-state
structures of all the complexes revealed a 2- chelation through pyridine nitrogen and selenium
atoms of ligand 1 to the zinc ion. Complex 2 proved to be a competent pre-catalyst for the
addition of the amine N–H bond to carbodiimides, isocyanates, and isothiocyanates. The reaction
scope was expanded to reactions of aliphatic and aromatic amines with phenylisocyanate and
phenylisothiocyanate in toluene solvents, which proceeded rapidly at room temperature with 5
mol% catalyst loading to yield (up to 99%) the corresponding derivatives of urea and thio-urea.
However, a temperature of 90°C was needed for the hydroamination of 1,3-
dicyclohexylcarbodiimide. We also report the most plausible mechanism of the hydroamination
reaction.
Keywords: P-N Ligands / Selenium / Sulfur / Carbodiimides / Isocyanate /Isothiocyanate / Zinc
2
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Introduction
Catalytic hydroamination reactions of multiple bonds of C=X (X= N, O, S) in the presence of a
metal catalyst, such as the addition of an N–H bond in an unsaturated C=N bond, have gained
popularity in the scientific community over the past decade, because they represent an atom-
economical route for the synthesis of various families of organic reactions [1]. Among these, the
catalytic addition of amine N–H bonds with isocyanates, isothiocyanates, and with
carbodiimides, which is known as guanylation,[2] is a highly efficient and atom-economical
route to produce a series of substituted urea, thio-urea, and guanidines derivatives. Substituted
urea and thio-urea have unique value in pharmaceutical chemistry, organometallic and
coordination chemistry, and organic synthesis [3]. Guanidine and guanidine derivatives are given
attention by organic chemists because of their most attractive structure, atom economy, and
importance in terms of organic and pharmaceutical chemistry [4]. These guanidines act as organo
super bases. When compared with other nitrogen compounds, guanidines are more basic, and
also more widely explored as a base catalyst for various organic transformations [5] and
organometallic and coordination chemistry [6]. Besides this, guanidine also act as building
blocks for several natural products [7]. Without the presence of a catalyst, the direct guanylation
reaction of primary aliphatic amines with carbodiimides is very difficult and can be performed
only under very harsh conditions; [8] in the case of isocyanate and isothiocyanate, the reaction
proceeds under normal temperature, but the yield is quite poor. An early attempt, well-known in
literature, was the guanylation of some aromatic amines with activated N,N′-diaryl-substituted
carbodiimide, promoted by tetra-butylammonium fluoride (TBAF) [9]. However, such a catalytic
process was hardly explored until Richeson et al. reported the first catalytic guanylation of
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primary aromatic amines with un-activated carbodiimides using titanium imido complexes in
2003 [10]. In addition, another protocol of reductive coupling of nitro compounds with amines
and CO, mediated by transition metals or elemental chalcogens (S, Se), is known [11]. Later, in
2006 Hou et al. reported the first catalytic guanylation of secondary amines with carbodiimides
in the presence of a half-sandwich rare-earth-metal alkyl complex [12].
In our ongoing research, we are involved in developing various metal complexes with a number
of ligand systems to utilize them in different organic transformations [13]. Imidazolin-2-iminato
titanium complex [(ImDippN)Ti(NMe2)3] (ImN = imidazolin-2-iminato, Dipp = 2,6-
diisopropylphenyl) [14] and bis(phosphinoselenoicamide) titanium (IV) complex
[{Ph2P(Se)NCH2CH2NPPh2(Se)}Ti(NMe2)2] [15] were recently employed for the
hydroamination of various amines to isocyanates, isothiocyanates, and carbodiimides. S
Nembenna et.al reported NHC supported magnesium and zinc complexes catalyzed guanylation
reactions [16] Very recently S Nembenna et.al reported catalyst free C-N bond by using amines
with carbodiimides [17]. As Ti (IV) and Zn (II) ions consist of the d0 and d10 electronic
configuration respectively, both the ions can’t use the metal d orbital for chelation with ligands.
Thus, some resemblance in electronic and subsequent catalytic properties may be expected
between titanium(IV) and zinc(II) ions. In addition, zinc is the more abundant, inexpensive and
relatively nontoxic metal in nature and its bio-affinity and nontoxicity make it suitable for
innumerable applications in chemical synthesis. Thus, we were encouraged to develop zinc
catalysts for the hydroamination of various amines to isocyanates, isothiocyanates, and
carbodiimides. 2-picolylaminophosphine was recently explored by Biricik et al. to prepare
palladium complexes using the C–C coupling reaction [18]. This ligand system is also utilized in
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iron and nickel chemistry to study their applicability in ethylene oligomerization [19]. There is
only one report on the development of a zinc complex with 2-picolylaminophosphine by the
Reiher and Westerhausen working group [20]. However, the bis-selenide derivative has not been
explored in zinc chemistry so far.
Here, we report the synthesis and structural details of three zinc complexes with the molecular
formula [2-{(Ph2P-(=Se)}2NCH2(C5H4N)ZnX2] [(X = Cl (2), Br (3a), I (4)], and their
application as pre-catalysts in the N–H addition of amines to readily available isocyantes,
isothiocynates, and carbodiimides, to produce a series of substituted urea, thiourea, and
guanidines derivatives in good yield. We also report the solid-state structure of 2-picolylamino-
(diphenylphosphinoselenoic)amide ligand [{(Ph2P-(=Se)}2NCH2(C5H4N)] (1).
