Combined NMR and DFT Study on the Complexation Behavior of

Article pubs.acs.org/Organometallics

Combined NMR and DFT Study on the Complexation Behavior of Lappert’s Tin(II) Amide Lies Broeckaert,† Jan Turek,§ Roman Olejník,§ Aleš Růzǐ čka,§ Monique Biesemans,‡ Paul Geerlings,† Rudolph Willem,‡ and Frank De Proft*,† †

Department of General Chemistry (ALGC) and ‡Department of Materials and Chemistry (MACH), High Resolution NMR Centre (HNMR), Vrije Universiteit Brussel (VUB), Pleinlaan 2, 1050 Brussels, Belgium § Department of General and Inorganic Chemistry, Faculty of Chemical Technology, University of Pardubice, Studentska 573, 53210 Pardubice, Czech Republic S Supporting Information *

ABSTRACT: The complexation chemistry of the stannylene Sn{N[Si(CH3)3]2}2, first reported by Lappert in the 1970s, was investigated by 119 Sn NMR chemical shift measurements. To this end, experimental NMR data and theoretical density functional theory (DFT) calculations were combined to get an insight into the interaction between the stannylene and various solvent molecules with σ- and/or π-coordinating power. Small variations in the measured 119Sn chemical shifts revealed a donor−acceptor interaction with the solvent molecules. In comparison to the noncoordinating solvent cyclohexane taken as a reference, a weak coordination was observed with aromatic solvent molecules (benzene and toluene) and a much stronger coordination with the σ-donors THF and pyridine. Pyridine was confirmed to be the strongest donor, as evidenced by its large upfield chemical shift Δδ(119Sn) of 635 ppm. The experimental chemical shifts were reproduced by DFT (NMR) calculations, demonstrating similar trends in the interaction strength with the σ- and π-donors. The stannylene Sn{N[Si(CH3)3]2}2 showed the ability to react with Fe(CO)5 and Fe2(CO)9 in the molar ratio 1/1 to provide L2SnFe(CO)4 complexes. With a molar excess of Fe2(CO)9, L2Sn[Fe(CO)4]2 was generated irreversibly. Upon prolonged UV irradiation in the presence of W(CO)6, in the molar ratio 1/1, a mixture of L2SnW(CO)5 and two (L2Sn)2W(CO)4 complexes was generated.

1. INTRODUCTION The importance of organotin chemistry has increased in the last few years.1 The chemistry of divalent Sn(II) compounds has been far less explored than that of their more common and more stable tetravalent Sn(IV) analogues.2−4 Stannylenes are neutral organic compounds with a divalent tin center and only six electrons in the Sn valence shell. The heavy (singlet) carbene analogues SnR2 can be stabilized by incorporating sterically hindered substituents on the central metal atom or by complexation with transition-metal complexes, such as iron and tungsten derivatives.5−10 The first stable dialkyl and diamido stannylenes were reported by Lappert and co-workers.11−26 The original Lappert compound Sn{CH[Si(CH3)3]2}2 (1a) was characterized by various techniques, including Mössbauer spectroscopy, in order to establish the tin atom valence.11 Structural studies have found that the tin(II) alkyl compounds generally exist as dimers with a metal−metal interaction in the solid state (and in the gas phase) and often display a dynamic monomer−dimer equilibrium in solution at ambient temperature.27 These dimers were considered as the first compounds with a formal SnSn double bond. Lappert considered the weak SnSn bond in the distannene (dimer) as a double dative bond, in which the lone pair on each monomer interacts with the formal vacant pz © 2013 American Chemical Society

orbital of the other stannylene molecule, resulting in a bent bond.15,28 Zilm and co-workers28 reported some thermodynamic parameters for the equilibrium between the distannene and the corresponding Lappert stannylene monomer on the basis of the temperature dependence of NMR chemical shifts (ΔH = 11 kcal mol−1 and ΔS = 28 cal mol−1 K−1).29 Kira and co-workers reported a cyclic analogue of Lappert’s stannylene,30 the first monomeric dialkylstannylene with coordination number 2 in the crystalline as well as in the solution state. The present report focuses mainly on the isoelectronic nitrogen analogue of the stannylene 1a, Sn{N[Si(CH3)3]2}2 (1b), likewise reported first by Lappert slightly later (Figure 1).13,15,17 The tin(II) amide 1b exists as a yellow-orange, lowmelting crystalline solid (mp 37−38 °C).13,15,17 Lappert and co-workers studied the structure of cyclic bis(amino)stannylene analogues of 1b and its reactivity with SnCl2 and a cyclic Si(II) diamide.31 In a recent study,32 we reported the results of theoretical DFT calculations on model dihalogenated germylenes and stannylenes interacting with σ- and π-donor molecules. The Received: December 20, 2012 Published: March 19, 2013 2121

dx.doi.org/10.1021/om3012344 | Organometallics 2013, 32, 2121−2134

Organometallics

Article

Figure 1. Lappert stannylenes Sn{CH[Si(CH3)3]2}2 (1a) and Sn{N[Si(CH3)3]2}2 (1b).

