Unusual activation pathways of amines in the reactions with molybdenum pentachloride

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Unusual activation pathways of amines in the

reactions with molybdenum pentachloride†

Cite this:DOI: 10.1039/c6nj03992h

Niccolo Bartalucci,^aMarco Bortoluzzi,^bFabio Marchetti, *^aGuido Pampaloni,^a

`

c

Silvia Schoch^aand Stefano Zacchini

*

The 1 : 1 molar reactions at room temperature of MoCl₅with aliphatic amines were investigated in

dichloromethane. Pyrrolidine, diethylamine and dibenzylamine underwent dehydrogenative oxidation

when allowed to react with MoCl₅; the compounds [MoCl₅{NCH(CH₂)₃}], 1, and [CH₃CHQNHEt][MoOCl₄],

2, were isolated in moderate to low yields from MoCl₅/pyrrolidine and MoCl₅/NHEt₂, respectively. The

chloride–amide complex [MoCl₄(NEt₂)], 3, was afforded in 65% yield from MoCl₅and Et₂NSiMe₃. The inter-

action of MoCl₅with Me₂NSiMe₃was accompanied by activation of the solvent, and the complexes

[MoCl₃(NMe₂)(k²-Me₂NCH₂NMe₂)], 4a, and [MoCl₃(NMe)(k²-Me₂NCH₂NMe₂)], 4b, co-crystallized from the

reaction mixture. The reactions of MoCl₅with a series of primary amines afforded mixtures of products,

t

and the Mo(^VI) chloride imido complexes [MoCl₄(NR)]₂(R = Cy, 5a; Bu, 5b) were isolated in ca. 40% yield

t

from MoCl₅/NH₂R (R = Cy, Bu). C–H bond activation may be viable in the reactions of MoCl₅with tertiary

amines: the compounds [(CH₂Ph)₂NQCHPh]₂[MoCl₆]ꢀCH₂Cl₂, 6, and [NHEt₃]₂[Mo₂Cl₁₀], 7, were obtained

from MoCl₅/tribenzylamine and MoCl₅/triethylamine, respectively. Pyrrolidine and tribenzylamine under-

went analogous activation pathways when allowed to react with [MoCl₃{OCH(CF₃)₂}]₂in the place of

MoCl₅. The isolated metal products were characterized by analytical and spectroscopic techniques,

in addition the structures of 1, 2, 4, 5a, 6ꢀCH₂Cl₂and 7 were ascertained by single crystal X-ray

diffraction studies. The organic products were identified by NMR and GC-MS after hydrolysis of the

reaction mixtures. DFT calculations were carried out in order to assist the IR assignments, and clarify struc-

tural and mechanistic aspects.

Received 19th December 2016,

Accepted 2nd May 2017

DOI: 10.1039/c6nj03992h

rsc.li/njc

tetrahalides,²and Nb and Ta pentahalides.³Nevertheless alter-

native reaction pathways may be working, and this is probably

Introduction

Homoleptic chlorides of high valent transition metals have been the reason why mixed chloride amide complexes have been more

employed as eﬀective catalytic precursors in a variety of organic frequently prepared by treatment of the parent metal chlorides

reactions.¹These compounds typically manifest their strong with lithium amides.⁴More precisely, when the metal chloride is

acidic nature towards primary and secondary amines, aﬀording allowed to react with an excess of primary amine, the formation

amido-derivatives via aminolysis reactions. This behaviour has of metal-amido species (M–NHR) may be followed by proton

been well established with reference to group

4

metal abstraction affording imido ligands (MQNR). This usually takes

place without a change in the oxidation state of the metal

centre.^2e,3d,5Alternatively, imido derivatives have been obtained

by the addition of a suitable base to the metal chloride/amine

system.^3c,6In some cases, high valent transition metal chlorides

act as single electron oxidant reagents towards amines, the

amines possibly converting into the relevant iminium cations.^3a,7

This kind of reactivity usually regards tertiary amines, and, for

a

`

Universita di Pisa, Dipartimento di Chimica e Chimica Industriale,

Via Moruzzi 13, I-56124 Pisa, Italy. E-mail: fabio.marchetti1974@unipi.it;

Web: http://www.dcci.unipi.it/fabio-marchetti.html; Tel: +39 050 2219245

b

c

`

Universita di Venezia Ca’ Foscari, Dipartimento di Scienze Molecolari e

Nanosistemi, Via Torino 155, I-30170 Mestre (VE), Italy

`

Universita di Bologna, Dipartimento di Chimica Industriale ‘‘Toso Montanari’’,

Viale Risorgimento 4, I-40136 Bologna, Italy. E-mail: stefano.zacchini@unibo.it;

Web: https://www.unibo.it/sitoweb/stefano.zacchini

instance, the redox interaction between TiCl₄and trialkylamines

† Electronic supplementary information (ESI) available: Fig. S1 and S2 show DFT- constitutes an efficient catalytic system serving diverse organic

transformations, such as the C–C coupling of esters,⁸the synthesis

calculated structures, while Table S1 contains the computed relevant bonding

of 2,5-diarylpyrroles⁹and a,b-unsaturated carbonyl compounds.¹⁰

parameters for 1. Cartesian coordinates of the DFT-optimized compounds are

1

reported in a separated .xyz file. CCDC 1491992 (1), 1491993 (2), 1491991 (4ꢀ₂CH₂Cl₂),

Being involved with the coordination chemistry of group 6 chlorides,

we have recently elucidated the reactions of WCl₆with limited

1491994 (5a), 1511406 (6ꢀCH₂Cl₂) and 1511407 (7) contain the supplementary

crystallographic data for the X-ray studies reported in this paper. For ESI and

crystallographic data in CIF or other electronic format see DOI: 10.1039/c6nj03992h amounts of tribenzylamine and triphenylamine, respectively.

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Both the reactions proceed with single electron transfer from

the organic reactant to the metal centre, followed by C–H bond

activation and intermolecular hydrogen migration.¹¹

In this overall scenario, information on the direct interaction of

molybdenum pentachloride, MoCl₅, with amines still remain rather

sparse in the literature. Simple coordination adducts were claimed to

be formed by combination of MoCl₅with tertiary aliphatic amines.¹²

On the other hand, based on elemental analyses, it was suggested

that the reactions with an excess of primary and secondary aliphatic

amines aﬀorded mixed chloride amide complexes, as expected from

aminolysis processes.¹²Later on, Nielson reported the Mo(V)

compound [Mo(NCMe₃)(NHCMe₃)Cl₂(NH₂CMe₃)]₂, containing

amino, amido and imido groups, as the prevalent product of

the reaction of MoCl₅with six equivalent of tert-butylamine in

benzene.⁵Similarly, [Mo(NCMe₃)Cl₃(NH₂CMe₃)]₂was obtained

from MoCl₅and Me₃SiNHCMe₃(1: 2 ratio).⁵

Scheme 1 MoCl₅-directed oxidation of dialkylamines.

In the present paper, we describe the results of our investi-

gation on the reactivity of MoCl₅with a selection of primary,

secondary and tertiary aliphatic amines, including N-(trimethyl-

silyl)dialkylamines, in a weakly coordinating solvent. A com-

parison with the analogous reactions of the Mo(^V) chloride

13

alkoxide compound MoCl₃[OCH(CF₃)₂]₂will be discussed.

Scheme 2 Isolation of metal complexes from the reactions of MoCl₅with

pyrrolidine and diethylamine.

Results and discussion

1. Reactions of MoCl₅with secondary amines

found in 2 confirms the occurrence of dehydrogenative oxidation

of diethylamine by means of MoCl₅(Scheme 1); the presence of

the oxido ligand within the counterion appears to be the result

of fortuitous hydrolysis of Mo–Cl bonds in the course of the

crystallization procedure.¹⁷Views of the X-ray structures of 1 and 2

are given in Fig. 1 and 2, the relevant bonding parameters being

reported in Tables 1 and 2.

Compound 1 represents a very rare example of crystallographi-

cally characterized MoCl₅L (L = organic molecule) complex.¹⁸The

Mo(^V) centre is octahedrally coordinated to five chlorides and one

pyrroline (3,4-dihydro-2H-pyrrole) ligand. Pyrroline is a valuable

compound¹⁹which has been produced from pyrrolidine by means

of various oxidants.²⁰

We were interested in the synthesis of Mo(^V) mixed chloride

amide compounds of the type MoCl_x(NR₂)_5ꢁx. According to the

information available in the literature,¹²we performed the reac-

tions of MoCl₅with a selection of secondary aliphatic amines.