Results and Discussion
Synthesis of Compound 1
2-picolylamino-(diphenylphosphinoselenoic)amide ligand [{(Ph2P-(=Se)}2NCH2(C5H4N)] (1)
was prepared according to the procedure prescribed in literature, using the two-step synthesis
[18]. The corresponding phosphinamine [(Ph2P)2NCH2(C5H4N)] (L) was prepared by using 2-
picolylamine and Ph2PCl in a 1:2 molar ratio in the presence of triethylamine. The bis-selenide
compound 1 was obtained by the oxidation of L with elemental selenium in toluene at 90°C (see
Scheme 1).
The multi-nuclear 1H, 13C, and 31P NMR spectra of compound 1 are in agreement with the details
reported in literature (supporting information) [18]. The solid-state structures of the
phosphinamine L and bis-selenide ligand 1 were confirmed using single-crystal X-ray diffraction
analysis. The phosphinamine L crystallizes in the triclinic space group P-1, with two molecules
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in the unit cell. However, the bis-selenide compound 1 crystallizes in the monoclinic space group
P21/c, with four molecules in the unit cell. The crystallographic and refinement parameters are
given in Table 1. The molecular structures of the phosphinamine L and bis-selenide compound 1
are given in Figures 1a and 1b respectively.
The N–P distances in both the compounds [1.736(2) and 1.710(2) Å for L and 1.720(5) and
1.710(5) Å for 1] are similar, indicating no significant change in the N–P distance after oxidation
of the phosphorus atom. The P–Se bond lengths [2.095(2) and 2.100(2) Å] are also in good
agreement with previously reported aminophosphinselenide compounds [21]. In compound 1, the
two selenium atoms are not coplanar to each other, and a dihedral angle of 51.38° between the
mean planes containing Se1, P1, N2 and Se2, P2, N2 was observed.
Synthesis of Zinc Complexes 2, 3a, 3b, and 4
The zinc complexes 2, 3a, and 4 could be obtained by the treatment of bis-selenide compound 1
with anhydrous zinc dihalide in chloroform as solvent at ambient temperature (see Scheme 2).
However, the methanol-coordinated zinc compound 3b was obtained by the reaction of
compound 1 and anhydrous ZnBr2 in methanol as solvent (see Scheme 2). All the new
complexes were characterized using spectroscopic and analytical techniques and the solid-state
structures of compounds 3a, 3b, and 4 were established by single-crystal X-ray diffraction
analysis.
In the FT-IR spectra, each complex shows a characteristic strong absorption band (570 cm-1 for
2, 580 cm-1 for complexes 3a, 3b, and 4) for the P=Se bond stretching, which is slightly shifted
from the ligand P=Se bond stretching frequency of 560 cm-1. The observed values are in
1
agreement with those reported in the literature [21]. In the H NMR spectra, measured in
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dimethylsufoxide-d6, each complex displayed a sharp singlet at H 4.74 ppm for 2, 4.74 ppm for
complexes 3a and 3b, and 4.74 ppm for complex 4 for methylene (CH2) protons present in the
backbone of the ligand. These chemical shift values are within the same range as that of the free
ligand 1 (H 4.75 ppm). A sharp singlet at P 69.2 for complex 2a, 69.3 for complex 3a, and 69.2
31
for complex 4a was observed in the P{1H} NMR spectra, which indicates that no significant
31
change in the P resonance signal occurred in any of the complexes after chelation of ligand 1
1
with the zinc ion (P 69.3 ppm for compound 1). A JP-Se coupling constant of 395 Hz (for
compound 1), 396 Hz (for complex 2), 391 Hz (for complex 3a), and 393 Hz (for complex 4)
was observed for each complex due to coupling of the 31P and 77Se nuclei. Signals in the 13C{1H}
are in the expected ranges and match the values reported in literature [21].
The solid-state structures of complexes 3a, 3b, and 4, determined by single-crystal x-ray
diffraction
analysis,
confirmed
the
attachment
of
the
2-picolylamino-
(diphenylphosphinoselenoic)amide ligand to the zinc ion. The crystals obtained for compound 2
were weakly diffracted in X-ray to generate a poor dataset. However, to make a visual
comparison, the structure of compound 2 is shown in the supporting information.
In the solid state, compounds 3a crystallizes in the triclinic space group P-1, with two molecules
of complex 3a and two molecules of chloroform in the unit cell. In contrast, the complex 3b
crystallizes in the monoclinic space group P21, with one molecule of 3b and one molecule of
ligand 1 in the asymmetric unit. However, complex 4 crystallizes in the monoclinic space group
P21, with two molecules in the unit cell. The crystallographic and refinement parameters are
given in Table 1. The molecular structures of complexes 3a and 3b are given in Figures 2a and
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2b respectively, whereas Figure 3 represents the molecular structure of 4. Selected bond lengths
[Å] and bond angles [o] of zinc complexes 3a, 3b, and 4 are given in Table 2.