existence of weak π-donor interactions with tin in organotin compounds, in particular, in tin(II) species, has been a matter for debate for many years (for a review see ref 33). In particular, such interactions have been observed recently in the crystalline state between the Cp*SnII cation and two of the four phenyl rings of the tetraphenylborate counteranion.34 The dominating electrophilic power of the simple metallylenes (MX2; M = Ge, Sn and X = F, Cl, Br, I) and the existence of weak π- and stronger σ-interactions with donor molecules has received strong support from our work.32 The stannylene complexes32 can be considered as model systems for the present investigation of Lappert’s stannylene 1b in different media with σ- and π-donor potential. Experimentally measured and theoretically calculated 119Sn chemical shifts are the main tools used here for assessing the coordination sphere around the central Sn(II) atom. Various theoretical 119Sn chemical shift studies in the past have focused mainly on Sn(IV) compounds.35−41 The importance of stannylene 1b is stressed by its use in the conversion of aldehydes and ketones to N,N-dialkyleneamines.42,43 It is also an important reagent for the synthesis of different tin(II) amides, guanidinates, and phenoxides44−46 and calcium amides.47

2. RESULTS AND DISCUSSION 2.1. Geometries. The optimized geometries of 1b and its complexes with σ- and π-donors are presented in Figure 2. In our recent study on the π-complexation of model stannylenes,32 an intermolecular distance of 3−3.5 Å between the aromatic donor molecules and the tin center in SnX2 (X = F, Cl, Br) was reported. The interaction occurs through electron donation from the π-electrons of the benzene to the formal “empty” p-orbital on the metal. In the complexes of Figure 2, the aromatic donors are more remote from the tin atom. This is a combined effect of a weak intermolecular interaction between the donor molecule and the stannylene and steric hindrance due to the bulky groups, partially preventing access to the tin atom in stannylene 1b. The shortest distance between one of the carbon atoms of a benzene or toluene molecule and the tin atom in the stannylene is 3.58 Å; this is larger than the distance between the N- or O-donor atom of the σ-donors pyridine and THF and the tin atom (d = 2.54 and 2.70 Å, respectively; below the corresponding sums of van der Waals radii, 3.72 and 3.69 Å). The lone pairs on N- and Odonor atoms are pointed toward the formal empty p-orbital on tin, coordinating via a classical Lewis acid−base interaction with the electrophilic tin atoms. 2.2. Stannylene Properties. Lappert and co-workers17 investigated the difference in reactivity between the stannylenes (Sn{CH[Si(CH3)3]2}2) (1a) and (Sn{N[Si(CH3)3]2}2) (1b)

Figure 2. Optimized geometries of the isolated stannylene (a) Sn{N[Si(CH3)3]2}2 (1b) and of its complexes with π- and σ-donors: (b) benzene; (c) toluene; (d) THF; (e) pyridine. The structure in (f) represents 1b in the presence of cyclohexane at the optimized interaction distance.

and observed a higher electrophilicity of the tin center in the dialkyl stannylene (C > N). The properties of the Lappert compounds Sn{CH[Si(CH 3 ) 3 ] 2 } 2 (1a) and Sn{N[Si(CH3)3]2}2 (1b) were probed using an NBO analysis and are compared in Table 1. The reactivity of the stannylenes can be explained by the calculated charges and electrophilicities. The NPA charges on the tin atoms were obtained from NBO analysis. The total NPA charges are global descriptors, reflecting a combination of both σ- and π-characteristics of the tin atom. From its higher positive (NPA) charge, it could be concluded at first glance that 1b is more electrophilic than 1a. However, Lappert observed a reverse reactivity in reactions with N-bases and concluded that the dialkyl stannylene was more electrophilic than the Sn(II) diamide.17 This can also be rationalized by the p-occupancy of the formal empty p-type NBO orbital on the Sn atom in both stannylenes, which is higher in 1b than in 1a, because of the lone pair π-donation from the N atoms into the empty p-type orbital of the Sn atom. This results in a lower orbital energy for both stannylenes, with 2122

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Table 2. Calculated and Experimental 119Sn NMR Chemical Shifts (in ppm) of Sn{N[Si(CH3)3]2}2 (1b)a

Table 1. Calculated Properties of the Lappert Stannylenes Sn{CH[Si(CH3)3]2}2, and Sn{N[Si(CH3)3]2}2a Lappert stannylene NPA charge q(Sn) occupancy formal empty porbital on Sn (au) energy formal empty p-orbital on Sn (au) global electrophilicity ω (kcal mol−1) local electrophilicity ω+(Sn) (kcal mol−1)