Surprisingly, these reactions did not furnish the expected products.

When MoCl₅was allowed to react with pyrrolidine, diethylamine

and dibenzylamine, in 1 : 1 molar ratios in CH₂Cl₂, mixtures of

metal products were obtained which could not be fully identified

(vide infra). In order to understand the destiny of the organic

material, we treated the respective solid residues with CDCl₃/

water.^14,15Subsequent NMR analyses pointed out the prevalent

formation of dehydrogenative oxidation products (Scheme 1). More

in detail, almost complete conversion of pyrrolidine to pyrroline

was ascertained, while ca. 50% of dibenzylamine was found

converted into the relevant imine. Pyrroline was detected in its

monomeric form, and no evidence for oligomerization was

found.¹⁶Acetaldehyde was the only product detected from

MoCl₅/NHEt₂, the aldehyde presumably generating from

N-ethylidene ethanamine in hydrolytic conditions (see below).

Several attempts were conducted in order to isolate metal

products from the highly moisture sensitive reaction systems. Hence,

[MoCl₅{NCH(CH₂)₃}], 1, and minor amounts of [CH₃CHQNHEt]-

[MoOCl₄], 2, were isolated as crystalline materials from

MoCl₅/pyrrolidine and MoCl₅/NHEt₂, respectively, and X-ray

characterized (Scheme 2).

A limited number of metal complexes containing pyrroline

are known and in all of them the pyrroline ligand is bonded

Compounds 1 and 2 contain a pyrroline ligand and a

N-ethylidenium ethanamine cation, respectively, coherently

with the NMR analyses on the corresponding hydrolyzed reac-

tion mixtures. In particular, the detection of [MeCHQNHEt]⁺as

Fig. 1 Molecular structure of 1. Displacement ellipsoids are at the 50%

probability level.

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H-bonds with the terminal Cl ligand of the anion [N(1)–H(1) 0.88 Å,

H(1)ꢀꢀꢀCl(4)#1 2.92 Å, N(1)ꢀꢀꢀCl(4)#1 3.770(3) Å, oN(1)H(1)Cl(4)#1

163.71; N(1)–H(1) 0.88 Å, H(1)ꢀꢀꢀCl(3)#1 2.99 Å, N(1)ꢀꢀꢀCl(3)#1

3.569(4) Å, oN(1)H(1)Cl(3)#1 125.11; N(1)–H(1) 0.88 Å, H(1)ꢀꢀꢀ

Cl(1)#2 2.92 Å, N(1)ꢀꢀꢀCl(1)#2 3.423(4) Å, oN(1)H(1)Cl(1)#2 117.71;

1

symmetry transformations used: #1 x + ¹₂, ꢁy + ₂¹, z ꢁ

;

2

1

#2 ꢁx + ¹₂, y ꢁ , ꢁz + ¹₂].

2

The [Mo₂O₂Cl₈]^2ꢁanion is located on an inversion centre

and displays a dimeric structure, approximately consisting of

two edge-sharing octahedra, as previously found in related

salts.²⁶The Mo(1)–O(1) bond [1.648(2) Å] reveals a strong

p-character, as expected for a Mo(^V)QO unit.²⁶The chloride

bridges are very asymmetric, being Mo(1)–Cl(4) [2.4055(9) Å], trans

to Cl(2), considerably shorter than Mo(1)–Cl(4)_1 [2.865(2) Å],

trans to the stronger oxido ligand.

The magnetic analyses of 1 and 2 were indicative of Mo(^V)

chloride derivatives.³³In the IR spectrum of 1, diagnostic

Fig. 2 Molecular structure of 2. Displacement ellipsoids are at the 50%

probability level. Symmetry transformations used to generate equivalent

atoms: ꢁx, ꢁy + 1, ꢁz + 1.

absorption for the imine group has been detected at 1572 cm^ꢁ1

.

The IR spectrum of 2 displays the absorptions related to the

[CQN] and [MoQO]³³moieties at ca. 1700 and at 989 cm^ꢁ1

respectively.

,

Table 1 Selected bond distances (Å) and angles (1) for 1

The oxidative dehydrogenation of secondary amines to

imines is a known process²⁷with implications in the hydrogen

storage issue,²⁸nevertheless this reaction is unusual in the

landscape of the chemistry of metal chlorides.²⁹In the present

case, it is presumable that the Mo(^V) centre exerts its oxidant

power³⁰towards the amine reactants, even though GC analyses

on MoCl₅/diethylamine and MoCl₅/pyrrolidine mixtures evidenced

the release of very low amounts of H₂as possible reduction

product. As a matter of fact, magnetic analysis on the pyrrolidine/

MoCl₅reaction residue suggested the presence of a mixture

of Mo(^V) and Mo(IV) species (m = 1.91 BM). The formation of

Mo(^IV) by-products is in agreement with the computed thermo-

dynamic variation for the reaction reported in eqn (1), DG =

ꢁ72.3 kcal mol^ꢁ1. Mo(IV) chloride was modelled on the basis of

the hexameric structure reported in the literature.³¹

Mo(1)–N(1)

Mo(1)–Cl(2)

Mo(1)–Cl(4)

N(1)–C(1)

C(1)–C(2)

C(2)–C(3)

2.193(3)

2.2804(9)

2.2730(10)

1.281(4)

1.481(5)

1.541(5)

Mo(1)–Cl(1)

Mo(1)–Cl(3)

Mo(1)–Cl(5)

N(1)–C(4)

2.3252(8)

2.3257(8)

2.2973(9)

1.477(4)

1.526(5)

C(3)–C(4)

Cl(1)–Mo(1)–Cl(3)

Cl(5)–Mo(1)–N(1)

Mo(1)–N(1)–C(4)

N(1)–C(1)–C(2)

C(2)–C(3)–C(4)

176.33(3)

177.62(7)

124.8(2)

115.4(3)

104.1(3)

Cl(2)–Mo(1)–Cl(4)

Mo(1)–N(1)–C(1)

C(1)–N(1)–C(4)

C(1)–C(2)–C(3)

C(3)–C(4)–N(1)

165.07(3)

125.3(2)

109.9(3)

102.0(3)

105.4(3)

Table 2 Selected bond distances (Å) and angles (1) for 2

Mo(1)–O(1)

1.648(2)

Mo(1)–Cl(1)

2.3718(10)

Mo(1)–Cl(2)

Mo(1)–Cl(4)

N(1)–C(1)

2.3380(10)

2.4055(9)

1.472(5)

Mo(1)–Cl(3)

Mo(1)–Cl(4_1)

N(1)–C(3)

2.3650(10)

2.865(2)

1.276(5)

1.467(6)

1.5Mo₂Cl₁₀+ NH(CH₂)₄- [MoCl₅{NCH(CH₂)₃}] + 2HCl

C(1)–C(2)

1.501(5)

C(3)–C(4)

1

+ Mo₆Cl₂₄

(1)

3

Cl(1)–Mo(1)–Cl(3)

O(1)–Mo(1)–Cl(4_1)

O(1)–Mo(1)–Cl(2)

C(1)–C(1)–N(1)

163.47(3)

175.47(3)

102.66(9)

109.8(3)

122.3(4)

Cl(2)–Mo(1)–Cl(4)

Cl(4)–Mo(1)–Cl(4_1)

Mo(1)–Cl(4)–Mo(1_1) 103.93

C(1)–N(1)–C(3) 125.1(4)

157.89(3)

76.07(3)

2. Reactions of MoCl₅with N-(trimethylsilyl)dialkylamines

With the idea in mind to access Mo(V) chloride amide com-

pounds, we moved to study the reactions of MoCl₅with variable

amounts of N-(trimethylsilyl)dialkylamines, these being expected

to act as clean Cl/NR₂exchangers.^5,32Indeed the 1 : 1 molar

reaction of MoCl₅with Et₂NSiMe₃in dichloromethane aﬀorded

the chloride amide compound [MoCl₄(NEt₂)], 3, which was

isolated in 65% yield after work up (eqn (2)).