In both complex 3a and 4, the coordination polyhedron was formed by the attachment of the
neutral [{(Ph2P-(=Se)}2NCH2(C5H4N)] ligand and two chloride ions to the zinc (II) ion, thus
providing the metal ion with a four-fold coordination. The bis-selenide ligand 1 coordinated to
the zinc (II) ion via chelation of pyridine nitrogen and one selenium atom attached to the
phosphorus atom. The uncoordinated amido nitrogen and second selenium atom present in the
ligand twisted away from the metal center due to steric crowding around the zinc ion.
Consequently, the central zinc ion adopted a distorted tetrahedral geometry. In each complex,
one twisted boat-shaped seven-membered metallacycle Zn1–N2–C5–C6–N1–P2–Se2 for
complex 3a and Zn1–N1–C1–C6–N2–P1–Se1 for 4 is formed. However, in complex 3b, the zinc
ion is chelated by one pyridine nitrogen of ligand 1 and one oxygen atom from a methanol
molecule to adopt a distorted tetrahedral geometry around the zinc ion. Thus, no metallacycle
was observed in complex 3b. However, weak hydrogen bonding between Br1…H1, Br2…H8
and Br2–H35a was observed in the solid state of complex 3b. The Zn1–N1 bond distance of
2.086(5) Å in complex 3a is slightly elongated than that in 3b [2.071(8) Å], presumably due to
ring strain of the seven-membered metallacycle. However, the analogous complex 4 has a similar
Zn–N bond distance [2.077(9) Å]. The Zn–Se bond distance in complex 3a [2.502(1) Å] is
slightly shorter than that of complex 4 [2.522(2) Å], presumably due to the smaller size of the
bromide ion around the zinc ion. However, the Zn-Se bond distances are in good agreement with
the literature values [22].
Catalytic Study
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Zinc complexes were used as pre-catalysts for the hydroamination of heterocumulenes such as
carbodiimides, isocyanates, and isothiocyanates at ambient temperature (see Scheme 3). The
hydroamination reactions were first screened with complexes 2, 3a, 3b, and 4 in toluene using 5
mol% of each complex and the results are given in Table 3. We found that complex 2, [2-
{(Ph2P(=Se)}2NCH2(C5H4N)ZnCl2], is the most active pre-catalyst among all the precursors
under identical conditions (see Table 3, entries 1, 4, 7), affording 99% conversion of the
isocyanate, isothiocyanate, and carbodiimide to produce corresponding urea, thio-urea, and
guanidine respectively. Only traces of product was detected in case of guanylation reaction of
dicyclohexylcarbodiimide and aniline (Fig S69-71) using free ZnX2 (X = Cl, Br, I) compounds
as catalyst. Thus ligand 1 have significant influence to enhance the catalytic activity. However,
50% of product could be obtained in reaction of phenylisocyanade and 2,6-diisopropyl amine
using ZnCl2 as catalyst over prolong reaction time (Fig S72). We also screened the effect of
solvent using toluene, THF, and CH2Cl2, and observed that toluene was the best solvent for the
reaction. The hydroamination reaction of isocyanates with aniline in the absence of any catalyst
did not yield any product even at elevated temperature (Table 1, entry 11).
The hydroamination reactions of several primary and secondary amines with three different
heterocumulenes catalyzed by zinc complex 2 (5 mol%) were investigated under mild
conditions. Catalytic experiments were performed using equimolar amounts of carbodiimides,
isocyanates or iso-thiocyanates, and amines. They were added to a toluene solution of 5 mol% of
zinc complex 2 under inert atmosphere (see Scheme 3). The reaction mixture of isocyanates and
iso-thiocyanate and amines was stirred continuously at room temperature for two hours. In the
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case of carbodiimides however, a temperature of 90°C and reaction time of 12 h were required.
1
The corresponding insertion products were isolated in each case and analyzed through H and
13C NMR and mass spectroscopy. Further, yields from the pure products were calculated. The
reactions displayed a broad substrate scope. The results the catalytic hydroamination to the three
different heterocumulenes are displayed in Table 4.
In most cases, complete substrate conversion resulting in the corresponding urea with excellent
chemoselectivity was achieved when various isocyanates were used. The reaction of phenyl
isocyanate with primary tert-butyl amine, aniline and secondary pyrrolidine, and N,N’-
diisopropylamine proceeded rapidly at room temperature and resulted in the selective production
of the corresponding urea at 99% yield (Table 4, entries 1–4). Substituted anilines containing
substituents with positive inductive effect too showed a very good conversion rate of 85–99%
under similar reaction conditions (Table 4, entries 5–7). However, we observed a sluggish
reaction when 2-fluroaniline was reacted with phenyl isocyanate, presumably due to deactivation
of the amine moiety (Table 4, entry 7). The catalytic hydroamination of amines was further
extended to two other isocyanates available with us: 4-chlorophenylisocyanate (ClPhNCO) and
p-tolylisocyanate (TolNCO). The secondary amines and the substituted anilines displayed
complete conversion, and produced corresponding urea compounds under similar reaction
conditions (Table 4, entries 8–12).
