Research article Received: 12 September 2014

Revised: 18 November 2014

Accepted: 24 November 2014

Published online in Wiley Online Library: 23 January 2015

(wileyonlinelibrary.com) DOI 10.1002/mrc.4201

Oligomeric complexes of some heteroaromatic ligands and aromatic diamines with rhodium and molybdenum tetracarboxylates: 13C and 15N CPMAS NMR and density functional theory studies Arkadiusz Leniak,a Bohdan Kamieńskia,b and Jarosław Jaźwińskia* Seven new oligomeric complexes of 4,4′-bipyridine; 3,3′-bipyridine; benzene-1,4-diamine; benzene-1,3-diamine; benzene-1,2diamine; and benzidine with rhodium tetraacetate, as well as 4,4′-bipyridine with molybdenum tetraacetate, have been obtained and investigated by elemental analysis and solid-state nuclear magnetic resonance spectroscopy, 13C and 15N CPMAS NMR. The known complexes of pyrazine with rhodium tetrabenzoate, benzoquinone with rhodium tetrapivalate, 4,4′-bipyridine with molybdenum tetrakistrifluoroacetate and the 1 : 1 complex of 2,2′-bipyridine with rhodium tetraacetate exhibiting axial–equatorial ligation mode have been obtained as well for comparison purposes. Elemental analysis revealed 1 : 1 complex stoichiometry of all complexes. The 15N CPMAS NMR spectra of all new complexes consist of one narrow signal, indicating regular uniform structures. Benzidine forms a heterogeneous material, probably containing linear oligomers and products of further reactions. The complexes were characterized by the parameter complexation shift Δδ (Δδ = δcomplex δligand). This parameter ranged from around 40 to 90 ppm in the case of heteroaromatic ligands, from around 12 to 22 ppm for diamines and from 16 to 31 ppm for the complexes of molybdenum tetracarboxylates with 4,4′-bipyridine. The experimental results have been supported by a density functional theory computation of 15N NMR chemical shifts and complexation shifts at the non-relativistic Becke, threeparameter, Perdew-Wang 91/[6-311++G(2d,p), Stuttgart] and GGA–PBE/QZ4P levels of theory and at the relativistic scalar and spin-orbit zeroth order regular approximation/GGA–PBE/QZ4P level of theory. Nucleus-independent chemical shifts have been calculated for the selected compounds. Copyright © 2015 John Wiley & Sons, Ltd. Keywords: coordination polymer; 15N CPMAS NMR; 13C CPMAS NMR; rhodium tetracarboxylate; DFT calculation

Introduction

344

Rhodium and molybdenum dimeric tetracarboxylates (Fig. 1), as well as other dimeric paddle-wheel-like molecules, attract interest of chemistry, spectroscopy, biology and related areas because of their complexation abilities.[1] These molecules enable the construction of supramolecular ensembles, such as triangles,[2] squares,[2–5] molecular wires,[6] fullerene-like structure[7] and coordination polymers.[8–19] The dimetallic cores can be linked either via equatorial substituents, by acid residues or via an axially bonded multifunctional ligand. The last approach results in the linear metal-organic oligomers or in two-dimensional and threedimensional networks, depending on the ligand, bifunctional or multifunctional. Some of these polymeric materials can absorb gases within the crystal lattice. Numerous one-dimensional linear polymers have been obtained by the combination of only a little dirhodium and dimolybdenum tetracarboxylates and organic ligands such as pyrazine, 4,4′-bipyridine, quinones and 1,4diazabicyclo[2.2.2]octane. The studies were focused mainly on the synthesis, single-crystal X-ray diffraction, electrochemistry and ability of gas absorption. In our previous works[20,21], we have obtained linear complexes of rhodium tetraacetate with a number of nitrogenous ligands:

Magn. Reson. Chem. 2015, 53, 344–352

pyrazine; pyrimidine; triazine; 1,4-diazabicyclo[2.2.2]octane; 1,3,5,7tetraazatricyclo[3.3.1.1[3,7]]decane; and a set of ethane-1,2-diamines and propane-1,3-diamines. The synthetic procedure involved mixing an acetonitrile solution of the ligand and rhodium(II) tetraacetate. Oligomeric products precipitated from the solutions as reddish fine powders. They were insoluble in organic solvents and did not form crystals suitable for X-ray diffraction analysis. As alternative methods, we employed elemental analysis and solidstate NMR spectroscopy (13C and 15N CPMAS NMR). The first method allowed estimating the stoichiometry of complexes, whereas the second one discerned pure compounds from mixtures. The 15N CPMAS NMR spectroscopy appears to supply the greatest amount of information. We observed either one narrow 15 N NMR signal or a set of signals in the spectra, depending on the ligand. The chemical shifts of the signals allowed us to determine the type of complexation.

* Correspondence to: Jarosław Jaźwiński, Institute of Organic Chemistry, Polish Academy of Sciences, Warsaw, Poland. E-mail: [email protected] a Institute of Organic Chemistry, Polish Academy of Sciences, Warsaw, Poland b Institute of Physical Chemistry, Polish Academy of Sciences, Warsaw, Poland

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Organic complexes of dirhodium and dimolybdenum: NMR and DFT studies

Figure 1. Ligands, rhodium and molybdenum dimeric salts investigated in the present work. Asterisks denote axial positions of complexation; acyl residues are located in the equatorial positions.

The present work is a continuation of our investigations. We combined rhodium tetracarboxylates and a range of nitrogenous ligands having two nitrogen atoms (Fig. 1) to obtain new dirhodium complexes. These complexes were then characterized by elemental analysis and CPMAS NMR spectroscopy. For comparison purposes and for assessing the utility of this method of investigation, we included four complexes previously described in the literature with known X-ray structures. Experimental results were supplemented with density functional theory (DFT) calculations, namely with structure modelling and estimation of NMR parameters, such as chemical shifts and complexation shifts.

