Intermolecular 119Sn, 31P Through-Space Spin–Spin Coupling in a

Article pubs.acs.org/IC

Intermolecular 119Sn,31P Through-Space Spin−Spin Coupling in a Solid Bivalent Tin Phosphido Complex Janet Arras,†,‡ Klaus Eichele,† Boris Maryasin,‡ Hartmut Schubert,† Christian Ochsenfeld,*,‡ and Lars Wesemann*,† †

Institut für Anorganische Chemie, Universität Tübingen, Auf der Morgenstelle 18, 72076 Tübingen, Germany Theoretische Chemie, University of Munich (LMU Munich), Butenandtstraße 7, 81377 Munich, Germany

‡

S Supporting Information *

ABSTRACT: A bivalent tin complex [Sn(NP)2] (NP = [(2Me2NC6H4)P(C6H5)]−) was prepared and characterized by Xray diffraction and solution and solid-state nuclear magnetic resonance (NMR) spectroscopy. In agreement with the X-ray structures of two polymorphs of the molecule, 31P and 119Sn CP/MAS NMR spectra revealed one crystallographic phosphorus and tin site with through-bond 1J(117/119Sn,31P) and through-space TSJ(117/119Sn,31P) spin−spin couplings. Density functional theory (DFT) calculations of the NMR parameters confirm the experimental data. The observation of throughspace TSJ(117/119Sn,31P) couplings was unexpected, as the distances of the phosphorus atoms of one molecule and the tin atom of the neighboring molecule (>4.6 Å) are outside the sum of the van der Waals radii of the atoms P and Sn (4.32 Å). The intermolecular Sn···P separations are clearly too large for bonding interactions, as supported by a natural bond orbital (NBO) analysis.

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pathways.8−11 One surprising result is that the vicinal 3J(1H,1H) in ethane (and the Karplus relationships12) seems to be intermediate between through-bond and through-space coupling because the coupling is transmitted through-space by the back-lobes of the C−H bonds (“through-tail”), circumventing the C−C bond.10 Hence, the concept of through-space coupling appears to be more common than generally anticipated. All that is required is the overlap of suitable orbitals. With this in mind, the suggestion by Tormena and coworkers13 to call this phenomenon “through-overlapping orbital coupling” rather than through-space coupling addresses this process more properly. An extension to this concept would be the observation of intermolecular spin−spin coupling. However, the observation of such couplings in the solution or gas phase would require some degree of relative preferential association of finite lifetime to allow the spin−spin coupling to manifest itself; therefore, some amount of attractive interactions such as in van der Waals dimers14−16 or across hydrogen bonds is required.6 In this respect, solid-state NMR spectroscopy offers the advantage that discrete molecules are held together in fixed relative orientations by crystal forces, in favorable cases allowing the study of intermolecular spin−spin couplings. Such intermolecular spin−spin couplings between two 31P nuclei (P···P = 3.384 Å) have been investigated theoretically by Alcorta and co-workers.17 While our investigation was in progress, Woolins and Ashbrook with co-workers18 reported an

INTRODUCTION One of the quantities most treasured in nuclear magnetic resonance (NMR) experiments is the electron-mediated indirect spin−spin coupling. The observation of spin−spin coupling establishes connectivity between the interacting nuclear spins and is usually taken as evidence for the existence of bonds between them,1 i.e., the mediating electrons are located in bonds. A manifestation of this concept is the use of the smallest number of intervening bonds, n, as a superscript to its symbol, nJ, to classify spin−spin coupling constants.2,3 However, this concept was challenged quite early on by reports4 of unexpectedly large indirect spin−spin coupling constants between nuclei that are separated by several bonds but are close to each other in space, and at least one of them features a lone pair (although cases involving only protons have also appeared5). This observation led to the concept of through-space coupling, in contrast to the concept of throughbond coupling. Although the term through-space coupling is often defined rather vaguely as coupling between nonbonded nuclei, the vast majority of examples are intramolecular,6 i.e., the atoms are connected by bonds by default. Justifiably, the term through-space coupling has also been called a misnomer7 because of its homonymous use for the direct dipole−dipole coupling, especially in the context of solid-state NMR spectroscopy. What is implied, at least, is the transmission of coupling information via electrons that are not formally involved in bonds. Currently, theoreticians are working on suitable tools to disentangle contributions from through-bond or through-space © XXXX American Chemical Society