Phenyl iso-thiocyanate (PhNCS) was used as the next substrate for hydroaminatin of amines. In
most cases, amines such as tert-butyl amine, pyrrolidine, and N,N’-diisopropylamine undergo
complete conversion easily to yield the corresponding thio-urea products (Table 4, entries 13–
15). Aniline and the substituted aryl amines such as MesNH2, DippNH2, 2-OMePhNH2, and 4-
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OMePhNH2, having an electron donating group, could be within an hour and at room
temperature—converted into their respective thio-urea derivatives smoothly (Table 1, entries 16–
20). In contrast, aryl amines with electron-withdrawing groups such as nitro-, fluoro-, chloro-,
and bromo- showed sluggish reactivity, generating a slightly lower yield under similar conditions
(Table 4, entries 21–24). Elevated temperatures were used, since the carbodiimides were less
active in comparison with isocyanates and isothiocyanates. We confined our scope of
hydroamination of aryl amines to N,N-dicyclohexylcarbodiimides (DCC). The reaction of aniline
PhNH2 with DCC at 60°C selectively afforded the guanidine complex 7a of 97% yield, which in
turn underwent a 1,3-hydride shift to yield the symmetric guanidine (see Scheme 2). Other aryl
amines with electron-withdrawing functional groups such as fluoro-, chloro-, iodo-, and nitro-
exhibited lesser conversion to afford the corresponding guanidines, due to the deactivation of the
aryl moieties (Table 4, entries 26–31). From this survey of the reactivity of different substrate
partners, we see the clear influence of electronic factors in both the reacting heterocumulenes
and amines.
Most Plausible Mechanism
Scheme 4 describes the most plausible mechanistic pathway for the insertion reaction between
heterocumulene and amines mediated by the zinc complex 2 as pre-catalyst. It is proposed that
isocyanate nitrogen initially undergoes Lewis acid base interaction to the zinc complex 2, leading
to the formation of transient penta-coordinated zinc species (I). Subsequent nucleophilic addition
of amine which is hydrogen bonded with uncoordinated selenium atom to the species (I) at the
electrophilic carbon forms another penta-coordinate zinc complex (II). In the final step, intra-
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molecular proton transfer in penta-coordinated zinc species results to the formation of guanidine
and pre-catalyst 2. A similar mechanistic cycle is also reported in literature [13].
Experimental
General Methods
All manipulations were performed in an atmosphere of dry nitrogen using Schlenk-type
glassware on a dual manifold Schlenk line, interfaced to a high vacuum (10-4 torr) line. THF was
pre-dried over Na wire and distilled under nitrogen from sodium and benzophenone ketyl prior to
use. Toluene was distilled under nitrogen from sodium. Chloroform was distilled over P2O5
1
under nitrogen. Methanol was purified using sodium metal under nitrogen gas. H NMR (400
MHz), 13C{1H} (100 MHz), and 31P{1H} NMR (161.9 MHz) spectra were recorded on a
BRUKER AVANCE III-400 spectrometer. BRUKER ALPHA FT-IR was used for FT-IR
measurement. Elemental analyses were performed on a BRUKER EURO EA at the Indian
Institute of Technology Hyderabad. 2-picolylaminophosphine (L) and 2-picolylamino-
(diphenylphosphinoselenoic)amide ligand [{(Ph2P(=Se)}2NCH2(C5H4N)] (1) were prepared
according to the slightly modified procedure prescribed in the literature [18]. The NMR solvents
CDCl3 and DMSO-d6 were purchased from Sigma Aldrich.
Preparation of [{(Ph2P-(=Se)}2NCH2(C5H4N)] (1)[18]
L: In a pre-dried 50 mL round-bottomed flask, chlorodiphenylphosphine was added slowly to a
solution of picolylamine and triethylamine in toluene at 0°C and the resulting white suspension
was stirred for four hours. The white precipitate was filtered. The solvent was evaporated from
filtrate in vacuum. A white-colored solid residue was obtained, which was re-crystallized in dry
THF.
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1: In a pre-dried 50 mL Schlenk flask, L (952mg) and elemental selenium (170mg) were heated
at 80°C in toluene for eight hours. Excess selenium was filtered through a G4 frit to collect the
filtrate. The solvent was evaporated in vacuum. A white solid residue was obtained and
compound 1 was re-crystallized from methanol. Colorless crystals appeared after one day.
Yield 0.99 g, 90%. 1H NMR (400 MHz, DMSO-d6) δ 8.20 (d, J = 4.7 Hz, 1H, Py), 8.02–7.96 (m,
8H, ArH), 7.43–7.39 (m, 4H, ArH), 7.36–7.34 (dd, J = 8 Hz 1H, Py), 7.32–7.28 (m, 8H, ArH),
7.15 (d, J = 7.8 Hz, 1H, Py), 6.99 (t, J = 5.1 Hz , 1H, Py) 4.75 (dd JHH = 15 Hz, 2H)
ppm.13C{1H} (100 MHz, DMSO-d6) 155.8 (ArC), 147.7 (ArC), 135.4 (ArC), 133.45 (ArC),
131.8 (ArC), 127.6 (ArC), 127.4 (ArC), 121.6 (ArC ), 54.9 (CH2) ppm. 31P{1H} NMR (161.9
MHz, DMSO-d6) δ 69.3 ppm. (C30H26N2P2Se2) (634.4) Calc. C 56.80, H 4.13, N 4.42; found C
56.41 H 3.93, N 4.21.
Preparation of Compound 2
In a 25 mL pre-dried Schlenk flask, ligand 1 (200 mg, 0.31 mmol) was dissolved in CHCl3 (10
mL), mixed with anhydrous zinc chloride (43 mg, 0.31 mmol), and heated at 65°C for six hours.