Results and discussion In the present work, 11 complexes have been obtained; six of them have heteroaromatic nitrogenous ligands 1–4, additional four with aromatic diamines 5–8 and one with benzoquinone 9. Within

experimental error, elemental analysis indicated the 1 : 1 stoichiometry for each of these complexes. The majority of them contained rhodium(II) tetraacetate Rh2Ac4 as the core substrate although complexes with rhodium tetrabenzoate Rh2Bz4, molybdenum tetraacetate Mo2Ac4 and tetrakistrifluoroacetate Mo2Tfa4 were obtained as well. Four of these complexes have been described in the literature, and their X-ray structures are known. These are 2,2′bipyridine with rhodium tetraacetate,[22] 4,4′-bipyridine with molybdenum tetrakistrifluoroacetate,[23] pyrazine with rhodium tetrabenzoate[16] and benzoquinone with rhodium tetrapivalate Rh2Piv4.[9,24] Bipyridine 3 and Rh2Ac4 form a 1 : 1 complex containing the ligand bound in the axial and equatorial positions (Fig. 2a). The remaining three complexes exhibit linear, polymeric structures. The structures of some oligomeric complexes of 4,4′-bipyridine with rhodium and molybdenum dimeric tetracarboxylates have been determined by X-ray diffraction. The complex of Rh2Piv4 consists a linear chain of ligands and dirhodium cores.[13] However, nitrogen atoms at both sites of the dirhodium unit are not entirely

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Figure 2. (a) Density functional theory optimized structure of the axial–equatorial complex of 2,2′-bipyridine with Rh2Ac4. (b) A part of the X-ray structure[13] of the axial complex of 4,4′-bipyridine with Rh2Piv4; CH3 groups were omitted for clarity. It is clearly visible that the N1, Rh1, Rh2 and N2 atoms are out of alignment (N1–Rh1–Rh2 and Rh1–Rh2–N2 angles are 173° and 179° respectively). (c), (d) A density functional theory optimized structure of the axial– equatorial complex of rhodium(II) tetraacetate with benzene-1,2-diamine with the O–H–N hydrogen bond (c) and no such bond (d). The latter is less stable by around 6.3 kcal/mol than the former. All geometry optimizations were performed at the Becke, three-parameter, Lee-Yang-Parr/[6-31G(2d), LANL2DZ] level of theory assuming isolated molecules in a vacuum.

A. Leniak, B. Kamieński and J. Jaźwiński Table 1.

15

N NMR data for some ligands and their complexes with dimeric rhodium and molybdenum saltsa

Ligand

Rhodium salt

1

2 3

Rh2Ac4 Mo2Ac4 Mo2Tfa4 Rh2Ac4 Rh2Ac4

4 5 6 7 8

Rh2Bz4 Rh2Ac4 Rh2Ac4 Rh2Ac4 Rh2Ac4

15

δ( N) Ligand 63.2 (CDCl3)

15

106.1 79.2 65.6, 108.8 145.1, 162.7, 90.5 341.9 346.1 343.5, 275.6,

64.8 (solid state)b 72.6 (DMSO-d6)c 48.9 (solid state) 329.5 (DMSO-d6) 324.4 (neat) 329.9 (CDCl3) 320.3, 323.2 328.2, 330.0 (solid state) 15

a15

15

δ( N) Complex ( N CPMAS NMR)

91.6, 142.8 159.5

345.8 340.3

15

93.3

15

Δδ( N) 42.9 16.0 2.4, 28.4, 31.0 44.0 72.5d 90.1d 41.6 12.4 21.7 13.6, 15.9 14.9, (+49.8)d 15

N NMR chemical shifts δ( N) (ppm) and complexation shifts Δδ( N) = δcomplex δligand (ppm) are given, where Δδ( N) were 15 estimated on the basis of N CPMAS NMR chemical shifts in complexes and corresponding ligand chemical shift taken from 15 15 the third column of the table. All N chemical shifts were given with respect to the signal of nitromethane (0.0 ppm). N NMR chemical shifts for ligands were obtained in the present work or taken from the literature (discussed in the Experimental section). The major signals were underlined. 15 b Chemical shift at 66.4 ppm (DMSO-d6) and 59.5 (CDCl3). The value of 64.8 ppm was used for the Δδ( N) estimation. 15 c Chemical shift at 78.9 ppm (CDCl3). The value of 72.6 ppm was used for the Δδ( N) estimation. d Values calculated for main signals only and on the basis of averaged chemical shift of the free ligand (δlig = 325.4 ppm). DMSO, dimethyl sulfoxide.

equivalent because of a deviation of one nitrogen atom from the N–Rh–Rh–N line (Fig. 2b). The complex of Mo2Tfa4, in turn, forms a lattice composed of infinite oligomeric chains and a 1 : 2 complex.[23] By analogy to these structures, a linear arrangement of ligands and dirhodium cores, with aligned Rh–Rh–N–N atoms, was expected for the oligomer of 4,4′-bipyridine 1 with Rh2Ac4 and pyrazine 4 with Rh2Bz4. A similar structure was expected for the complex of 3,3′-bipyridine 2. Because of the vicinal arrangement of nitrogen atoms in 3,3′-bipyridine and free rotation of phenyl rings around the C1–C1′ bond, the chain in this last complex

346

13

was expected not to adopt a linear arrangement but a twodimensional or three-dimensional zigzag conformation. The 13C and 15N CPMAS NMR data of the studied compounds were collected in the experimental section and in Table 1. Examples of 13C and 15N CPMAS NMR spectra are shown in Fig. 3. The 15N NMR spectrum of Rh2Ac4 complex with 2 consisted of one relatively narrow signal at 108.8 ppm. This signal was shifted by 44.0 ppm in relation to the signal of the free ligand ( 64.8 ppm). The 15N CPMAS NMR spectrum of Rh2Ac4 complex with 1 (δ of 106.1 ppm, Δδ of 42.9 ppm) and Rh2Bz4 complex with 4 (δ of

15

Figure 3. C and N CPMAS NMR spectra of oligomeric Rh2Ac4 complexes with 3,3-bipyridine 2 (left) and benzene-1,4-diamine 5 (right). Asterisks denote rotational side bands. Various mixing time allows identifying the signals of quaternary and protonated carbon atoms.