Received: March 8, 2016

A

DOI: 10.1021/acs.inorgchem.6b00573 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry intermolecular TSJ(77Se,31P) coupling (Se···P = 3.514 Å). It is generally accepted that the sum of the van der Waals radii poses an upper limit on the distance between the atoms.19 During the course of our investigations into the coordination properties of the bidentate phosphide ligand [(2-Me2NC6H4)P(C6H5)]− (NP),20 we discovered in solid-state 31P and 119Sn NMR spectra a textbook case of intermolecular through-space spin−spin coupling TSJ(119Sn,31P), where the distance between the two nuclei in separate molecules (4.6 Å) exceeds the sum of the van der Waals radii; however, the spin−spin coupling constants are about one-third of the intramolecular one-bond spin−spin coupling constant. The experimental results are augmented by density functional theory (DFT) calculations of NMR properties of the monomer and dimer constellations, which demonstrate that observable spin−spin coupling constants at even greater distances are feasible.

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RESULTS AND DISCUSSION Synthesis. The bisamino bisphosphide tin complex Sn(NP)2 (2) is obtained from the corresponding secondary phosphine NPH (1)20 in a two-step reaction in high yield (Scheme 1). First, the phosphine 1 is deprotonated using n-

Figure 1. ORTEP plot (50% displacement ellipsoids) of two molecules of tin complex 2 crystallized from THF (2(thf)) in the solid state. Hydrogen atoms, the phenyl rings (except for the ipso carbon atom) at phosphorus, and the methyl groups at nitrogen have been omitted for clarity. Selected distances (Å) and angles (°): Sna−Pa 2.6167(5), Sna−Na 2.632(2), Pa−Sna−Pb 102.73(2), Na−Sna−Nb 148.44(5), Pa−Sna−Na 70.68(4), Pa−Sna−Nb 89.50(4), Sna···Pc,d 4.6352(7), Snc···Pa,b 7.7041(9), Sna···Snc 5.7941(8).

Scheme 1. Synthesis of the Bisamino Bisphosphide Tin Complex Sn(NP)2 (2) from the Secondary Phosphine NPH (1)

BuLi in hexane; second, after removal of all volatile components in vacuo, two equiv of the resulting lithium phosphide is reacted with one equiv of SnCl2 in dry THF at room temperature. After crystallization at room temperature, the neutral tin complex 2 is isolated in 85% yield (see Supporting Information for more details). Compound 2 is stable toward heat and light and can be stored as a solid in an inert atmosphere for extended periods of time. It was characterized by elemental analysis, NMR spectroscopy in solution and in the solid state, single-crystal X-ray diffraction, and DFT calculations. Crystal Structures. Crystals of complex 2 show a tendency for polymorphism, offering the chance to study the same molecule in different environments: crystallization from THF affords crystals in the polar tetragonal space group I41cd (2(thf)), whereas crystals obtained from an n-hexane solution (2(hex)) belong to the centro-symmetric tetragonal space group P42/n. In both structures, the tin atom is located on a special position of crystallographic C2 symmetry; hence, the asymmetric unit consists of half of a molecule. The molecular structures in both space groups are very similar (cf. Figures 1 and 2, where selected distances and angles are shown). A

Figure 2. ORTEP plot (50% displacement ellipsoids) of two molecules of tin complex 2 crystallized from n-hexane (2(hex)) in the solid state. Hydrogen atoms, the phenyl rings (except for the ipso carbon) at phosphorus, and the methyl groups at nitrogen have been omitted for clarity. Selected distances (Å) and angles (°): Sna−Pa 2.6297(5), Sna−Na 2.583(2), Pa−Sna−Pb 101.17(2), Na−Sna−Nb 154.18(7), Pa−Sna−Na 71.43(4), Pa−Sna−Nb 91.99(4), Sna···Pc,d 4.6866(5), Snc···Pa,b 7.8310(5), Sna···Snc 5.8932(3).