The resulting white residue was further purified by washing with hexane and crystals suitable for
X-ray analysis were grown from CH3CN/CH3OH mixture at room temperature.
1
Yield: 220 mg, 90%. H NMR (400 MHz, DMSO-d6 ) δ 8.24 (d, J = 3.9 Hz, 1H, Py), 8.06–8.01
(m, 8H, ArH), 7.42–7.38 (m, 4H, ArH), 7.36–7.34 (dd, J = 8 Hz 1H, Py), 7.32–7.28 (m, 8H,
ArH), 7.19 (d, J = 7.7 Hz, 1H, Py), 7.04 (t, J = 5.2 Hz, 1H, Py) 4.80 (dd JHH = 15 Hz, 2H)
ppm.13C{1H} (100 MHz) 155.8 (ArC), 147.7 (ArC), 135.4 (ArC), 133.45 (ArC), 131.8 (ArC),
31
127.6 (ArC), 127.4 (ArC), 121.6 (ArC ), 53.3 (CH2) ppm. P{1H} NMR (161.9 MHz, DMSO-
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d6) δ 69.3 ppm. (C30H26Cl2N2P2Se2Zn) (2, 770.70) Calc. C 46.75, H 3.40, N 3.63; found C 46.49
H 3.28, N 3.52.
Preparation of Compounds 3a and 3b
In a 25 mL pre-dried Schlenk flask, ligand 1 (200 mg, 0.31 mmol) was dissolved in CHCl3 (10
mL, and for 3b 10 mL CH3OH), mixed with anhydrous zinc bromide (71 mg, 0.31 mmol), and
heated at 65°C for six hours. The resulting white residue was further purified by washing with
hexane and crystals suitable for X-ray diffraction analysis were grown from CH3CN/CH3OH
mixture at room temperature.
1
Yield: 260 mg, 96%. 3a: H NMR (400 MHz, DMSO-d6) δ 8.19 (d, J = 4.0 Hz, 1H, Py), 8.00–
7.95 (m, 8H, ArH), 7.43–7.39 (m, 4H, ArH), 7.37–7.35 (dd, J = 8 Hz 1H, Py), 7.32–7.28 (m, 8H,
ArH),7.13 (d, J = 7.8 Hz, 1H, Py), 6.98 (t, J = 4.9 Hz, 1H, Py) 4.76 (dd, JHH = 15 Hz, 2H) ppm.
13C{1H} (100 MHz) 155.8 (ArC), 147.7 (ArC), 135.4 (ArC), 133.45 (ArC), 131.8 (ArC), 127.6
(ArC), 127.4 (ArC), 121.6 (ArC ), 53.3 (CH2) ppm. 31P{1H} NMR (161.9 MHz, DMSO-d6) δ
69.2 ppm. (C30H26Br2N2P2Se2Zn) (3a. 859.6) Calc. C 41.92, H 3.05, N 3.26; found C 41.65 H
2.87, N 3.10. Complex 3b displayed identical NMR and elemental analysis values to those of
complex 3a due to the elimination of the methanol molecule during drying of the complex 3b
crystal under vacuum.
Preparation of Compound 4
In a 25 mL pre-dried Schlenk flask, ligand 1 (200 mg, 0.31 mmol) was dissolved in CHCl3 (10
mL), mixed with anhydrous zinc iodide (101 mg, 0.31 mmol), and heated at 65°C for six hours.
The resulting white residue was further purified by washing with hexane and crystals suitable for
X-ray analysis were grown from CH3CN/CH3OH mixture at room temperature.
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1
Yield: 285 mg, 95%. H NMR (400 MHz, DMSO-d6) δ 8.19 (d, J = 4.0 Hz, 1H, Py), 8.01–7.95
(m, 8H, ArH), 7.42–7.40 (m, 4H, ArH), 7.36–7.34 (dd, J = 8 Hz 1H, Py), 7.31–7.27 (m, 8H,
ArH), 7.13 (d, J = 7.8 Hz, 1H, Py), 6.98 (t, J = 4.9 Hz, 1H, Py) 4.74 (dd, JHH = 16 Hz, 2H) ppm.
13C{1H} (100 MHz) 155.8 (ArC), 147.7 (ArC), 135.4 (ArC), 133.45 (ArC), 131.8 (ArC), 127.6
(ArC), 127.4 (ArC), 121.6 (ArC ), 53.3 (CH2) ppm. 31P{1H} NMR (161.9 MHz, DMSO-d6) δ
69.2 ppm. (C30H26I2N2P2Se2Zn) (4, 953.6) Calc. C 37.79, H 2.75, N 2.94; found C 37.51 H 2.60,
N 2.83.