N(1)–C(3)–C(4)

Symmetry transformations used to generate equivalent atoms: ꢁx, ꢁy + 1,

ꢁz + 1.

through the N-atom,²¹apart one case in which pyrroline is Z²

coordinated via the imine bond.²²The Mo(1)–N(1) distance

[2.193(3) Å] is typical of a N(sp²)–Mo(V) dative bond,²³while

C(1)–N(1) [1.281(4) Å] is an imine double bond.^21,22,24

MoCl₅þ Et₂NSiMe₃! ½MoCl₄ðNEt₂Þꢂ þ Me₃SiCl

(2)

3

Compound 2 is an ionic one, composed of [MeCHQNHEt]⁺

cations and [Mo₂O₂Cl₈]^2ꢁanions. The structure of [MeCHQNHEt]⁺The synthesis of 3 originates from the selective Cl/NEt₂

is unprecedented, but closely related to the crystallographically exchange between the reactants. The use of two/three equi-

characterized cation [Me₂CQN(H)(Et)]⁺.²⁵The iminium N(1)–C(3) valents of Et₂NSiMe₃resulted in the formation of mixtures of

interaction [1.276(5) Å] displays a typical double bond character. products. Compound 3 was characterized by elemental analy-

In addition, the NH iminium group is involved in inter-molecular sis, magnetic analysis and IR spectroscopy. On account of the

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fact that crystallographic characterizations of Mo(V) chloride

amide complexes are still absent in the literature, we made

several attempts in order to obtain X-ray quality crystals of 3.

Unfortunately, these attempts were not successful. In the

absence of X-ray data, we performed DFT calculations aimed to

the prediction of the most stable structure. A range of possibi-

lities were considered (see Fig. S2, ESI,† structures 3A–3C); the

mononuclear structure (3-mono) and the dinuclear one with

the amido groups in relative trans-equatorial position (3A)

resulted the most stable ones, exhibiting strictly comparable

relative Gibbs energies (Fig. 3a and b). It should be noted that

the crystallographically characterized complex [NbCl₄(NEt₂)]

displays a dinuclear structure matching that of 3A.^3aDFT

calculations ruled out the possibility of Mo–Mo covalent inter-

action in 3A, being the triplet state more stable than the

corresponding singlet one by about 11 kcal mol^ꢁ1. This is in

agreement with the magnetic measurement performed on 3

(m = 1.44 BM), indicating the presence of isolated Mo(^V) centers.³³

The reaction of MoCl₅with one equivalent of Me₂NSiMe₃in

dichloromethane led to a mixture of metal compounds presum-

Fig. 4 Molecular structure of 4. Displacement ellipsoids are at the 50%

probability level.

Table 3 Selected bond distances (Å) and angles (1) for 4

Mo(1)–Cl(1)

Mo(1)–Cl(3)

Mo(1)–N(2)

N(1)–C(3)

2.3954(15)

2.3817(14)

2.255(5)

1.487(7)

1.484(8)

2.3941(16)

2.3790(16)

2.402(5)

1.496(7)

1.409(9)

Mo(1)–Cl(2)

Mo(1)–N(1)

Mo(1)–N(3)

N(2)–C(3)

2.4289(15)

2.332(5)

1.911(5)

1.500(7)

1.465(8)

2.4102(16)

2.235(5)

1.746(5)

1.481(7)

C(6)–N(3)

C(7)–N(3)

Mo(2)–Cl(4)

Mo(2)–Cl(6)

Mo(2)–N(5)

N(4)–C(10)

C(13)–N(6)

Mo(2)–Cl(5)

Mo(2)–N(4)

Mo(2)–N(6)

N(5)–C(10)

ably containing a prevalence of Mo(^V) centres (m = 1.33 BM).³³

A

crystallization procedure furnished few X-ray quality crystals (4).

These crystals consist of a 1 : 1 mixture of the Mo(^IV) complex

[MoCl₃(NMe₂)(k²-Me₂NCH₂NMe₂)], 4a, and the Mo(V) complex

[MoCl₃(NMe)(k²-Me₂NCH₂NMe₂)], 4b. The two structures are

shown in Fig. 4, while relevant bonding parameters are reported

Cl(1)–Mo(1)–Cl(2)

N(1)–Mo(1)–N(3)

Mo(1)–N(3)–C(6)

C(6)–N(3)–C(7)

Cl(4)–Mo(2)–Cl(5)

N(5)–Mo(2)–N(6)

175.26(5)

160.62(18)

124.9(4)

109.9(5)

169.13(6)

156.9(2)

Cl(3)–Mo(1)–N(2)

N(1)–Mo(1)–N(2)

Mo(1)–N(3)–C(7)

Cl(6)–Mo(2)–N(4)

N(4)–Mo(2)–N(5)

Mo(2)–N(6)–C(13)

158.21(13)

62.85(16)

124.5(4)

159.51(13)

61.62(17)

166.3(5)

in Table 3. In both 4a–b, the Mo centres display a distorted

octahedral geometry, being bonded to three chlorides in mer

position, a chelating k²-Me₂NCH₂NMe₂ligand and an imido

[NMe] (4a) or [amido] NMe₂(4b) ligand. As far as we are aware,

4a–b represent the first cases of structurally characterized

Mo-complexes with the Me₂NCH₂NMe₂ligand, and only a few

examples of Co, Ni, Fe and Re complexes with the same ligand

have been found within the Cambridge Crystallographic Data

Centre.³⁴The Mo(1)–N(1) [2.332(5) Å] and Mo(1)–N(2) [2.255(3) Å]

contacts of 2a are considerably longer than Mo(1)–N(3) [1.911(5) Å],

in view of the amido nature of the latter group.³⁵A further decrease

in the related Mo(2)–N(6) distance [1.746(5) Å] is observed in the

imido complex 2b, in keeping with previous findings.³⁶

The presence of Me₂NCH₂NMe₂in 4a,b is the result of the

involvement of the solvent (CH₂Cl₂) in the Cl/NMe₂exchange.³⁷

Accordingly, significant amounts of Me₂N(CH₂)₂NMe₂were

recovered after hydrolysis of the MoCl₅/Me₂NSiMe₃mixture in

1,2-dichloroethane.^37bNeither Me₂NCH₂NMe₂nor Me₂N(CH₂)₂NMe₂

were recognized from the reaction of MoCl₅with Me₂NSiMe₃in

heptane, although even in this case unambiguous characterization

Fig. 3 (a) DFT-optimized geometry of 3A, C-PCM/-B97X calculations.

Selected computed bond lengths (Å): Mo–N 1.878, 1.885; Mo–Cl(terminal)

2.295, 2.296, 2.316, 2.324, 2.325, 2.326; Mo–Cl(bridging) 2.504, 2.514, 2.713, of the metal products failed.

2.719; MoꢀꢀꢀMo 4.016. Selected computed angles (deg): N–Mo–Cl(terminal)

97.1, 100.5, 92.3, 92.8, 93.1, 94.6; Cl(bridging)–Mo–Cl(bridging) 79.5, 79.8; 3. Reactions of MoCl₅with primary amines

Mo–Cl(bridging)–Mo 100.3, 100.4. (b) DFT-optimized geometry of 3-mono,

C-PCM/-B97X calculations. Selected computed bond lengths (Å): Mo–N 2.022;

Mo–Cl(apical) 2.389; Mo–Cl(equatorial) 2.263, 2.287, 2.308. Selected computed

angles (deg): N–Mo–Cl(apical) 176.4; N–Mo–Cl(equatorial) 87.5, 90.1, 91.2.

t

i

The 1 : 1 reactions of MoCl₅with NH₂R (R = Cy, Bu, Pr, 2,6-

C₆H₃Me₂, CH₂Ph) were studied. These reactions aﬀorded, after

elimination of the volatile materials, paramagnetic solid

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Table 4 Selected bond distances (Å) and angles (1) for 5a

mixtures. After hydrolytic treatment of the latter, the starting

amines were identified (NMR) as largely prevalent components

in the respective organic phases. Only traces of NH₂QCHPh

were recognized from MoCl₅/NH₂CH₂Ph. These results indicate

that primary amines are generally not prone to oxidation

by molybdenum pentachloride, in contrast to what seen for

secondary amines. Numerous attempts were performed with

the aim of isolating clean metal products from MoCl₅/NH₂R.