Sn{CH[Si(CH3)3]2}2 (1a)

δ(119Sn)/ppm

Sn{N[Si(CH3)3]2}2 (1b)

+1.19 0.101

+1.37 0.179

−0.061

−0.083

29.5

28.1

23.1

22.0

a

The NPA charges on the central tin atom and the occupancies and energies of the vacant p-type NBO orbital on tin are given for the two stannylenes.

solvent (donor)

A

B

C

exptl

cyclohexane-d12

677

671

794

benzene-d6

641

706

740

toluene-d8

645

708

758

THF-d8

397

860

440

pyridine-d5

130

238

96

779 780b 767 768c 767d 770 770e 602 609f 145 144f

a

In the last column, the results of the reproducibility measurements are given below the first experimental 119Sn chemical shift values. Legend: (A) calculated 119Sn NMR chemical shifts (in ppm) of optimized 1/1 stannylene 1b donor complexes; (B) calculated 119Sn NMR chemical shifts (in ppm) of the isolated stannylene 1b in a COSMO solvent model, including the corresponding dielectric constant of the considered solvent; (C) calculated 119Sn NMR chemical shifts (in ppm) of the optimized 1/1 stannylene 1b donor complexes in a COSMO solvent model with the corresponding dielectric constant of the donor (solvent) molecule. bReproducibility test on day 488. cReproducibility test on day 484. dNew sample measured on day 486. eReproducibility test on day 485. fReproducibility test on day 487. gReproducibility test on day 25.

a lower p-occupancy and a higher p-energy for 1a, which displays both a higher global and local electrophilicity in comparison to 1b (see Table 1). In the experimental 119Sn NMR study below, we focus on stannylene 1b, because stannylene 1a is less stable photochemically,48 making it more difficult to keep 1a stable in solution for data accumulation over a long duration. In contrast, 1b is easier to handle and dissolves readily in all solvents of interest for the present study. 2.3. Stannylene−Solvent Donor Complexation: NMR Results. The reproducibility of the 119Sn chemical shift measurements was demonstrated by remeasuring the samples a few months and 1 year after their preparation, providing simultaneous information as to the long-term stability of the samples, all kept under moisture- and air-free conditions. The tin atom can expand its coordination sphere through weak interactions with (solvent) donor molecules in solution. Thus, nuclear shielding constants can be considered as a convenient indicator of the bonding pattern of the tin atom in the presence of potential donors. 119Sn shifts tend to lower resonance frequencies upon coordination expansion. The resulting chemical shift variations, Δδ(119Sn), reflect, at least qualitatively, the strength of interactions between the tin atom and the potential donor site, both intra- and intermolecular.49−51 Tin(II) 119Sn chemical shifts span a range of not less than 4000 ppm.49−51 δ( 119Sn) values are positive for stannylenes, because the considered reference is a tin(IV) compound, i.e. Sn(CH3)4, reflecting a generally lower nuclear shielding of Sn(II) in comparison to that in Sn(IV) compounds. 119Sn NMR chemical shifts can thus not only demonstrate the divalent character of tin atoms but also reveal additional coordination by σ- and π-donors. The measured 119 Sn chemical shift values of stannylene 1b in various deuterated solvents are presented in Table 2, together with 119 Sn NMR chemical shifts calculated under various conditions and given for comparison. As expected, the experimental 119Sn chemical shift value of 1b is the highest, +779 ppm, in the nondonating solvent cyclohexane-d12 (Table 2). Our values for benzene and THF (respectively 767 and 602 ppm) are in good agreement with literature values (respectively 771 and 624 ppm).52 The 119Sn chemical shift value of the whole series of complexes involving 1b is quite low in comparison to the literature values of 1a, which has shifts of well over 2300 ppm.51,53 This is tentatively assigned to the important donation from the N atom lone pair evidenced above by the high p-occupancy of the formal empty