General Procedure for the Catalytic Addition of Amines with RNCO and RNCS
A solution of primary/secondary amine (0.276 mmol) in toluene (1.0 mL) was added drop-wise
into the reaction mixture of RNCO/RNCS (0.276 mmol) and zinc complex 2 (0.0138 mmol).
This was done in a 25 mL dry Schlenk flask under an atmosphere of nitrogen. The reaction
mixture was stirred for 1–2 hours at room temperature (25°C). The solvent was thereafter
evaporated under vacuum. This resulted in a white solid compound being obtained in each case.
The conversion of amines was calculated from isolated pure products.
General Procedure of Catalytic Addition for Amines with Carbodiimide
A solution of primary/secondary amine (0.276 mmol) in toluene (1.0 mL) was added drop-wise
into the reaction mixture of carbodiimide (0.276 mmol) and complex 2 (0.0138 mmol). This was
done in a 25 mL dry Schlenk flask under an atmosphere of nitrogen. The reaction mixture was
stirred for 8–12 hours at 90˚C. The solvent was thereafter evaporated under vacuum. A white
solid compound was obtained in each case, which was purified by washing several times with n-
hexane. The conversion of amines was calculated from isolated pure products.
Characterization of the Products:
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The data for insertion products 5a, 5b, 5c, 5d, 5e, 5f, 5m, 5h, 5i, 5j, 5k, 5l, and 6a, 6b, 6c, 6d,
6e, 6f, 7a, 7b, 7c, 7d, 7e, 7f, and 7g are already described in literature.13,14,15 The 1H and 13C{1H}
NMR spectra and mass spectra of insertion products, 5a, 5b, 5c, 5d, 5e, 5f, 5m, 5h, 5i, 5j, 5k, 5l,
and 6a, 6b, 6c, 6d, 6e, 6f, 6g, 6h, 6j, 6k, 6l, 7a, 7b, 7c, 7d, 7e, 7f, 7g, and complexes 1 and 2, 3a,
and 4 are given in the Supporting Information.
X-Ray Crystallographic Studies of Compounds L, 1, 3a, 3b and 4
Single crystals of compounds L and 1 were obtained from concentrated toluene and
dichloromethane respectively. However, complexes 3a, 3b, and 4 were re-crystallized from a
mixture of CH3CN and CHCl3 (1:1) at room temperature. In each case a crystal of suitable
dimensions was mounted on a CryoLoop (Hampton Research Corp) with a layer of light mineral
oil at 293 (2) K. All measurements were made on an Oxford Supernova X-calibur Eos CCD
detector with graphite monochromatic Cu-Kα (1.54184 Å) (for complexes L and 1) or Mo-Kα
(0.71069 Å) (for complexes 3a, 3b, and 4) radiation. Crystal data and structure refinement
parameters are summarized in Table 1. The structures were solved by direct methods (SIR2004)
[23] and refined on F2 by full-matrix least-squares methods, using SHELXL-97 [24]. Non-
hydrogen atoms were anisotropically refined. Hydrogen atoms were included in the refinement
on calculated positions riding on their carrier atoms. The function minimized was [w(Fo2 –
Fc2)2] (w = 1 / [2 (Fo2) + (aP)2 + bP]), where P = (Max(Fo2,0) + 2Fc2) / 3 with 2(Fo2) from
counting statistics. The function R1 and wR2 were (||Fo| – |Fc||) / |Fo| and [w(Fo2 – Fc2)2 /
(wFo4)]1/2 respectively. The ORTEP-3 program was used to draw the molecule.
Crystallographic data (excluding structure factors) for the structures reported in this paper have
been deposited with the Cambridge Crystallographic Data Centre as supplementary publication
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no. CCDC 1540980-1540984. Copies of the data can be obtained free of charge on application to
CCDC, 12 Union Road, Cambridge CB21EZ, UK [fax: + (44)1223-336-033; email:
deposit@ccdc.cam.ac.uk].
Conclusion
We have synthesized
a
series of zinc complexes using 2-picolylamino-
(diphenylphosphinoselenoic)amide ligand. The solid-state structure each of complexes 3a and 4
exhibited a seven-membered zinc metallacycle through the chelation of selenium and pyridine
nitrogen atoms of ligand 1. Zinc complex 2 acted as a competent pre-catalyst for the
hydroamination of primary/secondary amines to carbodiimides, isocyanates, and isothiocyanates
to yield guanidine, urea, and thio-urea derivatives. It also exhibited an unusual tolerance toward
several heteroatoms such as oxygen, sulfur, and nitrogen, in addition to a number of functional
groups such as NO2, F, Cl, and I, thus manifesting its versatility as a pre-catalyst for this
reaction. A most plausible mechanism is also proposed.
Acknowledgments
This work was supported by the Science and Engineering Research Board (SERB), Department
of Science and Technology (DST), Government of India, under project no. (SB/S1/IC/045/2013).
A.H. and K.B. thank CSIR, India, for their PhD fellowships.
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Table 1. Crystal Structure and Data Refinement Parameters of L, 1, 3a, 3b and 4a.
Compound
CCDC No.