molecular fit using PLATON21 results in an RMS fit for bonds of 0.013 Å and 1.675°. In the neutral monomers, the bivalent tin atom is coordinated by two phosphorus and two nitrogen atoms, generating two puckered five-membered chelate rings in the envelope conformation with the tin atom in the flap and angles between the flap and base of 59.46° for 2(thf) and 54.20° for 2(hex). The geometry about the tin atom corresponds to that of a distorted trigonal bipyramid22 with the nitrogen atoms in axial positions. One equatorial position is occupied by the putative free-electron pair. In addition to being a dissymmetric molecule, the two phosphorus atoms are chiral and pyramidal; the sums of the angles about phosphorus are 291.57° for 2(thf) and 292.18° for 2(hex). B

DOI: 10.1021/acs.inorgchem.6b00573 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry In both polymorphs, the Sn−P distances of 2.6167(5) and 2.6297(5) Å are similar to the Sn−N distances of 2.583(2) and 2.632(2) Å, respectively, which is a situation also encountered in the structurally similar complex [{2,6-(CH2NMe2)2C6H3}(Ph)P]2Sn.23 Typical Sn−P distances in Sn(II) phosphides range from 2.5783(11) to 2.7902(8) Å,23−28 whereas Sn−N distances featuring the Sn(II)−NMe2Ar moiety range from 2.408(3) to 2.668(3) Å.24,25,29−31 Compared to 2, the situation is reversed in the bisamido bisphosphino complex (PNP)SnN(SiMe3)2 (PNP = N-[2-P(iPr)2-4-methylphenyl]2N−), where the Sn−P distances are clearly elongated (2.9698(7) and 2.8520(7) Å), and the Sn−N distances significantly shortened (2.142(2) and 2.206(2) Å).32 In 2(thf), the polymorph crystallized from THF, the closest neighboring molecule shown in Figure 1 is generated by the c glide plane; hence, the enantiomers that alternate along this direction and the P−Sn−P fragments form an almost planar herringbone arrangement. Neighboring herringbone strings are rotated by 90°, but due to the nature of the polar space group, all P−Sn−P fragments point in the positive c direction. In the structure of the polymorph crystallized from 2(hex), the second molecule shown in Figure 2 is generated by the 42 axis in the direction of the crystallographic c axis, which generates a string of the same enantiomer along this direction, each one rotated by 90° with respect to the preceding molecule. The packings of 2 in the solid state (Figures 1 and 2) reveal discrete molecules; the intermolecular Sna···Snc distances indicate that the molecules are shifted by almost 6 Å with respect to each other. The Sna···Pc,d distances in excess of 4.6 Å are longer than those of the sum of the van der Waals radii (3.96 Å), tin (2.17 Å), and phosphorus (1.80 Å),33 even for the recent greater values obtained for tin (2.42 Å) and phosphorus (1.90 Å), resulting in a sum of 4.32 Å.34 In both polymorphs, the intermolecular distances are very similar, except for those affected by the consecutive 90° twists in 2(hex) as compared to the respective distances of 2(thf). NMR Spectroscopy in Solution. In solution, the 1H, 13 C{1H}, 31P{1H}, and 119Sn{1H} NMR spectra at room temperature indicate that the C2 symmetry of the monomer found in the crystal structure is maintained: both phosphide ligands are chemically equivalent. Only the two diastereotopic methyl groups at nitrogen result in two distinct broad signal sets in 1H and 13C{1H} NMR spectra at room temperature. The broadening of the methyl peaks is attributed to the slow mutual exchange,20 confirmed by the 1H EXSY NMR spectrum (Supporting Information). The 31P{1H} NMR spectrum shows a sharp singlet flanked by 117,119Sn satellites (Figure 3a), while the 119Sn{ 1H} NMR spectrum features a triplet with 1 119 J( Sn,31P) = 834 Hz (Figure 3b). The mutual exchange of diastereotopic methyl groups at nitrogen requires breaking of the weak tin−nitrogen interaction, rotation of the NMe2 group followed by inversion at nitrogen, and recoordination.20 This is in line with observations of terminal/chelating exchange of bifunctional PN ligands by Izod et al.25,35 and trifunctional PN2 ligands by Ř eznič́ ek et al.23 In principle, the exchange of methyl groups could also be achieved by an exchange of axial nitrogen atoms brought about by one of the rearrangement modes of a trigonal bipyramid,36 but this would also require inversion at phosphorus. No further dynamic processes were evident in the 1 H and 31P{1H} NMR spectra in the temperature range from −80 to 80 °C, and there was no evidence of other diastereomers caused by different configurations at phosphorus