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Magn. Reson. Chem. 2015, 53, 344–352

Organic complexes of dirhodium and dimolybdenum: NMR and DFT studies 90.5 ppm, Δδ of 41.6 ppm) exhibited similar features. The latter was similar to Rh2Ac4 complex with 4 published previously (δ of 89.3 ppm, Δδ of 41.6 ppm).[20] In contrast, much smaller Δδ parameter, of 16.0 ppm, has been measured for Mo2Ac4 complex with 1. It is worth noting that in the case of Rh2Tfa4 complexes of pyridine and picolines, the greater Δδ(15N) values, from around 68 to 77 ppm, were observed in the solution.[25] Complexation of 4,4′-bipyridine 1 with Mo2Tfa4 yielded an interesting material, which is expected to be a convenient model for the testing of analytical methods. Crystals of this complex consist of the two species, the oligomeric (1 : 1)n infinite chains and 2 : 1 complexes.[23] Indeed, the 15N CPMAS NMR spectrum of this material showed two signals, the minor one at 65.6 ppm (Δδ of 2.4 ppm) assigned to the non-complexed edge of bipyridine and the dominant multiplet at 92.5 ppm, arising from complexed nitrogen atoms. The latter consists of two components, the major one at 91.6 ppm (Δδ of 28.4 ppm) assigned to nitrogen atoms in oligomeric chains and a minor one at 93.3 (Δδ of 31.0 ppm) assigned to the atoms in the 2 : 1 complex. Thus, the 15N CPMAS NMR technique allowed distinguishing three types of nitrogen atoms in the material. Because of the difficulties with unambiguous assignment of signals in 13C CPMAS NMR spectra, the Δδ(13C) parameter can be estimated only roughly. For example, in the case of the oligomer of Rh2Ac4 and 4 taken as a typical example, Δδ(13C) varied from 4.9 ppm (Ci) to +6.0 ppm (CH carbon atoms in the ligand). Different symmetry of the ligand and rhodium tetracarboxylate molecules causes chemical non-equivalency of acyl residues in the dirhodium core in the solid state. However, in the case of the complexes of Rh2Ac4 with 1 and 2, only broadening of CH3 and CO2 signals occurred (a line width of around 2–3 ppm). The 13C NMR spectrum of the complex of Mo2Ac4 and 1 consists of two signals of CO2, at 183.4 and 182.3 ppm, and two signals of the CH3 group with unequal intensity, at 25.1 and 23.3 ppm. The CO2 signal in the oligomeric complex of Rh2Bz4 with 4 appeared as a singlet. In conclusion, the signal dispersions did not exceed around 3 ppm in the case of heteroaromatic ligands. 2,2′-Bipyridine forms a 1 : 1 complex with Rh2Ac4 with the ligand bound in axial and equatorial positions (Fig.2a). The structure of this compound has been determined by an X-ray technique.[22] The 15N CPMAS NMR spectrum consisted of two signals, at 145.1 and 162.7 ppm (Δδ of 72.5 and 90.1 ppm). Based on DFT

calculations (discussed later in the text), the signals have been assigned to the equatorially and axially bonded nitrogen atoms respectively. Each of these peaks consists of two components with unequal intensities; this splitting can be explained by the presence of polymorphs in the analysed material. Signals of CO2 and CH groups consisted of three peaks within the range of between 185.1 to 192.1 ppm and between 18.1 and 26.5 ppm respectively. The corresponding signal dispersions were 7.0 and 8.4 ppm. The large dispersion of 15N and 13C NMR signals appears to be a characteristic feature of the complexes with axial–equatorial ligand arrangement. In the rhodium complexes with the amines RNH2(Rh), the substituents adopt a tetrahedral-like arrangement and are not co-linear with the Rh–Rh chain. Consequently, benzene-1,4- 5 and benzene1,3-diamines 6 were expected to form linear oligomers with a zigzag chain. Because of the freedom of rotation of the molecules around R–N bonds in a solution, the chain is expected to form a mixture of conformers in equilibrium. However, it is not the case of the solid state where the chain is rigid, adopting either the regular or chaotic arrangement. Although the structure of an isolated molecule does not correspond to this occurring in the crystalline state, we attempted to model some of the complexes of these two diamines assuming a molecule in vacuum and a regular arrangement of the chain. In order to simplify the task, rhodium(II) tetraformate Rh2form4 was assumed to be a substrate, and chains containing two ligand units and three dirhodium cores were constructed. The optimized structures were collected in Fig. 4. Dirhodium cores can be attached either on one side or on the opposite sides of a diamine aromatic ring (Z and E arrangements, respectively). Each arrangement resulted in a different shape of the chain. Each chain, in turn, can adopt various conformations. Three conformers were considered for Rh2form4 complex of 5. The analysis of conformer distribution according to Boltzmann revealed 69%, 25% and 6% of their proportion. In the case of Rh2form4 complex of 6, two structures were taken into account. Structure b-I appeared to be the preferred one. The CPMAS NMR spectra of Rh2Ac4 complex with 5 were shown in Fig. 3. The 15N NMR spectrum shows one narrow signal at 341.9 ppm (Δδ of 12.4 ppm). The signals of CO2 and CH3 carbon atoms consist of two peaks with unequal intensities, at 191.5 and 192.5 ppm, and 23.1 and 24.0 ppm respectively. Signal dispersion did not exceed 1 ppm. Corresponding NMR parameters of the

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Figure 4. The arrangements of N–Rh–Rh–N chains in selected rotamers of hypothetical 3 : 2 complexes of Rh2form4 with benzene-1,4-diamine 5 (a), benzene-1,3-diamine 6 (b) and benzene-1,2-diamine 7 (c). HCO2 residues have been omitted for simplicity. The optimizations were performed assuming isolated molecules in vacuum at the Becke, three-parameter, Lee-Yang-Parr/[6-31G(2d), LANL2DZ] level of theory.