Empirical
Formula
Formula
Weight
L
1
3a
1540981
C31H27Br2Cl3N2P2 C61H56Br2N4OP4-
3b
1540983
4
1540982
1540980
1540984
C30H26I2N2P2Se2-
Zn
C30H26N2P2
476.47
C30H26N2P2Se2
634.39
Se2Zn
Se4Zn
1526.01
978.95
953.58
T (K)
λ (Å)
Crystal System
Space Group
a / Å
293 (2)
293(2)
293 (2)
293(2)
293(2)
0.71073
Monoclinic
P 21
9.9343 (2)
15.0894(3)
11.3133(2)
90
107.199(2)
90
1620.06(6)
2
1.54184
Triclinic
P -1
1.54184
Monoclinic
P 21/c
9.7110 (2)
17.4530(3)
18.5312(5)
90
111.053(2)
90
2931.13(12)
4
0.71073
Triclinic
P -1
0.71073
Monoclinic
P 21
9.2580(4)
17.4638(6)
19.0130(9)
90
101.102(4)
90
3016.5(2)
2
10.2786(8)
10.5273(7)
13.1034(9)
70.173(6)
77.655(6)
70.590(7)
1249.82(15)
2
9.2688(6)
13.0282(7)
15.7475(9)
88.824(4)
75.432(5)
82.955(5)
1826.46(19)
2
b / Å
c / Å



V / Å3
Z
Dcalc (g cm-3)
µ (mm-1)
1.266
1.731
1.438
4.342
1.780
5.185
1.955
5.032
1.254
2.569
0.27 x 0.21 x
0.19
Colorless /
Block
0.28 x 0.26 x 0.19
Crystal Size
0.33 x 0.31 x 0.23 0.32 x 0.25 x 0.21
0.28 x 0.26 x 0.19
Colorless / Block
Colorless / Block
Color / Shape
Colorless / Block
Colorless / Block
Theta range for
data collection
Reflections
collected /
3.61 to 71.52
deg.
8301/ 4754
[R(int) =
0.0262]
3.598 to 71.172
deg.
2.925 to 29.037
deg.
4.8330 to 71.444
deg
15291/11468
[R(int) = 0.0337]
3.249 to 29.025
deg.
12122 / 5576
[R(int) = 0.0269]
16576/ 8428
[R(int) = 0.0418]
10310/ 6175
[R(int) = 0.0421]
unique
Completeness
to theta = 71.25
Absorption
99.8%
97.7 %
99.9 %
99.8 %
99.8 %
Multi-scan
Multi-scan
Multi-scan
Multi-scan
Multi-scan
correction
1.00000 and
0.53572
Max. and min.
transmission
1.000 and
0.83536
1.000 and
0.79676
1.000 and
0.62322
1.000 and
0.36128
Full-matrix
least-squares
on F2
Full-matrix least-
Refinement
method
Full-matrix least-
squares on F2
Full-matrix least-
squares on F2
Full-matrix least-
squares on F2
squares on F2
11468/3/696
1.030
Data / restraints
/ parameters
Goodness-of-fit
on F2
4754 / 0 / 307
5576 / 0 / 326
8428 / 0 / 388
1.024
6175 / 1 / 353
0.995
1.098
1.051
Final R indices
[I>2(I)]
R indices (all
data)
R1 = 0.0545,
wR2 = 0.0919
R1 = 0.0901,
wR2 = 0.1058
R1 = 0.0546,
wR2 = 0.1584 wR2 = 0.2450
R1 = 0.0716,
wR2 = 0.1711
R1 = 0.0725,
R1 = 0.0643,
wR2 = 0.1402
R1 = 0.1145,
wR2 = 0.1682
R1 = 0.0496,
wR2 = 0.0909
R1 = 0.0682,
wR2 = 0.1041
R1 = 0.0828,
wR2 = 0.2587
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Largest peak
and hole e.Å-3
0.720 and -
0.328
3.603 and -0.468
3.603 and -0.468
1.529 and -1.041
0.838 and -0.850
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Table 2. Comparison of selected bond lengths [Å] and bond angles [°] of different zinc
complexes 2, 3a, 3b, and 4.
3a
3b
2.071(8)
--------
4
2.077(9)
2.5222(17)
2.101(3)
-----
Zn–N
Zn–Se
2.086(5)
2.5018(11)
2.1501(17)
--
P–Se
2.094(3)
2.028(7)
--------
Zn–O1
N2–Zn–Se2
Se2–Zn–X2
N2–Zn–X2
107.78(13)
112.36(14)
105.41(15)
106.1(2)
107.86(6)
102.4(3)
--------
109.9(2)
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Table 3. Comparison of activity of zinc catalysts 2, 3a, and 4 for the hydroamination of
heterocumulenes with PhNH2.