Figure 3. 31P (a) and 119Sn (b) NMR spectra in solution (top) and in the solid state (bottom). Spectra are drawn on the same kHz scale relative to the centers of the multiplets. The higher multiplicity of the solid-state NMR spectra due to an additional TSJ(119Sn,31P) or TS 117,119 J( Sn,31P) is indicated.

or tin.24,25 The value of the measured 1J(119Sn,31P) (834 Hz) is smaller than those reported for other Sn(II) phosphides (875− 1682 Hz).23−26,37−39 NMR Spectroscopy of Solids. To bridge the results from the crystal structures and NMR spectroscopy in solution, we also acquired solid-state 31P and 119Sn cross-polarization magicangle spinning (CP/MAS) NMR spectra. The isotropic peaks in the CP/MAS NMR spectra of 2(thf) are compared to their solution counterparts in Figure 3, and Table 1 summarizes the results. The 31P CP/MAS NMR spectrum of 2(thf) shows, in addition to the features expected from the solution NMR spectrum, viz., the singlet due to phosphorus bonded to NMR inactive isotopes of tin and the doublet of satellites due to C

DOI: 10.1021/acs.inorgchem.6b00573 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Principal Componentsa of the 31P and Literature 31

P

compound

δiso (ppm)

b

−50.0 −55.1d −55.1e −60.7f −59.4 −59.5 −10.2i,j −13.4j,k 410.5l 472m 356n 530 −190 129p

1 solution 2 solutionc 2(thf) 2(hex) 2 calcg

(PNP)SnN(SiMe3)2h 119

Sn

119

2 solutionc 2(thf) 2(hex) 2 calco (PNP)SnN(SiMe3)2h [Sn(NMe2)2]2h

Sn NMR Chemical Shift Tensors of Compound 2 and Examples from the δ11 (ppm)

δ22 (ppm)

δ33 (ppm)

Ω (ppm)

κ

23 −6 5 7 27 22

−39 −28 −36 −37 0 −13

−149 −148 −146 −149 −58 −48

171 143 151 156 85 70

0.27 0.69 0.45 0.44 0.35 0.00

802 612 853 239 673

532 608 616 46 233

82 −158 119 −855 −520

720 770 734 1094 1193

0.25 0.98 0.35 0.65 0.26

a δ11 ≥ δ22 ≥ δ33, isotropic chemical shift δiso = (δ11 + δ22 + δ33)/3, span Ω ≈ δ11 − δ33, skew κ = 3(δ22 − δiso)/Ω, −1 ≤ κ ≤ 1. bIn C6D6, rt, ref 20. cIn toluene-d8, rt. d1J(119Sn,31P) = 834 Hz, 1J(117Sn,31P) = 799 Hz. e1J(117,119Sn,31P) = 828 Hz, TSJ(117,119Sn,31P) = 261 Hz, 2J(31P,31P) = 45 Hz. D = 289 Hz, orientations (α, β, γ) of chemical shift tensors: (0,−87,−72) and (0,−93,−108). f1J(117,119Sn,31P) = 815 Hz, TSJ(117,119Sn,31P) = 273 Hz. g Structure optimized in C1, chemical shifts obtained using the calculated shielding of PH3 with δ(31P, PH3) = −266.1 ppm,47 see the Supporting Information. hPNP = N-[2-P(iPr)2-4-methylphenyl]2N−, ref 32. i1J(117/119Sn,31P) = 500 Hz. j2J(31P,31P) = 211 Hz. k2J(117/119Sn,31P) = 650 Hz. l1 119 J( Sn,31P) = 834 Hz. m1J(119Sn,31P) = 845 Hz, TSJ(119Sn,31P) = 263 Hz. n1J(119Sn,31P) = 816 Hz, TSJ(119Sn,31P) = 277 Hz. oChemical shifts with respect to the calculated magnetic shielding of SnMe4, see the Supporting Information. p1J(119Sn,14N) = 305 Hz, 1J(119Sn,14N) = 150 Hz, 1 119 J( Sn,14N) = 140 Hz.