A. Leniak, B. Kamieński and J. Jaźwiński complex of 6 were similar: δ(15N) of 346.1 ppm, Δδ(15N) of 21.7 ppm, the signal of CO2 consisting of two unequal peaks, at 191.7 and 192.5 ppm, and the signal at 23.2 ppm (singlet) arising from CH3. Observed Δδ(15N) parameters were typical for Rh complexes of amines; narrow signals suggested a regular structure of the oligomer. Benzidine 8 was expected to form a complex analogous to 5 with a similar arrangement of the functional group. However, important differences arose. The 15N NMR spectrum of this complex consists of two signals, a major signal at 340.3 ppm (Δδ of 14.9 ppm) and a minor one at 275.6 ppm (Δδ of +49.8 ppm). The latter could not be assigned to an NH2 group but to a product containing another nitrogenous functional groups, for instance, N=C. The major signal appeared as unresolved multiplets, indicating the number of non-equivalent nitrogen atoms in the molecule. In addition, the CH3 groups produced four signals within the range of between 35.5 and 23.2 ppm (dispersion of 12.2 ppm). These features indicated that the product was heterogeneous and contained, apart from linear oligomers, the products of further reactions including the rearrangement of dirhodium cores. The 15N NMR spectrum of the Rh2Ac4 complex with 7 consists of two narrow signals with equal intensities, at 343.5 and 345.8 ppm. The distance between signals, measuring 2.3 ppm (117 Hz), excludes the 1J(103Rh,15N) coupling as the cause of signal splitting (the magnitude of such coupling constant did not exceed around 30 Hz); these signals evidently arise from two chemically non-equivalent nitrogen atoms. The presence of two 15N NMR signals begs the question which kind of complex was formed: the 1 : 1 with axial–equatorial ligation (Fig. 2c,d) or the (1 : 1)n linear oligomer. The NMR data, in fact, did not allow unambiguous conclusion. We tentatively identified this compound as a linear oligomer taking into account the following: (i) The dispersion of 15N NMR signals was small, of 2.3 ppm, in comparison with the dispersion of Rh2Ac4 complex with 3, of 17.6 ppm. (ii) Similarly, the dispersion of CO2 and CH3 signals was less than that in complex of 3, of 3.6 and 3.0 ppm versus 7.0 and 8.4 ppm respectively. (iii) Complexation shifts were similar to those observed in linear complexes of 5 and 6, whereas in the case of axial–equatorial complex of 3, complexation shifts were greater than those in corresponding linear oligomers of 1 and 2. (iv) The complex precipitate immediately from reaction mixture, whereas the formation of axial–equatorial complex took usually at least a dozen minutes. The two possible arrangements of dirhodium units in the 2 : 1 complex of 3 were presented in Fig. 4c-I and c-II. The first structure, having E configuration of metal substituents, is preferred. In contrast to the c-II structure, this fragment can form regular oligomeric chains. The complex of benzoquinone 9 with Rh2Piv4 was the last one studied. This complex does not contain nitrogen atoms but exhibits

348

Figure 5. Fragment of the X-ray structure of Rh2Piv4 complex of 9. The (CH3)3CCO2 residues were omitted for clarity.

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interesting structural features: the ligand molecules are bonded via carbonyl oxygen atom and via double C=C bond of the quinone ring (Fig. 5).[24] In this case, the ‘carbonyl’ region of 13C CPMAS NMR spectrum appeared to be the most interesting. This part of the spectrum contains four signals arising from CO2(Rh) residues, within the range of 201.9 to 197.3 ppm, and two signals of C=O groups of the ligand, at 193.7 ppm (Δδ(13C) of +9.1 ppm) and at 184.2 ppm (Δδ of 0.4 ppm). The former was assigned to complexed C=O group; its Δδ parameter agree with our previous findings.[26]