Time
(h)
1
Yield
(%)
99
Entry Amine
Heterocumulenes
Catalyst Solvent
T(oC)
RT
1
PhNH2 PhNCO
Toluene
2
PhNH2 PhNCO
PhNH2 PhNCO
PhNH2 PhNCS
PhNH2 PhNCS
PhNH2 PhNCS
RT
2
2
Toluene
Toluene
70
81
3a
4
RT
RT
RT
RT
3
2
Toluene
Toluene
Toluene
Toluene
Toluene
4
1
99
65
2
5
1.5
1.5
8
3a
4
6
75
PhNH2
DCC
7
90
97
55
2
PhNH2
8
12
3
90
DCC
3a
2
PhNH2 PhNCS
PhNH2 PhNCS
RT
9
70
60
THF
RT
10
3
CH2Cl2
Toluene
Toluene
2
11
12
PhNH2 PhNCO
PhNH2 DCC
1
6
90
90
trace
trace
None
ZnCl2
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Table 4. Formation of insertion products of various amines and heterocumulenes catalyzed by
zinc complex 2.a
Yield (%)b
Entry
Hetero-cumulenes
PhNCO
Amine
tBuNH2
Product
(isolated yield)c
99 (95)c
1
2
3
5a
5b
5c
5d
5e
5f
5g
5h
5i
5j
5k
5l
6a
PhNCO
Pyrrolidine
N,N’-iPr2amine
99 (95)
99 (96)
PhNCO
4
5
6
PhNCO
PhNCO
PhNCO
Aniline
MesNH2
Dipp-NH2
99 (95)
85 (80)
90 (86)
7
8
9
PhNCO
Cl-PhNCO
Cl-PhNCO
Cl-PhNCO
TolNCO
o-F-C6H4NH2
Et2NH
Trace
99 (96)
97 (93)
95 (90)
96 (92)
95 (91)
Pyrrolidine
MesNH2
10
11
12
Pyrrolidine
MesNH2
TolNCO
13
14
PhNCS
PhNCS
tBuNH2
99 (96)
99 (95)
Pyrrolidine
6b
15
16
17
PhNCS
PhNCS
PhNCS
N,N’-iPr2-amine
PhNH2
99 (96)
99 (95)
90 (86)
6c
6d
MesNH2
6e
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18
19
20
PhNCS
PhNCS
PhNCS
DippNH2
90 (85)
95 (91)
90 (85)
6f
2-methyl
aniline
6g
6h
4-methoxy
aniline
21
22c
23
PhNCS
PhNCS
PhNCS
o-NO2-C6H4NH2
o-F-C6H4NH2
o-Cl-C6H4NH2
89 (84)
90 (84)
90 (84)
6i
6j
6k
6l
7a
7b
7c
7d
24
25
26
PhNCS
o-Br-C6H4NH2
PhNH2
25 (20)
97 (92)
60 (55)
CyNCNCy
CyNCNCy
o-NO2 C6H4NH2
27
28
29
CyNCNCy
CyNCNCy
CyNCNCy
m-NO2-C6H4NH2
o-F-C6H4NH2
63 (58)
57 (52)
60 (55)
o-Cl-C6H4NH2
7e
7f
30
31
CyNCNCy
CyNCNCy
o-I-C6H4NH2
p-I-C6H4NH2
70 (64)
72 (68)
7g
Reaction conditions: Zinc complex 2 (5 mol%), toluene solvent, room temperature, 2 hours
b
(optimized reaction time). Conversions were obtained from integration of signals in the 1H NMR
c
spectra on the basis of consumption of amine. Yields were calculated from isolated pure products
isolated and purified by washing with n-pentane. For 7a–7g, reactions were performed at 90°C for
12 hours.
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Figures 1a, and 1b. ORTEP drawing of the molecular structures of L (left) and 1 (right);
ellipsoids are drawn to encompass 30% probability while omitting hydrogen atoms for clarity.
Selected bond lengths [Å] and bond angles[o]: L: P1–N2 1.736(2), P2–N2 1.710(2), P1–C7
1.827(3), N2–C6 1.474(3), N1–C1 1.333(4), C1–C2 1.373(4), P2–N2–P1 121.66(12), N2–P1–C7
104.7(3), C5–N1–C1 117.5(3). 1: P1–Se1 2.094(16), P1–N2 1.719(5), P2–N2 1.711(5), P1–C7
1.818(6), N2–C6 1.491(7), N1–C1 1.347(8), C1–C2 1.373(11), N2–P1–Se1 113.15(17), P2–N2–
P1 127.1(3), C7–P1–Se1 112.5(2), N2–P1–C7 104.7(3), C5–N1–C1 117.5(3).
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Figures 2a and 2b. ORTEP drawing of the molecular structures of complexes 3a (left) and 3b
(right); ellipsoids are drawn to encompass 30% probability while omitting hydrogen atoms for
clarity. Selected bond lengths [Å] and bond angles [°]: 3a: Zn1–Se2 2.5018(11), Zn1–N2
2.086(5), P2–N1 1.706(5), P1–C13 1.800(6), N1–C6 1.506(7), N2–C1 1.341(8), N2–Zn1–Br1
109.57(17), P2–Se2–Zn1 99.37(5), Br1–Zn1–Br2 115.33(4), Br2–Zn1–Se2 112.36(4), N2–Zn1–
Br1 109.57(15). 3b: Zn1–O1 2.028(7), Zn1–N1 2.071(8), P2–N2 1.726(7), P1–C13 1.800(9),
N2–C6 1.496(10), N1–C1 1.359(12), N1–Zn1–O1 96.1(17), Br1–Zn1–Br2 115.72(6), Br2–Zn1–
O1 105.0(2), N1–Zn1–Br1 109.9(2).
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