These attempts were successful in two cases, thus the Mo(^VI)

imido chloride complexes [MoCl₄(NR)]₂(R = Cy, 5a; R = ^tBu, 5b)

were isolated in ca. 40% yields by crystallization procedures

Mo(1)–Cl(1)

Mo(1)–Cl(3)

Mo(1)–Cl(4)

N(1)–C(1)

C(2)–C(3)

2.2966(10)

2.4562(10)

2.3194(9)

1.444(4)

1.517(5)

1.524(5)

Mo(1)–Cl(2)

Mo(1)–Cl(3_1)

Mo(1)–N(1)

C(1)–C(2)

C(3)–C(4)

C(5)–C(6)

2.2996(10)

2.6996(11)

1.689(3)

1.543(5)

1.540(5)

1.522(5)

C(4)–C(5)

C(6)–C(1)

1.546(5)

Cl(1)–Mo(1)–Cl(3)

N(1)–Mo(1)–Cl(3_1)

Mo(1)–N(1)–C(1)

166.62(3)

172.23(9)

175.3(2)

Cl(2)–Mo(1)–Cl(4)

Cl(3)–Mo(1)–Cl(3_1)

Mo(1)–Cl(3)–Mo(1_1)

165.62(3)

79.03(3)

100.97(3)

t

from MoCl₅/NH₂R (R = Cy, Bu), Scheme 3.

The synthesis of the Mo(^VI) based imido complexes 5a,b

represents a very unusual result in the context of the chemistry

of metal halides with aliphatic amines. Indeed imido ligands

have been typically introduced in metal complexes via deprotona-

tion of amines or amides (see Introduction).³⁹This synthetic

strategy does not imply any change in the oxidation state of the

metal centre, unless specific oxidative co-reactants are involved.⁴⁰

Imido ligands have been generated also by metathesis of metal

oxide chlorides with isocyanates⁴¹and by addition of azides,

RN₃, to metal complexes. The latter approach has been success-

fully employed for the synthesis of [Mo^(VI)Cl₄(NR)(thf)] from

[Mo^(IV)Cl₄(thf)₂],⁴²the oxidation of the metal centre being permitted

by N₂release.

DFT calculations were performed on the MoCl₅/NH₂Cy

system, in order to supply a plausible reaction pathway. The

formation of [MoCl₄(NCy)]₂is probably preceded by that of the

intermediate Mo(^V) dimeric complex [MoCl₄(NHCy)]₂(Fig. 6), from

Mo₂Cl₁₀and NH₂Cy by HCl elimination (DG = ꢁ43.9 kcal mol^ꢁ1).

A possible pathway going from [MoCl₄(NHCy)]₂to [MoCl₄(NCy)]₂

could involve, in principle, the formation of H₂as a by-product

(eqn (3)). This reaction should be slightly thermodynamically

unfavourable (DG = ca. 0.9 kcal mol^ꢁ1) and, accordingly, GC

analyses on MoCl₅/diethylamine and MoCl₅/pyrrolidine mix-

tures pointed out the formation of only minor amounts of H₂. It

has to be observed that the H₂dissociation from primary amines

is not a common feature, being achieved only by powerful

oxidative systems.⁴³

Compound 5b was previously obtained in modest yield by

Cl₂oxidation of a Mo(V) imido precursor.³⁸Here, it was

identified by elemental analysis and single crystal X-ray analysis.

The novel 5a was characterized by IR and NMR spectroscopy,

and the structure was elucidated by a single crystal X-ray

diﬀraction study. 5a is diamagnetic, and displays an intense IR

band at 1239 cm^ꢁ1, accounting for the [MoQN] moiety. The

ORTEP molecular structure of 5a is shown in Fig. 5, while

relevant bonding parameters are reported in Table 4.

The structure of 5a is closely related to that previously

reported for 5b,³⁸showing an almost identical geometry and

bonding parameters. In 5a, the Mo(^VI) centres display a distorted

octahedral geometry, being bonded to one imido, three terminal

and two edge bridging Cl ligands. The Mo(1)–N(1) contact

[1.689(3) Å] is rather short, as expected for a Mo(^VI)–N multiple

bond, and has a strong trans influence. Thus, Mo(1)–Cl(3)_1

[2.6996(11) Å] is considerably elongated compared to Mo(1)–Cl(3)

[2.4562(10) Å], which is trans to a terminal chloride, and

Mo(1)–N(1)–C(1) [175.3(2)1] is almost linear. The molecule is

located on an inversion centre and, hence, only half of it is

present within the asymmetric unit of the unit cell.

Scheme 3 Formation of Mo(VI) imido chloride complexes from the reac-

tions of MoCl₅with primary amines.

[MoCl₄(NHCy)]₂- [MoCl₄(NCy)]₂+ H₂

(3)

Fig. 6 DFT-optimized geometry of [MoCl₄(NHCy)]₂, C-PCM/-B97X calcula-

tions. Selected computed bond lengths (Å): Mo–N 1.853, 1.853; Mo–Cl(terminal)

2.275, 2.275, 2.318, 2.318, 2.325, 2.325; Mo–Cl(bridging) 2.499, 2.499, 2.695,

2.695; Moꢀ ꢀ ꢀMo 4.030; N–H 1.029, 1.029. Selected computed angles (deg):

N–Mo–Cl(terminal) 101.4, 101.4, 88.4, 88.4, 92.3, 92.3; Cl(bridging)–Mo–

Cl(bridging) 78.3, 78.3; Mo–Cl(bridging)–Mo 101.7, 101.7.

Fig. 5 Molecular structure of 5a. Displacement ellipsoids are at the 50%

probability level. Symmetry transformations used to generate equivalent

atoms: ꢁx + 1, ꢁy + 2, ꢁz + 2.

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Table 5 Selected bond distances (Å) and angles (1) for 6

According to the calculations, a more plausible route from

[Mo^(V)Cl₄(NHCy)]₂to [Mo^(VI)Cl₄(NCy)]₂involves the dispropor-

tion of the Mo(^V) reactant to Mo(VI) and Mo(IV), eqn (4). This

Mo(1)–Cl(1)

Mo(1)–Cl(3)

N(1)–C(2)

C(1)–C(11)

C(3)–C(31)

2.365(2)

Mo(1)–Cl(2)

N(1)–C(1)

N(1)–C(3)

C(2)–C(21)

2.362(2)

2.4169(17)

1.306(11)

1.487(11)

1.486(13)

1.496(10)

1.504(11)

1.473(12)

hypothesis is supported by the calculated DG = ꢁ8.7 kcal mol^ꢁ1

,

and by the fact that magnetic analyses on the reaction residues

were in alignment with the presence of mixtures of Mo(^VI) and

Mo(^IV) species.³¹

C(1)–N(1)–C(2)

C(2)–N(1)–C(3)

N(1)–C(2)–C(21)

121.6(8)

117.9(7)

125.3(9)

C(1)–N(1)–C(3)

N(1)–C(1)–C(11)

N(1)–C(3)–C(31)

111.6(7)

113.5(7)

115.8(8)

1

3

2[MoCl₄(NHCy)]₂- [MoCl₄(NCy)]₂+ 2NH₂Cy + Mo₆Cl₂₄

(4)

4. Reactions of MoCl₅with trialkylamines

The 1 : 1 molar reaction of MoCl₅with tribenzylamine aﬀorded a

precipitate whose Cl and magnetic analysis suggested the prevalent

formation of a Mo(^V) compound, presumably [MoCl₅{N(CH₂Ph)₃}],

in admixture with Mo(^IV) containing side products (Scheme 4).

A crystallization procedure allowed to isolate the iminium salt

[(CH₂Ph)₂NQCHPh]₂[MoCl₆]ꢀCH₂Cl₂, 6, containing Mo(IV) anions,

that was X-ray characterized (Fig. 7 and Table 5).