1

J(117,119Sn,31P), an additional pair of peaks displaced symmetrically about the central peak and separated by 261 Hz. Upon close inspection, another pair of peaks is also apparent as shoulders at the base of the central peak, but their appearance was anticipated (vide infra). More striking are the differences in multiplicity in the 119Sn{1H} solution and 119Sn CP/MAS solidstate NMR spectra: the triplet of the solution NMR spectrum forms a triplet of triplets in the solid-state NMR spectrum. The sizable additional splitting of 263 Hz could result from spin− spin coupling with two 14N nuclei (cf. Table 1, [Sn(NMe2)2]2), but the observed multiplet is not in agreement with that expected for two equivalent 14N nuclei29,32,40 and fails to explain the additional satellites in the 31P CP/MAS NMR spectrum. Invoking an intermolecular through-space spin−spin coupling, we found TSJ(117,119Sn,31P) appeared to be the only reasonable explanation left; i.e., the 119Sn nucleus shows not only spin−spin coupling to the two directly bonded 31P nuclei but also through-space coupling to two 31P nuclei in the next molecule. In this respect, the isotopic dilution of 117,119Sn is actually an advantage because it results in localized 117,119Sn[31P]2[31P]2 five-spin systems rather than an infinite [117,119Sn[31P]2]∞ chain, simplifying the appearance to some extent. The five-spin system is still of a higher order, in principle, but due to small intermolecular 31P,31P couplings, can be considered as first order. The phenomenon of TSJ(117,119Sn,31P) through-space coupling is investigated in more detail by the DFT calculations described below. A representation of the electronic structure about a nucleus more detailed than that of the isotropic chemical shift alone is provided by the principal components of the chemical shift tensor of that nucleus. The principal components describe the dependence of the chemical shift on the orientation of the molecule in the applied magnetic field and can be obtained from an analysis of the intensities of the spinning sidebands at moderate and slow spinning rates.41 The full 31P CP/MAS spectrum of 2(thf) is shown in Figure 4. According to the

Figure 4. Solid-state 31P CP/MAS NMR spectra of a powder sample of compound 2(thf) recorded at 81.0 MHz. Top: full spectrum at a rotation frequency of 2 kHz; the asterisk marks the isotropic region. Bottom: spinning rate dependence of the isotropic peak obtained at (from top) 1.5, 2.0, and 4.0 kHz, observed experimentally (left) and with the corresponding simulations (right, see text).

crystal structure, both phosphorus nuclei are crystallographically equivalent but are not magnetically equivalent. The orientations of the chemical shift tensors of both nuclei will be such that, for an arbitrary orientation of the molecule with respect to the external magnetic field, different instantaneous chemical shift differences between both 31P nuclei will result. Depending on the orientation of the molecule in the external magnetic field, both phosphorus nuclei may constitute an A2, AB, or AX spin system. In such cases, MAS fails to completely remove the homonuclear spin−spin interactions, and spinning rate-dependent line shapes result.42 This is demonstrated at the bottom of Figure 4, where the isotropic region of the slowD

DOI: 10.1021/acs.inorgchem.6b00573 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 2. Calculateda and Experimental Spin−Spin Coupling Constants (in Hertz) Determined for Compound 2 property 1

119

31

J( Sn, P)

TS

J(119Sn,31P)

2

J(31P,31P)

structure

DSO

PSOb

FC + SDc

total

experimentald

2(thf) 2(hex) 2(thf) 2(hex) 2(thf) 2(hex)