Estimation of complexation shifts Δδ The calculations were performed by various computation methods, both relativistic [zeroth order regular approximation (ZORA)] and non-relativistic, applying the B3LYP (Becke, threeparameter, Lee-Yang-Parr) and GGA–PBE functionals. Two model structures were used: the hypothetical axial–equatorial complex of 2,2′-bipyridine 3 with Rh2form4 and the known complex of 3 with Rh2Ac4, having either optimized or the Xray geometry. Computed 15N NMR shielding constants were converted to chemical shifts using either reference shielding of nitromethane or applying the correlation method. The first method resulted in ‘raw’ chemical shifts δraw and the latter in the so-called scaled chemical shifts δscal.[27] The calculated values were collected in Table 2. Chemical shifts obtained from shielding constants by the use of reference shielding of σ CH3NO2 were quoted for comparison purposes, although this method provided underestimated values, bearing a systematic error of several dozen ppm. Only scaled chemical shifts appear to be suitable for further discussion. The calculations performed at the GGA–PBE/QZ4P level of theory using optimized structures provided the values ranging from 142.2 to 144.8 ppm (Neq) and from 158.9 to 165.3 ppm (Nax) depending on the method, whereas the measurements revealed the values of 145.1 and 162.7 ppm. No significant difference was noted between chemical shifts computed for Rh2Ac4 and Rh2form4 complexes. Thus, the latter can serve as a simplified model of the rhodium salt in calculations. The application of the X-ray geometry resulted in overestimated chemical shift values of around 155 and 175 ppm. Different results were obtained in the case of the B3LYP/[6311++G(2d,p), Stuttgart] method. The calculations performed for Rh2form4 complex revealed the 15N NMR chemical shifts of 144.7 and 152.0 ppm for Nax and Neq, respectively, in opposition to the GGA–PBE results. Nearly equal chemical shifts for both signals were obtained for Rh2Ac4-3 structure regardless of its geometry, X-ray or optimized. Ambiguous results were obtained for the B3LYP/[cc-pVTZ, Stuttgart] method as well. In order to resolve the uncertainties, we have considered two other parameters, the difference between signals (dispersion) δax δeq and complexation shifts Δδ, assuming signal order to be arising from each calculation. The GGA– PBE/QZ4P calculations yielded dispersions from around 15 to 23 ppm, close to the experimental value (17.6 ppm). The dispersion obtained from B3LYP calculations ranged from around 2 to +7 ppm. Similarly, the GGA–PBE/QZ4P-estimated Δδ parameters were slightly closer to the experimental data than those obtained from B3LYP calculations. Finally, the calculations performed with the use of the B3LYP functional

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Organic complexes of dirhodium and dimolybdenum: NMR and DFT studies 15

15

Table 2. Calculated N NMR shielding constants σ (ppm), N NMR chemical shifts δ (ppm) and selected bond lengths d (nm) of some complexes of 2.2′bipyridine 3a δax

δeq

δax

δeqb d(RhRh)

Theory level

Rhodium salt

d(RhNax)

d(RhNeq)

B3PW91/[6-311G++(2d,p), Stuttgart]c B3PW91/[cc-pVTZ, Stuttgart]c GGA–PBE/QZ4P non-relativistice GGA–PBE/QZ4P scalar ZORAe GGA–PBE/QZ4P spin-orbit ZORAe B3PW91/[6-311G++(2d,p), Stuttgart]c B3PW91/[cc-pVTZ, Stuttgart]c GGA–PBE/QZ4P non-relativistice GGA–PBE/QZ4P scalar ZORAe B3PW91/[6-311G++(2d,p), Stuttgart]c B3PW91/[cc-pVTZ, Stuttgart]f GGA–PBE/QZ4P non-relativisticf GGA–PBE/QZ4P scalar ZORAf 15 N CPMAS NMR

Rh2form4

37.0( 119.7)[ 144.7]

28.9( 127.8)[ 152.0]

+7.3

0.2518

0.2191

0.2088

Rh2form4 Rh2form4 Rh2form4 Rh2form4 Rh2Ac4

34.1( 34.7( 26.6( 9.2( 38.3(

119.5) 106.9)[ 115.9)[ 130.2)[ 118.4)[

159.7] 164.5] 158.9] 143.5]

31.6( 50.2( 49.1( 25.1( 35.6(

122.0) 91.4)[ 144.8] 93.4)[ 142.9] 114.3)[ 143.2] 121.1)[ 146.0]

+2.5d 14.9 21.6 15.7 +2.5

0.2525 0.2514 0.2516 0.2506

0.2159 0.2127 0.2127 0.2203

0.2053 0.2025 0.2056 0.2094

Rh2Ac4 Rh2Ac4 Rh2Ac4 Rh2Ac4

35.0( 33.6( 25.7( 31.3(

118.6) 107.9)[ 160.7] 116.8)[ 165.3] 125.4)[ 149.8]

37.4( 51.6( 49.8( 33.2(

116.2) 90.0)[ 143.5] 92.7)[ 142.2] 123.5)[ 148.1]

2.4d 17.2 23.1 1.7

0.2509 0.2497 0.2475g

0.2161 0.2130 0.2120g

0.2047 0.2018 0.2039g

Rh2Ac4 Rh2Ac4 Rh2Ac4 Rh2Ac4

26.9( 19.6( 13.4( 145.1/

126.7) 122.0)[ 174.2] 129.1)[ 177.1] 162.7

35.5( 118.1) 38.5( 103.1)[ 156.0] 35.7( 106.8)[ 155.7]

8.6d 18.2 21.4 ±17.6

0.2475g 0.2475g

0.2120g 0.2120g

0.2039g 0.2039g

Ligand 3 σ(δscal): 97.6[ 69.1] B3PW91/6-311++G(2d,p); 108.9[ 71.3] GGA-PBE/QZ4P non-relativistic; 109.2[ 72.9] GGA–PBE/QZ4P scalar ZORA; exp: 72.6 (DMSO), 78.9 (CDCl3). 15 a Calculated N NMR shielding values σ (ppm), chemical shift values δraw = σ CH3NO2 σ x (ppm, in parentheses) and scaled chemical shifts δscal = aσ + b (ppm, square brackets) were shown. The parameters σ CH3NO2, a and b were collected in Table 3. b Dispersion δax δeq calculated from scaled chemical shifts. c The geometries of molecules were optimized at the Becke, three-parameter, Lee-Yang-Parr[6-31G(2d), LANL2DZ] theory level. 15 d Theory level calculated on the basis of non-scaled N NMR chemical shifts. e The geometries of molecules were optimized at the same level of theory as NMR calculations, that is at the GGA–PBE/QZ4P level applying nonrelativistic, scalar and spin-orbit ZORA methods. f Calculations were performed using the X-ray geometry. g Distance taken from the X-ray structure. B3PW91, Becke, three-parameter, Perdew-Wang 91; ZORA; zeroth order regular approximation.

were recognized as too inaccurate. Consequently, the GGA– PBE/QZ4P calculations were taken as the basis for signal assignment, Nax and Neq.