The conversion of tribenzylamine into the relevant iminium

cation was previously realized by WCl₆-directed C–H activation,

initiated by amine to metal single electron transfer, and

finally affording the salts [(CH₂Ph)₂NQCHPh][WCl₆] and

[NH(CH₂Ph)₃][WCl₆].^11bThe formation of 6 probably follows a

similar pathway, and evidence for the presence of the ammonium

[NH(CH₂Ph)₃]⁺in the MoCl₅/N(CH₂Ph)₃mixture was supplied by

Fig. 8 Molecular structure of [NHEt₃]₂[Mo₂Cl₁₀], 7, with key atoms

labeled. Displacement ellipsoids are at the 50% probability level. Symmetry

transformation used to generate equivalent atoms: ꢁx + 2, ꢁy + 2, ꢁz.

an IR absorption at 1595 cm^{ꢁ1 11b}

on the hydrolyzed reaction mixture, ca. one third of the amine

.

According to NMR analysis

reactant was converted into the iminium upon interaction with

MoCl₅, the iminium being detected as benzaldehyde PhCHO

after hydrolysis.

The 1 : 1 molar reaction of MoCl₅with NEt₃proceeded with

prevalent Mo(^V) to Mo(IV) reduction (Scheme 4), and a crystalli-

zation procedure allowed to isolate some crystals of the Mo(^IV)

salt [NHEt₃]₂[Mo₂Cl₁₀], 7 (Fig. 8, Tables 6 and 7).⁴⁴Actually,

crystals of 7 contain a mixture of the Mo(^IV) anion [Mo₂Cl₁₀]^2ꢁ

(80%) and the Mo(V) anion [Mo₂(O)₂Cl₈]^2ꢁ(20%), in agreement

with the proposed reaction scheme. Coherently with previous

findings, the protonation source aﬀording 7 might be a C–H

activation process analogous to that suggested above for MoCl₅/

N(CH₂Ph)₃.¹²Indeed the IR spectrum of the reaction residue

contained two weak bands at 1680 and 1648 cm^ꢁ1, possibly

ascribable to [CQN] containing species. Nevertheless, the

possibility that fortuitous hydrolysis contributed to the genera-

tion of [NHEt₃]⁺must not be ruled out.

Scheme 4 Reactions of MoCl₅with trialkylamines.

Table 6 Selected bond distances (Å) and angles (1) for 7

Mo(1)–Cl(1)

Mo(1)–Cl(3)

Mo(1)–Cl(5)

N(1)–C(1)

N(1)–C(5)

C(3)–C(4)

2.312(3)

2.229(3)

2.584(3)

1.503(12)

1.527(12)

1.518(13)

Mo(1)–Cl(2)

Mo(1)–Cl(4)

Mo(1)–Cl(5)_1

N(1)–C(3)

C(1)–C(2)

C(5)–C(6)

2.327(3)

2.341(3)

2.471(3)

1.511(12)

1.509(13)

1.501(14)

Fig. 7 Molecular structure of [(CH₂Ph)₂NQCHPh]₂[MoCl₆], 6, with key

atoms labeled. Displacement ellipsoids are at the 50% probability level.

Symmetry transformation used to generate equivalent atoms: ꢁx + 2,

ꢁy + 1, ꢁz + 1.

Cl(1)–Mo(1)–Cl(5)_1 171.63(10) Cl(3)–Mo(1)–Cl(5)

174.89(12)

82.25(8)

Cl(2)–Mo(1)–Cl(4)

Cl(1)–Mo(1)–Cl(3)

172.61(11) Cl(5)–Mo(1)–Cl(5)_1

95.69(12)

Mo(1)–Cl(5)–Mo(1)_1 97.75(8)

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Table 7 Hydrogen bonds for 7

oxidation may take place during the interaction of MoCl₅with

primary amines, in view of the specific nature of the latter possibly

acting as sequential source of one H⁺cation and one H atom.

Furthermore, although Et₂NSiMe₃has been proved to act as a clean

amido group transferor towards MoCl₅, such behaviour seems to

be strictly dependent on electronic and steric features. Thus, the

reaction of MoCl₅with Me₂NSiMe₃proceeds in a non selective way,

possibly involving the solvent in the Cl/NMe₂exchange process.

The partial substitution of chloride ligands with hexafluoroisoprop-

oxide groups did not substantially alter the activation capability

towards amines of the Mo(^V) centre.

D–Hꢀ ꢀ ꢀA

d(D–H)

d(Hꢀ ꢀ ꢀA)

2.78

2.58

d(Dꢀ ꢀ ꢀA)

3.520(9)

3.283(8)

+(DHA)

N(1)–H(1)ꢀ ꢀ ꢀCl(2)#1

0.91

138.9

134.3

N(1)–H(1)ꢀ ꢀ ꢀCl(4)#2

Symmetry transformations used to generate equivalent atoms: #1 ꢁx + 1,

ꢁy + 2, ꢁz + 1; #2 x, y, z + 1.

Experimental

Warning! The metal compounds reported in this paper are

highly moisture-sensitive, thus rigorously anhydrous condi-

tions were required for the reaction, crystallization and separa-

tion procedures. The reaction vessels were oven dried at 140 1C

prior to use, evacuated (10^ꢁ2mmHg) and then filled with

argon. MoCl₅(99.9%) was purchased from Strem and stored

under argon atmosphere as received. [MoCl₃{OCH(CF₃)₂}₂] was

prepared according to the literature procedure.¹³The organic

reactants were commercial products (Apollo Sci., Sigma Aldrich

or TCI Europe) of the highest purity available, dried over P₄O₁₀

and stored under argon atmosphere. Solvents were distilled

from P₄O₁₀under argon atmosphere before use. Infrared

spectra were recorded at 298 K on a FT IR-Perkin Elmer

Spectrometer, equipped with UATR sampling accessory. Magnetic

susceptibilities (reported per Mo atom) were measured at 298 K

on solid samples with a Magway MSB Mk1 magnetic susceptibility

balance (Sherwood Scientific Ltd). Diamagnetic corrections were

Scheme 5 Reactions of a Mo(V) chloride alkoxide with amines.

5. Reactions of MoCl₅with [MoCl₃{OCH(CF₃)₂}₂]

Waldvogel and coworkers previously found that the substitu-

tion of chloride ligands in MoCl₅with hexafluoropropoxide

moieties supplied superior performances to the resulting metal

species in promoting the coupling reactions of arenes.¹³

We came interested to see whether the same strategy could

enhance the activation capability of the molybdenum frame

towards amines.

Thus the complex [MoCl₃{OCH(CF₃)₂}₂], bearing a dinuclear

structure analogous to that of MoCl₅, was synthesized following

the literature procedure.¹³The reactions of [MoCl₃{OCH(CF₃)₂}₂]

with amines were carried out in the same conditions as those

employed for MoCl₅. A better conversion of tribenzylamine into

the iminium was realized with [MoCl₃{OCH(CF₃)₂}₂] (Scheme 5).

Otherwise, [MoCl₃{OCH(CF₃)₂}₂] manifested a lower activation

power than MoCl₅towards pyrrolidine, in fact only limited

conversion to pyrroline was ascertained (Scheme 5). A highly

air sensitive solid was isolated from the reaction with NH₂Cy,

bearing a magnetic susceptibility value typical for Mo(^V) species.

The IR spectrum did not show any intense band around 1240 cm^ꢁ1

(compare IR spectrum of 5a). These data suggest that the replace-

ment of chloride ligands with hexafluoropropoxide moieties

somehow inhibits the generation of the group [Mo^VIQNCy].

introduced according to Konig.⁴⁵Carbon, hydrogen and nitrogen

¨

analyses were performed on a Carlo Erba mod. 1106 instrument.

The chloride content was determined by the Mohr method⁴⁶on

solutions prepared by dissolution of the solid in aqueous KOH at

boiling temperature, followed by cooling to room temperature

and addition of HNO₃up to neutralization. NMR spectra were

recorded at 298 K on a Bruker Avance II DRX400 instrument

equipped with a BBFO broadband probe. The chemical shifts for

¹H and ¹³C were referenced to the non-deuterated aliquot of

the solvent, while the chemical shifts for ²⁹Si were referenced to

external tetramethylsilane. NMR assignments were assisted by

DEPT experiments and ¹H,¹³C correlation measured using

gs-HSQC and gs-HMBC experiments. Gas chromatographic ana-

lyses were performed at 50 1C with a Dani 3200 gas chromato-

graph, equipped with a molecular sieves packed capillary column

(2 m; 0.25 in ID), and using argon as the gas carrier ( p = 1.5 atm).

GC-MS analyses were performed on a HP6890 instrument, inter-

faced with MSD-HP5973 detector and equipped with Phenonex

Zebron column.