−0.4 −0.4 −0.3 −0.2 0.2 0.2

42.8 40.1 0.7 0.8 0.9 1.1

701.4 688.4 −305.9 −290.4 37.1 46.3

743.9 728.1 −305.4 −289.8 38.2 47.6

845 816 263 277 45 -

PBE0/QZ4P plus relativistic corrections using first-order VWN potential for coupling constant calculations. bWith FC + SD cross-terms. cWith PSO cross-terms. dWith the exception of 2J(31P,31P), the signs of the coupling constants have not been determined.

a

spinning 31P CP/MAS NMR spectra of 2(thf) are plotted along with simulations using Simpson43 (the simulations include only the 31P spin pair). Evidently, the 117,119Sn satellites appear to be affected more strongly by this recoupling effect.44 The mutual orientations of 31P chemical shift tensors with respect to the internuclear 31P···31P vector obtained from the simulations (Table 1) indicate that the least-shielded direction is close to the internuclear vector, in agreement with DFT calculations (vide infra). Also, the simulation indicates that the sign of the 2J(31P,31P) coupling constant is the same as that of the dipolar coupling constant D = (μ0/4π)(γ2P−31ℏ/2π)⟨r−3 PP ⟩, i.e., positive (Table 2). The full 119Sn CP/MAS NMR spectrum of 2(thf) is shown in Figure 5; the principal components of the

and smaller than those of the other 4-fold coordinated tin compounds (PNP)SnN(SiMe3)2 and [Sn(NMe2)2]2.32 Density Functional Theory Calculations. Because the observation of a sizable through-space spin−spin coupling over such a great distance was initially puzzling, the experimental NMR spectroscopic evidence was augmented by quantum chemical calculations using the Amsterdam density functional (ADF) package,48 whereas Turbomole49 was used for geometry optimizations. In the first step, to assess the effect of packing on the intramolecular NMR properties and because the crystal structures of 2(thf) and 2(hex) pointed toward the presence of discrete molecules, the crystal structure coordinates were used as starting points for geometry optimizations of an isolated gas phase molecule at the PBE0-D3/def2-QZVP level of theory.50−52 Both converged to a very similar structure that was slightly closer to 2(hex) than 2(thf). Relativistic effects in the NMR calculations were considered by means of the twocomponent zeroth-order regular approximation (ZORA, scalaronly and scalar plus spin−orbit relativistic effects) method.53−57 Chemical shifts were obtained by using magnetic shieldings of suitable reference compounds calculated at the same level of theory (SnMe4 for 119Sn and PH3 for 31P). Details, coordinates, and magnetic shieldings are listed in the Supporting Information, whereas the resulting calculated 31P and 119Sn chemical shifts are compared to experimental values in Table 1. In the case of the 31P NMR data, this method actually yields values of δiso(31P) = −59.4 and −59.5 ppm that agree well with the experimental δiso(31P, 2(hex)) = −60.7 ppm; however, it has to be noted that an isolated gas phase molecule was compared to a solid. The spans of the 31P and 119Sn chemical shift tensors calculated for the isolated molecule agree quite well with those observed experimentally. As observed experimentally in the 31P CP/MAS NMR spectrum of 2(thf), the least-shielded direction at phosphorus is close to the P···P vector (i.e., perpendicular to the “lone pair” and the P−C bonds). In summary, there seems to be good agreement in modeling the crystal structure using an isolated molecule. In the second step, two neighboring molecules in the crystal structures of 2(thf) and 2(hex) were used, without further geometry optimization, to calculate intra- and intermolecular spin−spin coupling constants; the essential results are collected in Table 2 and compared to the values observed experimentally (additional coupling constants are summarized in the Supporting Information). There are four terms contributing to the total spin−spin coupling constant: Fermi-contact (FC), diamagnetic spin−orbit (DSO), paramagnetic spin−orbit (PSO), and spin-dipole (SD). 58,59 Within the ZORA approximation, the FC, SD, and PSO terms contain crossterms with the others. In this work, we report the DSO, the PSO, and the total FC + SD term. There is good agreement between the calculated and observed magnitudes. In particular,

Figure 5. Experimental (top) and simulated (bottom) solid-state 119Sn CP/MAS NMR spectra of a powder sample of 2(thf) recorded at 74.7 MHz. Rotation frequency: 8 kHz. The asterisk indicates the isotropic region (cf. Figure 3).