p-orbital at tin. Different 119Sn chemical shift values are observed for the different solvents, all upfield shifts being assignable to donor−acceptor interactions of variable strength. Only a small upfield 119Sn chemical shift of a few ppm is observed when the stannylene is dissolved in aromatic solvents lacking σ-donating atoms, such as benzene and toluene. The weakness of this effect for benzene and toluene is in line with the very weak π-interactions with tin in the model compounds addressed in our previous work.32 Accordingly, a larger 119Sn upfield shift holds for the Sn atom of 1b in σ-donor solvents such as THF and pyridine. Not unexpectedly, a larger upfield shift was observed in the latter than in the former solvent (Table 2). The experimental chemical shift value reported by Wrackmeyer49−51 and Piers53 for 1b in C6D6, 776 ppm, is in satisfactory agreement with our value (Table 2). Our result is also in line with that of Kira et al.,54 showing reversible complexation of their cyclic dialkylstannylene with tetrahydrofuran (THF) with ΔH = −7.0 kcal mol−1 and ΔS = −48 cal mol−1 K−1, attributed to the complexation being energetically favored but sterically disfavored. Cotton et al. emphasized the Lewis acidity of stannylenes in a colorless weak 1/1 complex between the (red) stannylene 1a and pyridine.19 As already mentioned, our DFT optimizations of the complexes pointed out that the distance between the N-donor atom in pyridine and Sn in the stannylene (d(Sn−N) = 2.54 Å for stannylene 1b) is shorter than the distance between the O-donor atom from THF and Sn (d(Sn−O) = 2.70 Å). This result confirms the stronger σ-interaction with pyridine, in line with the higher upfield shift observed for pyridine, in comparison to that for THF. Hahn and co-workers55 studied symmetrically and unsymmetrically N,N′-substituted benzimidazolin-2-stannylenes and observed upfield 119Sn shifts in THF-d8, in comparison to samples in C6D6. 2123

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Organometallics

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Table 3. Experimental δ(119Sn) and Calculated NMR Chemical Shift Values of Sn{N[Si(CH3)3]2}2 (1b) in Pure Benzene-d6, Pure Pyridine-d5, and Different Benzene− Pyridine Solvent Mixturesa

The 119Sn chemical shifts were calculated using different approaches with various levels of complexity. In general, the trends in the 119Sn chemical shifts (Table 2) calculated in entry A, simply assuming an optimized 1/1 stannylene 1b/donor complex, are in moderate agreement with the experimental values. The calculated 119Sn chemical shifts of 1b in cyclohexane, benzene, toluene, and THF appear systematically lower in comparison to the experimental values. In contrast, for pyridine, the agreement between calculated and experimental values can be considered as good. The differences between experimental and calculated 119Sn chemical shifts can be accepted if it is considered that the experimental values were obtained in an excess solvent continuum, while the calculated values were obtained from energy optimized 1/1 donor/ stannylene complexes that probably overestimate the strength of the donor−Sn interaction in comparison to the actual solvent continuum situation in the experimental sample. This proposal is in line with the observation that such is not the case for pyridine, where it is experimentally well-known56−58 that a strong donor−acceptor interaction exists between the N lone pair and the Sn(II) atom, even under conditions of no solvent excess, explaining why the difference between experimental and calculated value is much smaller. Calculating the 119Sn chemical shifts of 1b in isolated form and just using a COSMO continuum solvent model59 (Table 2, entry B) slightly improves the values for benzene and toluene but worsens the disagreement with the experimental values for THF and pyridine. The latter is ascribed to the σ-donating capacities of THF and pyridine being improperly accounted for by a simple dielectric solvent correction without a 1/1 1b/solvent complex. The overall agreement between calculated and experimental 119 Sn chemical shifts is most satisfactory when optimized 1b/ solvent 1/1 complexes are considered additionally, including the COSMO solvent model (Table 2, entry C). This emphasizes the importance of the contribution to the chemical shift of specific 1/1 1b/solvent complexation being included in addition to the appropriate dielectric medium. In the case of pyridine, the 1/1 complexation is obviously dominant, since solvent dielectric constant inclusion does not really improve the chemical shift, in comparison to its experimental value, resulting, in contrast, in a larger difference between calculated and experimental values. In order to address more specifically the special case of pyridine, the 119Sn chemical shifts of the stannylene 1b were measured in benzene-d6/pyridine-d5 mixtures of variable composition, in order to get a deeper insight into the relative magnitudes of σ- and π-interactions so as to assess at least qualitatively the strength of pyridine as a σ-donor. The results are shown in Table 3. Table 3 clearly reveals the huge dominance of σ- over πinteractions. Experimentally, pyridine is known to be a very strong σ-donor,56−58 while benzene is a pure π-donor. The interaction between the stannylene and pyridine donor molecules in the pure pyridine-d5 sample results in the very high upfield 119Sn shift of not less than Δδ = 623 ppm, in comparison to pure benzene-d6. The dominance of the pyridine σ-interaction over the benzene π-donor interaction with the stannylene is confirmed by the 119Sn chemical shifts found for 50/50 and 99/1 benzene/pyridine solvent mixtures, since their values remain significantly closer to the pure pyridine value. Further evidence for the strong σ-donating power of pyridine is provided by the still large upfield 119Sn shift of ca. 100 ppm even when only a very small quantity of pyridine is present in a

solvent (donor)

δ(119Sn)/ppm

pure pyridine-d5

145 144b 153 154c 239 240c 668 670d 679 767 768e 767 130f 96g 112h

50% pyridine-d5 1% pyridine-d5