Conclusions

MoCl₅is a commonly known transition metal chloride, that has

been increasingly employed in synthetic organic chemistry.

We have elucidated the reactions of MoCl₅with a variety of

aliphatic amines, some of the reactions showing unusual

features. Thus, while high valent metal chlorides generally react

with primary and secondary amines via classical aminolysis, elec-

tron interchange processes dominate with MoCl₅. More in detail,

the Mo(^V) centre undergoes single electron reduction by converting

a series of secondary amines into the relevant imines. A similar

1. Reactions of MoCl₅with secondary amines

(A) NMR studies. A suspension of MoCl₅(120 mg, 0.439 mmol)

pathway works in the reactions of MoCl₅with trialkylamines, in in CH₂Cl₂was treated with pyrrolidine (0.037 mL, 0.443 mmol). The

contrast with previous literature reports. Conversely Mo(^V) to Mo(VI) mixture was allowed to stir at room temperature for 48 h, then the

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volatile materials were removed in vacuo. An aliquot of the analysis on the resulting organic phase evidenced the presence

resulting powdery solid underwent magnetic analysis: w_M^corr

=

of NHEt₂only [¹H NMR (CDCl₃): d = 2.65 (m, 2H, CH₂);

1.53 ꢃ 10^ꢁ3cgsu, m_eﬀ= 1.92 BM. The residue was dissolved in 1.10 ppm (t, 3H, CH₃). ¹³C{¹H} NMR (CDCl₃): d = 44.1 (CH₂);

CDCl₃(1 mL), and NH_3(aq)(40% w/w, 3 mmol) was added. The 15.5 ppm (CH₃)].

mixture was left stirring for 18 h. Then the organic phase was

(B) Isolation of [MoCl₃(NMe₂)(j²-Me₂NCH₂NMe₂)], 4a, and

analyzed by NMR spectroscopy. ¹H NMR (CDCl₃): d = 7.63 [MoCl₃(NMe)(k²-Me₂NCH₂NMe₂)], 4b. A suspension of MoCl₅

(m, CH, pyrroline⁴⁷), 3.87 (m, CH₂, pyrroline), 2.88 (m, NCH₂, (370 mg, 1.35 mmol) in CH₂Cl₂(15 mL) was treated with

pyrrolidine), 2.56 (m, CH₂, pyrroline), 1.82 (m, CH₂, pyrroline), Me₂NSiMe₃(0.22 mL, 1.35 mmol), then the mixture was stirred

1.71 ppm (m, CH₂, pyrrolidine). Approximate pyrroline/pyrroli- at room temperature for 18 h. The resulting dark red solution

dine ratio = 5. The reactions of MoCl₅(0.50 mmol) with one was eliminated of the volatiles, thus the residue was washed

molar equivalent of diethylamine and dibenzylamine, respec- with hexane (2 ꢃ 20 mL) and dried in vacuo. IR (solid state):

tively, were carried out by the same procedure described for 3154m, 3103m, 2984m, 2932m, 2965s, 2458m, 1596m, 1574m,

MoCl₅/pyrrolidine. NMR analyses were as follows. From MoCl₅/ 1463vs, 1410m, 1258m, 1116w, 1027m, 1011s, 949m, 923w,

NHEt₂: ¹H NMR (CDCl₃): d = 9.83 (q, ³J_HH= 2.93 Hz, MeCHQO); 886s, 800vs, 768w cm^ꢁ1. Magnetic measurement: w^c_M^orr

=

3

2.24 ppm (d, J_HH= 2.93 Hz, MeCHQO). From MoCl₅/ 7.35 ꢃ 10^ꢁ4cgsu, m_eﬀ= 1.33 BM. Few crystals of 4a–b suitable

3

NH(CH₂Ph)₂: ¹H NMR (CDCl₃): d = 9.83 (q, J_HH= 2.93 Hz, for X-ray analysis were obtained directly from a dichloromethane

3

MeCHQO); 2.24 ppm (d, J_HH

=

2.93 Hz, MeCHQO). reaction solution, layered with hexane and settled at ꢁ30 1C.

NH(CH₂Ph)₂/PhCH₂NQCHPh ratio = ca. 1.

(C) NMR studies. The reaction of MoCl₅(0.70 mmol) with

(B) Isolation of [MoCl₅{NCH(CH₂)₃}], 1, and [CH₃CHQNHEt]- Me₂NSiMe₃(0.70 mmol) was performed also in 1,2-dichloroethane

[MoOCl₄], 2. The reaction of MoCl₅(250 mg, 0.915 mmol) with at reflux temperature and in heptane at room temperature. The

pyrrolidine (0.078 mL, 0.934 mmol) was carried out in dichloro- distinct reaction residues were treated with CDCl₃(1 mL) and then

methane (10 mL) for 48 h. The final solution was filtrated in order with NH_3(aq)(40% w/w, 0.20 mL). Dimethylamine was clearly NMR

to remove some solid, then the solution was concentrated to 3 mL, identified in the organic phases after 18 h (¹H: d = 2.38 ppm; ¹³C: d =

layered with hexane and stored at ꢁ30 1C. Green crystals of 3 45.9 ppm);⁴⁸moreover, a significant amount of Me₂NCH₂CH₂NMe₂

suitable for X-ray analysis were recovered after one week. Yield (ca. 1 : 1 ratio respect to Me₂NH) was found in the mixture obtained

113 mg (36%). Anal. calcd for C₄H₇Cl₅MoN: C, 14.03; H, 2.06; N, from the 1,2-dichloroethane reaction.

4.09; Cl, 51.78. Found: C, 14.20; H, 1.98; N, 4.03; Cl, 51.60. IR

(solid state): 1572m (CQN) cm^ꢁ1. Magnetic measurement: w^c_M^orr

9.70 ꢃ 10^ꢁ4cgsu, m_eﬀ= 1.53 BM.

=

3. Reactions of MoCl₅with primary amines

(A) Isolation of [MoCl₄(NR)]₂(R = Cy, 5a; Bu^t, 5b). General

Crystals of 2 were obtained by a procedure analogous to that procedure: the appropriate amine was added to MoCl₅in

described for 1.

CH₂Cl₂(ca. 20 mL) in a Schlenk tube, and the mixture was

2 (dark yellow crystals). Yield 35 mg (10%), from MoCl₅allowed to stir at room temperature for 48 h. The resulting

(293 mg, 1.07 mmol) and NHEt₂(0.111 mL, 1.07 mmol). Anal. solution was filtered in order to remove some solid, concen-

calcd for C₄H₁₀Cl₄MoNO: C, 14.74; H, 3.09; N, 4.30; Cl, 43.52. trated to ca. 5 mL, layered with hexane and settled at ꢁ30 1C.

Found: C, 14.57; H, 3.11; N, 4.35; Cl, 43.12. IR (solid state): Crystals of 5a and 5b suitable for X-ray analyses were recovered

1700br (n_CQN+ d_N–H), 989s (MoQO) cm^ꢁ1. Magnetic measure- after ca. 1 week.

ment: w^c_M^orr= 9.70 ꢃ 10^ꢁ4cgsu, m_eff= 1.53 BM.

5a (red solid). Yield 139 mg (38%), from MoCl₅(300 mg,

1.10 mmol) and NH₂Cy (0.126 mL, 1.10 mmol). Anal. calcd for

C₆H₁₁Cl₄MoN: C, 21.52; H, 3.31; N, 4.18; Cl, 42.34. Found: C,

2. Reactions of MoCl₅with N,N-dialkyltrimethylsilylamines

(A) Synthesis and characterization of [MoCl₄(NEt₂)], 3. A 21.37; H, 3.39; N, 4.11; Cl, 42.16. IR (solid state): 2932s, 2856m,

suspension of MoCl₅(455 mg, 1.67 mmol) in CH₂Cl₂(15 mL) 1448s, 1426sh, 1338s, 1291s, 1266m, 1239s (MoQN), 1166m,

was treated with Et₂NSiMe₃(0.32 mL, 1.69 mmol), then the 1139w, 1099w-m, 1005s, 921w-m, 860w-m, 851m-s, 793w, 736w,

1

mixture was stirred at room temperature for 18 h. The resulting 709w-m cm^ꢁ1. Magnetic measurement: diamagnetic. H NMR

dark solution was filtered in order to remove some solid, then it (CD₂Cl₂): d = 5.47 (br, 1H, CH); 2.52–2.04 (m, 8H, CH₂);

was eliminated of the volatiles. The residue was washed with 1.60 ppm (m, 2H, CH₂). ¹³C{¹H} NMR (CD₂Cl₂): d = 86.0 (CH);

pentane (2 ꢃ 20 mL) and dried in vacuo. Yield 337 mg (65%). 32.0, 24.8, 23.7 ppm (CH₂).