chemical shift tensor obtained from the analysis of spinning sideband intensities45,46 are listed in Table 1, and a spectrum calculated using these parameters is shown at the bottom of Figure 5. The simulation neglects the effect of the 119Sn,31P dipolar coupling (D = −1019 Hz); given the total width of the spectrum (ca. 54 kHz), its effect is estimated to be small. The crop of crystals of the polymorph crystallized from 2(hex) was not sufficient to obtain a 119Sn CP/MAS NMR spectrum; hence, a microcrystalline powder sample was precipitated from a THF solution with n-hexane. This sample contained a minor amount of 2(thf). The 31P and 119Sn CP/MAS NMR data are also presented in Table 1, and the corresponding spectra are provided in the Supporting Information. The 119Sn chemical shift anisotropies of 2(thf) and 2(hex) are relatively moderate E

DOI: 10.1021/acs.inorgchem.6b00573 Inorg. Chem. XXXX, XXX, XXX−XXX

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CONCLUSIONS In summary, we discovered a highly interesting example of intermolecular through-space TSJ(117/119Sn,31P) spin−spin coupling in the solid state where the separation between the communicating nuclei exceeds the sum of the van der Waals radii yet results in a TSJ(117/119Sn,31P) that is still about a third of 1J(117/119Sn,31P). Using DFT calculations, we were able to identify the FC + SD term as the dominating term of the through-space coupling mechanism. The model used in the calculations consists of two molecules taken from the crystal structure to allow the calculation of through-bond and throughspace 119Sn,31P coupling constants. In this model, the effects of the infinite crystal structure are neglected. Accounting for the crystal effects could be an interesting aspect of future studies. Nevertheless, the observed trends are already reproduced quite well by the simpler model calculations.

the calculations confirm that, despite the large intermolecular 119 Sn,31P separation of about 4.6 Å, a TSJ(119Sn,31P) may result that is greater than one-third of 1J(119Sn,31P). For all calculated spin−spin coupling constants, the Fermi contact/spin dipolar contribution is the most important contribution by far. In the third step, to study the effect of the intermolecular distance on the magnitude of TSJ(119Sn,31P), we chose a model system composed of two molecules from the 2(hex) structure, but reduced them in size by using hydrogen atoms instead of the phenyl groups at phosphorus, methyl groups at nitrogen, and ethylenediyl groups instead of the aromatic ring systems that bridge P and N (see Supporting Information). In this model system, we varied the intermolecular distance, expressed as Sn···Sn separation, from 4.89 to 7.09 Å (corresponding to intermolecular Sn···P distances of 3.807−5.788 Å) and carried out calculations of 1J(119Snc,31Pc,d) and TSJ(119Sna,31Pc,d). The results are illustrated in Figure 6, and the actual data are given

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00573. Experimental procedures, spectral data, and computational details (PDF) Crystallographic information (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS C.O. acknowledges financial support from the DFG (Oc35/41). REFERENCES

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Figure 6. Calculated dependence of 1 J( 119 Sn c , 31 P c,d ) and TS 119 a 31 c,d J( Sn , P ) on the intermolecular Sna···Pc,d distances in the structure of 2(hex) (cf. Figure 2).

in the Supporting Information. We chose the 2(hex) structure instead of the 2(thf) structure because the 90° twist between the two molecules in the former avoids having all four phosphorus atoms coplanar, as in the latter, potentially providing an additional indirect P···P−Sn pathway for TS 119 a 31 c,d J( Sn , P ), at least at close distance. The calculations demonstrate that, even if the Sn···P distance is elongated by another angstrom, a TSJ(119Sna,31Pc,d) of −44 Hz could still be observable in MAS NMR spectra. At all distances covered by the calculations, DSO and PSO contributions are minor, and the coupling mechanism is dominated by the FC + SD term. A natural bond orbital (NBO) analysis indicates there is no covalent interaction between the [Sn(NP)2] molecules at the experimental distance (see the Supporting Information for more details). F

DOI: 10.1021/acs.inorgchem.6b00573 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.inorgchem.6b00573 Inorg. Chem. XXXX, XXX, XXX−XXX