Nucleus-independent chemical shifts in axial– equatorial complexes

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In order to understand NMR properties of complexes with axially and equatorially bonded ligands, we performed the calculations of nucleus-independent chemical shifts (NICS) in some complexes. This parameter is a measure of shielding value in the selected points of a molecule. The sign and magnitude of this parameter allow concluding on aromatic, anti-aromatic or non-aromatic character of rings in cyclic compounds.[28] Calculations have been performed for three points: the first located in the centre of Rh–N–C–C–N rings, the second in the centre of aromatic rings of ligands and the last one between the two nitrogen atoms in non-complexed ligands. These NICS values were denoted by NICS(0), NICS(1) and NICS(2) respectively. The calculations performed for the complex of 2,2′-bipyridine 3 with Rh2Ac4 resulted in NICS(0) of 6.3 and 12.7 ppm depending on the geometry of complex, optimized one or taken from X-ray data. Corresponding NICS(1) for two bipyridine rings adopted the values of 3.9 and 4.5 ppm (optimized geometry) and +2.9 and +1.3 ppm (X-ray geometry). Similar calculations performed for axial–equatorial Rh2Ac4 complexes of benzene-1,2-diamine (Fig. 2c) and 1,2-

ethylenediamine H2N(CH2)2NH2[21] provided the value of 0.9 and 1.8 ppm. In order to separate individual contributions to NICS, the calculations for non-complexed 3 were performed in the next step of the work. The calculations performed for the fully optimized structure of 3 exhibiting a twisted arrangement of aromatic rings (N–C1– C1′–N′ dihedral angle of 35.7°) resulted in NICS(1) and NICS(2) of 6.0 and +5.6 ppm respectively. For the structure with frozen N– C1–C1′–N′, dihedral angle (0°) and then relaxed corresponding values of 5.3 and 6.5 ppm were obtained. Finally, the NISC(1) of 3.2 and 3.5 ppm depending on the ring and NICS(2) of 15.6 ppm were obtained for the molecule having the experimental geometry (N–C1–C1′–N′ dihedral angle of 0.7°), taken from the Xray structure of the complex as it is. Similar procedure applied to non-complexed 7 having the geometry as in the complex yielded the NICS(1) and NICS(2) of 8.7 and 0.2 ppm respectively. In the case of a hypothetical axial 1 : 1 complex of Rh2form4 and 4,4′bipyridine, the calculations revealed NICS(1) of 3.9 for complexed ring of the ligand and 5.3 ppm for the non-complexed one. In order to obtain reference data, the calculations of NICS(1) for benzene (aromatic) and cyclobutadiene (anti-aromatic) have been performed, applying the same level of theory. The NICS(1) of 7.9 and +26.4 ppm have been obtained for these molecules respectively. The analysis of NICS values for this group of compounds leads to the two conclusions: (i) A major contribution to NICS(0) arises from ligand molecules. The formation of Rh–N–C–C–N ring

A. Leniak, B. Kamieński and J. Jaźwiński appeared to have a secondary meaning. (ii) The complexation decreases the aromatic character of bipyridine rings.

Conclusions i) 4,4′-Bipyridine 1, 3,3′-bipyridine 2 and pyrazine 4 as well as benzene-1,4-diamine 5, benzene-1,3-diamine 6 and benzene-1,2-diamine 7 form oligomeric complexes with rhodium tetracarboxylates, exhibiting a regular structure. In contrast, benzidine 8 forms a heterogeneous product containing the linear oligomer and the addition of products of further reactions. Elemental analysis revealed a 1 : 1 molar ratio of the ligand and the dirhodium core in the complexes. ii) Nuclear magnetic resonance, namely 13C and 15N CPMAS NMR techniques, appears to be a useful tool for investigating polymeric products in question. Especially, the number of signals in 15N NMR spectra and their chemical shifts and shape allow concluding on a regular or heterogeneous structure of the complex, on their axial or axial–equatorial binding modes and on the presence of non-complexed sites in the material. iii) As it was demonstrated, crystalline materials yielded narrow 15 N NMR signals. In contrast, amorphous mixtures were expected to produce broad, irregular signals. However, the presence of the cluster of narrow signals in a spectrum can put ambiguities, whether the material is heterogeneous or crystalline with non-equivalent ligand molecules in the unit cell (cf. the 15N CPMAS NMR spectrum of Rh2Ac4 adduct of 8). Another uncertainty arises from the low sensitivity of this technique and from its sensitivity to correctness of experimental parameters. As a consequence, some 15N signals can be missed (we were not able to detect 15N CPMAS NMR signals of some free ligands by applying a natural 15N abundance approach, despite numerous attempts). iv) The 13C CPMAS NMR technique provided supplementary information on the structures; the signals of CO2 and CH3 groups of the dirhodium core and the signals dispersion were the most diagnostic. v) Complexation resulted in the change of 15N NMR chemical shifts (Δδ parameter), from around 40 to 90 ppm in the case of heteroaromatic ligands and from around 12 to 22 ppm for diamines. The Δδ value appears to be lower in the case of Mo2Ac4 complex with 1, measuring from around 16 to 31 ppm only. vi) Density functional theory-computed 15N NMR shielding constants followed by the scaling procedure reproduced 15N NMR chemical shifts and 15N complexation shifts qualitatively.