Anal. calcd for C₄H₁₀Cl₄MoN: C, 15.50; H, 3.25; N, 4.52; Cl,

5b (dark red solid). Yield 129 mg (43%), from MoCl₅(265 mg,

45.76. Found: C, 15.33; H, 3.12; N, 4.66; Cl, 45.53. IR (solid 0.970 mmol) and NH₂Bu^t(0.103 mL, 0.975 mmol). Anal. calcd for

state): 3068m, 2983m, 1574m, 1452m-s, 1424m, 1389m-s, C₄H₉Cl₄MoN: C, 15.55; H, 2.94; N, 4.53; Cl, 45.91. Found: C,

1254m-s, 1193w, 1158w, 1038m-s, 842vs, 766s cm^ꢁ1. Magnetic 15.29; H, 3.02; N, 4.38; Cl, 45.70. Magnetic measurement:

measurement: w^c_M^orr= 8.60 ꢃ 10^ꢁ4cgsu, m_eﬀ= 1.44 BM. In a diamagnetic.

diﬀerent experiment, the reaction solution obtained from

(B) Magnetic analyses and NMR studies. General procedure:

MoCl₅(ca. 0.5 mmol) and one equivalent of Et₂NSiMe₃was dried MoCl₅(0.50 mmol) and the appropriate amine (0.50 mmol) were

in vacuo. CDCl₃(1 mL) was added to the residue in air, and the allowed to react in CH₂Cl₂(ca. 20 mL) for 72 h. Thus the volatile

obtained mixture was treated with NH_3(aq)(40% w/w, 0.20 mL). materials were removed in vacuo, and the residue was washed

The mixture were left stirring at room temperature for 18 h. NMR with pentane (2 ꢃ 20 mL). An aliquot of the solid underwent

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magnetic analysis: from MoCl₅/NH₂Cy, w_M= 7.5 ꢃ 10^ꢁ4cgsu; 5. Reactions of [MoCl₃{OCH(CF₃)₂}₂] with amines

from MoCl₅/NH₂Bu^t, w_M= 6.7 ꢃ 10^ꢁ4cgsu; from MoCl₅/NH₂Pr,

i

The reactions of MoCl₃[OCH(CF₃)₂]₂(ca. 0.80 mmol) with amines

w_M= 5.5 ꢃ 10^ꢁ4cgsu; from MoCl₅/NH₂(2,6-C₆H₃Me₂), w_M

=

(1 molar equivalent) were carried out by using the same condi-

tions described for the corresponding MoCl₅/amine reactions.

(A) From [MoCl₃{OCH(CF₃)₂}₂] and pyrrolidine. Brown

solid. IR (solid state): 3185m, 3111m, 2909w, 1579w, 1461m,

1366m-s, 1272m-s, 1228s, 1214s, 1175vs, 1123m, 1102vs, 1019w,

4.2 ꢃ 10^ꢁ4cgsu; from MoCl₅/NH₂CH₂Ph, w_M= 1.0 ꢃ 10^ꢁ3cgsu.

Another aliquot (ca. 50 mg) was treated with CDCl₃(1 mL) and

then with a large excess of NH_3(aq)(40% w/w). The mixture was

left stirring for 18 h, hence the organic phase was analyzed

by NMR, showing the presence of the starting amine. A

minor amount of [NHQCH(Ph)] (about 15% respect to NH₂Bz)

was detected from MoCl₅/NH₂CH₂Ph [d(¹H) = 8.35 ppm

(s, 1 H, CH)].

972vs, 895w, 885w, 851s, 803w, 749s, 735m-sh, 686vs cm^ꢁ1

.

Magnetic measurement: w^c_M^orr= 9.77 ꢃ 10^ꢁ4cgsu, m_eﬀ= 1.53 BM.

After treatment with NH_3(aq): pyrroline and pyrrolidine (molar

ratio 1.4 : 1).

(B) From [MoCl₃{OCH(CF₃)₂}₂] and cyclohexylamine. Brown

solid. IR (solid state): 3145m-br, 2939w-m, 2860w, 1582w-m,

1477m, 1451w-m, 1374w, 1352w, 1282s, 1228w-m, 1184s, 1104vs,

4. Reactions of MoCl₅with trialkylamines

(A) Reaction of MoCl₅with N(CH₂Ph)₃: isolation of

Q

1015m, 991m, 919w, 892m, 857m, 802w, 761m-s, 687vs cm^ꢁ1

.

[(CH₂Ph)₂N CHPh]₂[MoCl₆], 6. MoCl₅(250 mg, 0.915 mmol)

Magnetic measurement: w^c_M^orr= 8.53 ꢃ 10^ꢁ4cgsu, m_eﬀ= 1.43 BM.

was added to a solution of N(CH₂Ph)₃(263 mg, 0.915 mmol) in

CH₂Cl₂(15 mL). The mixture was stirred at room temperature

for 18 h. Hexane (80 mL) was added, and the resulting pre-

cipitate was separated from the colourless solution, washed

with pentane (2 ꢃ 10 mL) and dried in vacuo. Yield 420 mg. IR

(solid state): n = 3034m-br, 2763w, 1640w-m (CQN, 6), 1595w-m,

1497w-m, 1451s, 1415m, 1340w, 1266w, 1214w-m, 1087w, 1002m,

990m, 924m, 750vs, 736s-sh, 697vs cm^ꢁ1. Magnetic measurement:

w_M^corr= 1.29 ꢃ 10^ꢁ3cgsu, m_eﬀ= 1.76 BM. Anal. calcd for

C₂₁H₂₁Cl₅MoN: Cl, 31.62. Found: Cl, 29.40. Crystallization from

CH₂Cl₂/hexane at ꢁ30 1C aﬀorded few crystals of 3 suitable for

X-ray analysis. Anal. calcd for anal. Calcd for C₄₂H₄₀Cl₆MoN₂: C,

57.23; H, 4.57; N, 3.18; Cl, 24.13. Found: C, 57.11; H, 4.70; N, 3.22;

After treatment with NH_3(aq): cyclohexylamine.

(C) From [MoCl₃{OCH(CF₃)₂}₂] and tribenzylamine. Dark

red solid. IR (solid state): 3066w, 3036w, 2962w, 1639m, 1597m,

1497w-m, 1457m, 1379w, 1287m, 1260w-m, 1225m, 1216m,

1175s, 1124w-m, 1100s, 989s, 894w, 837w, 803w, 750vs, 734m-s,

698vs, 685s cm^ꢁ1. Magnetic measurement: w^c_M^orr= 1.47 ꢃ 10^ꢁ3

cgsu, m_eﬀ= 1.88 BM. After treatment with NH_3(aq):N(CH₂Ph)₃and

PhCHO (molar ratio 1 : 3).

6. GC analyses

Samples for gas chromatographic analyses were prepared as

follows: a mixture of MoCl₅(ca. 1 mmol) and the appropriate

amine (1 molar equivalent) in CH₂Cl₂(10 mL) was stirred at

room temperature for 48 h in a Schlenk tube tapped with a

silicon stopper. Then an aliquot of the reaction atmosphere was

withdrawn by a 1 mL syringe through the stopper, and injected

into the GC instrument. The yield of H₂formation was esti-

mated based on analyses of gaseous standard mixtures contain-

ing known amounts of H₂. From MoCl₅/pyrrolidine: 1% yield;

MoCl₅/NHEt₂: 1% yield; from MoCl₅/NH₂Cy: 2% yield; from

MoCl₅/NH₂Bu^t: 1% yield.

Cl, 23.90. IR (solid state): n = 1640s (CQN) cm^ꢁ1

.

(B) Reaction of MoCl₅with NEt₃: isolation of [NHEt₃]₂-

[Mo₂Cl₁₀], 7. This reaction was performed by using a procedure

analogous to that described for MoCl₅/N(CH₂Ph)₃, from MoCl₅

(280 mg, 1.02 mmol) and NEt₃(0.145 mL, 1.04 mmol). Yield

298 mg (light-red precipitate). Anal. calcd for C₆H₁₅Cl₅MoN: Cl,

47.35. Found: Cl, 37.05. IR (solid state): n = 3150m (N–H, 7),

3051w, 2984w-m, 2943w, 1680m, 1648m, 1452s, 1395s, 1352m,

1287w, 1265m, 1170w, 1153w, 1103w, 1081w, 1029, 1006m-w,

832w, 804w, 786, 732vw, 700m-s cm^ꢁ1. Magnetic measurement:

w^c_M^orr= 1.93 ꢃ 10^ꢁ3cgsu, m_eff= 2.16 BM. Crystallization from

CH₂Cl₂/hexane at ꢁ30 1C afforded 7 as dark-red crystals suit-

7. X-ray crystallographic studies

able for X-ray analysis. Yield 96 mg, 25%. Anal. calcd for Crystal data and collection details for 1, 2, 4ꢀ0.5CH₂Cl₂, 5a,

₁₂H₃₂Cl₁₀Mo₂N₂: C, 19.20; H, 4.30; N, 3.73; Cl, 47.22. Found: 6ꢀCH₂Cl₂and 7 are listed in Table 8. The diﬀraction experi-

C, 19.31; H, 4.15; N, 3.52; Cl, 46.98. ments were carried out on a Bruker APEX II diﬀractometer

C

(C) NMR studies. According to the procedure described equipped with a CCD detector and using Mo-Ka radiation

above for MoCl₅/pyrrolidine, organic solutions were obtained (l = 0.71073 Å). Data were corrected for Lorentz polariza-

by allowing the solid residues isolated from MoCl₅/N(CH₂Ph)₃tion and absorption eﬀects (empirical absorption correction

and MoCl₅/NEt₃, respectively, to react with CDCl₃/NH_3aqin SADABS).⁴⁹The structures were solved by direct methods and

contact with air. NMR analyses were as follows.

From MoCl₅/N(CH₂Ph)₃. ¹H NMR (CDCl₃):

refined by full-matrix least-squares based on all data using

d

=

10.02 F².⁵⁰All non-hydrogen atoms were refined with anisotropic

(s, PhCHQO); 7.90, 7.67–7.36 (Ph); 3.90 ppm [s, N(CH₂Ph)₃]. displacement parameters. All hydrogen atoms were fixed

¹³C NMR{¹H} (CDCl₃): d = 192.4 (PhCHQO); 139.9, 136.7, 134.4, at calculated positions and refined by a riding model.

129.7, 129.1, 128.5, 128.3, 127.1 (Ph); 53.9 ppm [N(CH₂Ph)₃]. The crystals of 4ꢀ0.5CH₂Cl₂are twinned with twin matrix

Ratio N(CH₂Ph)₃/PhCHQO 2 : 1.

1 0 0 0 ꢁ1 0 0 0 ꢁ1 and refined batch factor 0.1434(12). The

From MoCl₅/NEt₃. ¹H NMR (CDCl₃): d = 2.43 [m, N(CH₂CH₃)₃]; asymmetric unit of the unit cell of 6ꢀCH₂Cl₂contains one half

3

0.97 ppm [t, J_HH= 7.13 Hz, N(CH₂CH₃)₃]. C NMR{¹H} (CDCl₃): of a [MoCl₆]^2ꢁanion (located on an inversion centre), one

d = 44.5 [N(CH₂CH₃)₃]; 11.8 ppm [N(CH₂CH₃)₃].

[(CH₂Ph)₂NQCHPh]⁺cation (on a general position) and one

This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2017

New J. Chem.

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1

Table 8 Crystal data and measurement details for 1, 2, 4ꢀ₂CH₂Cl₂, 5a, 6ꢀCH₂Cl₂and 7

1

2

4ꢀ₂CH₂Cl₂

5a

6ꢀCH₂Cl₂

7

Formula

FW

T, K

C₄H₇Cl₅MoN

342.30

100(2)

0.71073

Orthorhombic

Pna2₁

11.8263(17)

8.2825(12)

10.2388(15)

90

1002.9(3)

4

2.267

C₈H₂₀Cl₈Mo₂N₂O₂C₁₄H₃₉Cl₈Mo₂N₆

C₁₂H₂₂Cl₈Mo₂N₂

669.80

100(2)

0.71073

Triclinic

%

P1

6.6315(17)

7.648(2)

11.839(3)

106.915(2)

96.903(3)

102.798(3)

549.1(2)

1

C₄₃H₄₂Cl₈MoN₂

966.33

100(2)

0.71073

Monoclinic

P2₁/c

10.0833(9)

13.2029(10)

16.6033(12)

90

103.818(6)

90

2146.4(3)

2

1.495

0.837

C₁₂H₃₂Cl_9.6Mo₂N₂O_0.4

651.74

100(2)

0.71073

Monoclinic

P2₁/n

8.449(3)

13.113(4)

9.937(3)

90

766.99

100(2)

0.71073

Monoclinic

P2₁/c

7.9913(5)

27.2271(8)

13.7004(8)

90

742.99

100(2)

0.71073

Orthorhombic

Pbca

13.8611(7)

13.6266(7)

14.1240(6)

90

2667.7(2)

4

1.850

l, Å

Crystal system

Space group

a, Å

b, Å

c, Å

a, 1

b, 1

g, 1

102.064(3)

90

1076.6(6)

2

90.321(4)

90

2980.9(3)

4

Cell volume, Å³

Z

D_c, g cm^ꢁ3

m, mm^ꢁ1

F(000)

2.011

2.161

1.079

1.574

2.026

2.115

328

2.577

660

1.907

1474

636

1540

948

Crystal size, mm

y (limits)1

Reflections collected 10 237

0.19 ꢃ 0.16 ꢃ 0.12 0.19 ꢃ 0.12 ꢃ 0.10 0.19 ꢃ 0.15 ꢃ 0.10 0.19 ꢃ 0.16 ꢃ 0.12 0.14 ꢃ 0.12 ꢃ 0.10 0.18 ꢃ 0.16 ꢃ 0.13

3.00–26.99

2.61–27.00

11 565

0.75–26.00

34 636

1.83–27.00

6004

1.99–25.02

30 144

2.54–27.00

30 112

Independent

reflections

2180 [R_int= 0.0392] 2351 [R_int= 0.0489] 5833 [R_int= 0.0613] 2382 [R_int= 0.0385] 3791 [R_int= 0.2415] 2888 [R_int= 0.0282]

Data/restraints/

parameters

2180/1/100

2351/0/100

5833/0/272

2382/0/109

3791/2/253

2888/0/124

Goodness on fit on F²1.083

1.041

1.063

1.093

0.970

1.212

R₁(I 4 2s(I))

wR₂(all data)

0.0227

0.0398

0.0278

0.0682

0.0461

0.1092

0.0317

0.0760

0.0627

0.1378

0.0803

0.1913

Largest diﬀ. peak

0.416/ꢁ0.480

0.756/ꢁ0.615

1.293/ꢁ0.724

0.882/ꢁ0.765

0.530/ꢁ1.061

2.147/ꢁ2.430

and hole, e Å^ꢁ3

CH₂Cl₂molecule disordered over two equally populated and Acknowledgements

symmetry related (by an inversion centre) positions. The C-

The University of Pisa is acknowledged for financial support.

Francesco Del Cima is gratefully acknowledged for the execu-

tion of the GC analyses.

atoms of the disordered solvent molecule was refined isotro-

pically. The C–Cl distances of the CH₂Cl₂molecule were

restrained to be 1.75 Å (DFIX command in SHELXL, s.u.

0.02). The asymmetric unit of the unit cell of 7 contains one

[NHEt₃]⁺cation (on a general position) and one half of a

References

2ꢁ

[Mo₂Cl₁₀

]

anion (on an inversion centre). The crystals are

contaminated by the oxide derivative [Mo₂(O)₂Cl₈]^2ꢁ(20%)

and this has been included in the final refinement.

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Unusual activation pathways of amines in the reactions with molybdenum pentachloride

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