Experimental

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All reagents were commercially available and have been used as received. Previously described complexes of 2,2′-bipyridine with rhodium tetraacetate,[22] 4,4′-bipyridine with molybdenum tetrakistrifluoroacetate,[23] pyrazine with rhodium tetrabenzoate[16] and benzoquinone with rhodium tetrapivalate Rh2Piv4[9,24] have been obtained as crystalline materials by published procedures. Their X-ray structures are known and were deposed in the Cambridge Structural Database, reference codes WEJZIU (1 with Mo2Tfa4), VIWGEN and VIVGEN10 (3 with Rh2Ac4), XUVMOQ and XUVMOQ01 (4 with Rh2Bz4), and PATLOL (9 with Rh2Piv4). New

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oligomeric complexes were obtained by the mixing of the two CH3CN solutions of rhodium tetraacetate (100/150 mg and 0.23/ 0.34 mM in 5-cm3 CH3CN) and ligand (a threefold molar excess in 1-cm3 of CH3CN). The mixture was stirred for a few hours, and the complex (fine, reddish powder) was filtered, washed with CH3CN and dried in vacuum. The samples were measured without additional purifications as bulk materials. Because of the solubility of the ligand and the rhodium salt in CH3CN, this procedure assured the presence of only oligomeric materials in the precipitate. The molar ratio of ligand to rhodium acetate has been estimated by elemental analysis (C, H, and N) according to a previously described method.[20,21] Complexes with phenylenediamines were synthesized under argon atmosphere because to their sensitivity to oxygen in a solution. Elemental analysis (%), Rh2Ac4 complexes: 1 C 37.03, H 3.58, N 5.13, Rh2Ac4 : 1 = 1 : 1.10; 2 C 36.49, H 3.69, N 5.03, Rh2Ac4 : 2 = 1 : 1.04; 5 C 31.03, H 3.75, N 5.34, Rh2Ac4 : 5 = 1 : 1.08; 6 C 31.16, H 3.73, N 5.25, Rh2Ac4 : 6 = 1 : 1.08; 7 C 30.45, H 3.61, N 5.07, Rh2Ac4 : 7 = 1 : 0.98; 8 C 38.08, H 4.08, N 5.77, Rh2Ac4 : 8 = 1 : 1.00; Mo2Ac4 complex: 1 C 37.72, H 3.68, N 4.26, Mo2Ac4 : 1 = 1 : 1.05. All CPMAS NMR spectra were recorded on a Bruker 500 Avance2 spectrometer at room temperature (295–298 K) using a 4-mm broadband probe and by applying the Hartmann–Hahn crosspolarization pulse sequence (Bruker’s ‘cp.av’ pulse programme). Typically, a spin rate of 10 KHz was applied, although for some purposes (signal identifications) spin rates of 6 and 8 kHz were used. The 13C CPMAS NMR spectra were acquired using the parameter values of 31.25 kHz (250 ppm) for spectral width, 15 ms for acquisition time, 30 s for relaxation delay and spin lock contact time of either 2 ms (detection of all signals) or 0.04 ms (detection of CH and CH3 signals only); typically, 1024 scans were acquired. In the case of 15N CPMAS NMR, the parameters have been adjusted and optimized using glycine, and then applied to the further samples. Namely, the typical parameter values were set at 38.5 kHz (760 ppm) for spectral width, 35 ms for acquisition time, 30 s for relaxation delay and 4 ms for spin lock contact time, and ca. 8000 scans were collected. If necessary, two parameters, the relaxation delay and spin contact time, were re-adjusted by a trial and error method. The spectra were transformed using the 8-K matrix; digital spectral resolutions from 3 to 5 Hz per point were achieved. All CPMAS spectra were referred to glycine signals: 43.3 ppm with respect to TMS, 0 ppm (13C), and 347.6 ppm with respect to CH3NO2, 0 ppm (15N). Nitrogen-15 chemical shifts were collected in Table 1; 13C CPMAS NMR data for ligands and complexes were given in the following text. Reference chemical shifts of free ligands have been measured in the present work or taken from the literature[29–32] including 13C CPMAS NMR data of quinone.[33] 13

C CPMAS NMR (ppm) data for ligands and complexes:

1: 145.7, 149.3 Ci, CH, 123.3 CH; (Rh2Ac4-1)n: 192.0 CO2(Rh), 151.7 CH, 144.4 Ci, 121.6 CH, 23.6 CH3(Rh); (Mo2Ac4-1)n: 183.4, 182.3 CO2(Mo); 150.8br CH, 119.8br CH, 25.1, 23.3 CH3(Mo); (Mo2Tfa4-1, the 1 : 2 complex and oligomer): 165.0–167.5 CO2 (Mo); 148.6br CH, 143.5, 142.4 Ci, 119.8br CH. 2: 149.1 CH, 133.6 Ci, CH, 125.3 CH, 122.7 CH; (Rh2Ac4-2)n: 190.7 CO2(Rh), 149.9 CH, 145.6 CH, 132.7 Ci, CH, 126.3 CH, 23.3 CH3(Rh). 3: 154.8 Ci, 149.0 CH, 135.8 CH, 123.6 CH, 120.7 CH; Rh2Ac4-3: 192.1, 184.5, 185.1 CO2(Rh), 158.9 CH, 156.1, 154.8 Ci, 147.4 CH, 138.5 CH, 126.9, 125.2 CH, 26.5, 24.0, 21.7, 20.0, 18.1 CH3(Rh). 4: n.m.; (Rh2bz4-4)n: 185.3 CO2(Rh) 146.9 CH, 129.5 m CH(Rh). 5: 141.0, 138.9, 138.6 m Ci, 117.6 m, CH; (Rh2Ac4-5)n: (192.5), 191.5 CO2(Rh), 141.5 Ci, 121.1 CH, 24.0, (23.1) CH3(Rh).

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Organic complexes of dirhodium and dimolybdenum: NMR and DFT studies Table 3. The σ CH3NO2 (ppm), a and b parameters applied for the scale conversiona Theory level for NMR calculations B3PW91/[cc-pVTZ, Stuttgart] B3PW91/[6-311G++(2d,p), Stuttgart] GGA–PBE/QZ4P (non-relativistic) GGA–PBE/QZ4P (scalar ZORA) GGA–PBE/QZ4P (spin-orbit ZORA)

σ CH3NO2

a

153.6 156.7 141.56 142.47 139.38

n.a. 0.88( 0.87( 0.87( 0.87(

b

0.90) 0.96) 0.96) 0.99)

n.a. 155( 178) 166( 193) 167( 190) 166( 168)

a

All parameters were taken from Leniak and Jaźwiński[27]; a and b coefficients refer to free ligands and complexes (in parentheses). B3PW91, Becke, three-parameter, Perdew-Wang 91; ZORA, zeroth order regular approximation ; n.a., not available.

6: n.m.; (Rh2Ac4-6)n: (192.5), 191.7, 188.9 CO2(Rh), 149.6 Ci, 127.7, 119.8, 115.5 CH, 23.2 CH3(Rh). 7: n.m.; Rh2Ac4-7: 192.9, 189.8, 189.3 CO2(Rh), 136.3, 135.6 Ci, 124.1, 122.1, 121.2 CH, (24.5), 22.5, (21.5) CH3(Rh). 8: 147.2, 144.8 Ci, 132.2 Ci, 127.0 m, Ci, CH, 18.9, 117.2, 115.3 m, CH; (Rh2Ac4-8)n: 191.6–188.9 m CO2(Rh), 163.3 CH, 146.4–143.0 m Ci, 134.1, 132.6 Ci, 126.4 m CH, 121.0 m CH, (35.5), (29.9), 24.5, (23.3) CH3(Rh). 9: [33] 184.6 C(1)=O, C(4)=O, 139.4, 137.8 C2, C4; 127.7, 127.3 C5, C8; 137.0, 135.8 C6, C7; 131.5 C9, C10; (Rh2Piv4-9)n: 201.9, 200.9, 198.6, 197.3 CO2(Rh); 193.7, 184.2 C=O; 134.2, 132.0, 128.4, 124.9, 122.0 CH; 41.6, 40.8, 40.3 C(CH3)3; 28.4, 27.6 C(CH3)3

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Acknowledgements The calculations of NMR parameters have been performed in the frame of the project no. N N204 266739 (2010–2013) of the Ministry of Science and Higher Education of Poland.

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The abbreviation n.m. stands for not measured; minor components are in parentheses. The calculations were performed with Gaussian 09[34] and Amsterdam Density Functional programme suite.[35] All calculations were performed assuming isolated molecules. The first approach included the optimization of geometry using the B3LYP functional with the 6-31G(2d) basis set for C, H, N and O atoms and the Los Alamos National Laboratory basis set of Double Zeta quality (LAN2DZ) for Rh, and the Gauge-Independent Atomic Orbital (GIAO) NMR calculations using the Becke, three-parameter, Perdew-Wang 91 functional with the 6-311G++(2d,p) basis set for C, H, N and O atoms and the Stuttgart Relativistic Small Core (RSC) designed for an Effective Core Potential (ECP) revision 1997 basis set for Rh atoms.[36] In the case of CH3NO2, the referenceshielding constant was computed using the previous methods and assuming the integral equation formalism variant of polarizable continuum model of solvation assuming CH3NO2 as the solvent. Alternatively, the calculations were carried out using the GGA-PBE functional (Generalized Gradient Approximation with Perdew-Burke-Emzerhof correlation and exchange corrections) with valence quadrupole zeta and 4 polarization functions, relativistically optimized (QZ4P)[35] applying the non-relativistic, scalar ZORA and spin-orbit ZORA methods. The optimization of geometries of all molecules and 15N NMR-shielding calculations was performed at the same level of theory, assuming isolated molecules. For comparison purposes, some calculations were performed using the X-ray geometry. The shielding scale was converted to the chemical shift scale by two ways: by the use of reference shielding according to the equation δ = (σ CH3NO2 σ x) and by the correlation method using the linear equation δ = aσ + b, where δ and σ x denote computed 15N NMR chemical shifts and shielding constants, respectively, σ CH3NO2 denotes the reference 15N NMR shielding constants of CH3NO2 computed at the same level of theory as in the case of compounds in question, and a and b refer to coefficients determined by linear regression analysis for each data set. Depending on the method of scale conversion, calculated 15N

NMR chemical shifts δ were denoted as δraw and δscal respectively. The σ CH3NO2 (ppm), a and b parameters were collected in Table 3. The calculations of NICS have been performed according to the described method[28] using the Gaussian ‘ghost atom’ option. Compound geometries have been taken from X-ray or optimized structure by using the B3LYP functional, the LANL2DZ basis set for Rh and 6-31G(2d) basis set for the remaining atoms. NICS have been computed using the Becke, three-parameter, Perdew-Wang 91 functional, the Stuttgart basis set placed on Rh and 6-311++G (2d,p) basis set placed on C, H, N and O atoms. Coordinates of ghost atoms have been found as non-weighted mean of the heavy atoms coordinates, either of the ring in question or the two nitrogen atoms in the ligands. According to NMR chemical shift convention,[28] computed values were taken with the opposite sign.

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Oligomeric complexes of some heteroaromatic ligands and aromatic diamines with rhodium and molybdenum tetracarboxylates: 13C and 15N CPMAS NMR and density functional theory studies.

Seven new oligomeric complexes of 4,4'-bipyridine; 3,3'-bipyridine; benzene-1,4-diamine; benzene-1,3-diamine; benzene-1,2-diamine; and benzidine with ...
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