Synthesis, crystal structure and hydrogen bonding of a Cp*Rh(III) complex bearing pyridyl azine ligands: a combined experimental and DFT approach

Abstract We report the synthesis of a Cp*Rh complex bearing a pyridyl azine ligand with a free amine (NH2) group. The reaction of [Cp*RhCl2]2 with ligand (L) formed a cationic complex with the formula [RhIII(η5-C5Me5)(L)к2 (N,N’)Cl]SbF6 (1) that was characterized by IR, UV-Vis, 1H, 13C{1H} NMR, etc. The SC-XRD analysis of the complex revealed the condensation of an acetone molecule with the azine NH2 group, thus forming an imine –C=N– bond in the ligand, giving [RhIII(η5-C5Me5)(L’)к2 (N,N’)Cl]SbF6 (2). The coordination geometry around rhodium in 2 is pseudo-tetrahedral with coordination sites occupied by a Cp* moiety in an η5 manner, a terminal chloride, and a pyridyl azine ligand in a bidentate N,N’ fashion through N(pyridine) and N(azine) nitrogen atoms occupying the four vertices of the tetrahedron. The crystal packing is mainly directed by moderate and weak hydrogen bonds including C–H⋯F, C–H⋯Cl, N–H⋯Cl and C–H⋯N interactions. The overall crystal packing and the significant role of hydrogen bonding in the supramolecular assembly has been discussed by performing Atoms-In-Molecules (AIM) and Natural Bond Orbital (NBO) analyses.


Introduction
Interest in the chemistry of half-sandwich arene Ru, Cp Ã Rh and Cp Ã Ir complexes positions these complexes as versatile subjects of organometallic chemistry, in part due to their crucial and potential applications [1][2][3].These Cp Ã Rh and Cp Ã Ir complexes are studied as an alternative to platinum-based drugs because of their high water solubility and utility as catalysts for various organic reactions [4][5][6].These complexes possess the general formula [Cp Ã MCl(LL 0 )] (M ¼ Rh, Ir and LL' ¼ various bidentate ligands), where the chemistry with different NN 0 , NO and NS ligands has been explored.The choice of the ligands plays a crucial role in enabling reactivity of these complexes, which in turn affects their biological activities [7].Studies have also shown that ancillary ligands control the reactivity towards biomolecules via hydrogen bonding and hydrophobic interactions, and the counter ion affects solubility and permeability [8].Also, the counterions in these complexes exhibit several types of intermolecular interactions, which leads to the overall formation of supramolecular architectures [9].Taking inspiration from this, a number of ruthenium complexes were synthesized with hydrogen bonding substituents attached to the arene ligand and evaluated for their biological activity [10].Sadler et al. reported Cp Ã Ir complexes with chelating C,N and NN 0 donor ligands, which were shown to possess strong anti-proliferative activity [11,12].Very recently, the antiplasmodial activities of substituted cyclopentadienyl rhodium and iridium complexes of 2-(2-pyridyl)benzimidazole were reported by Chellan et al. [13].
Azine ligands (diazine and pyridyl azine) are of importance to coordination chemistry.These ligands are easily prepared by condensation of aldehyde, ketone or nitrile groups with hydrazine hydrate and possess a flexible (-N-N-) bond.Due to its rotational flexibility and multiple donor sites, these ligands can yield a rich library of coordination compounds [14].The azine (-N-N-) ligand has been found to act as a bridge to bring two metal ions in proximity, thus yielding polynuclear complexes [14].
In this regard, many diazine ligands together with transition metal azido complexes have been utilized to obtain 1D, 2D and 3D polymers exhibiting interesting magnetic behavior [15,16].Also, several homo and dinuclear half-sandwich p-cymene Ru(II), Cp Ã Rh(III) and Cp Ã Ir(III) complexes bearing pyridyl azine ligands have been reported with interesting coordination modes [17].In this work, we report the synthesis and theoretical studies of a Cp Ã Rh(III) complex bearing a pyridyl azine ligand with free NH 2 groups.

Physical methods and materials
The reagents used were of commercial quality and used without purification.RhCl 3 ÁnH 2 O was purchased from Arora Matthey Limited.Pentamethylcyclopentadiene, 2-cyanopyridine and hydrazine hydrate were purchased from Sigma-Aldrich.Precursor [Cp Ã RhCl 2 ] 2 was prepared according to published procedures [18].The pyridyl azine ligand used in this work has been previously reported [17c]. 1 H and 13 Cf 1 Hg NMR spectra were recorded on a Bruker Advance II 400 MHz spectrometer using CDCl 3 ; chemical shifts were referenced to TMS.Chemical shift values, d, are reported in ppm (parts per million) and coupling constants (J) in Hertz.Electronic spectra were recorded on a Perkin-Elmer Lambda 25 UV/Visible spectrophotometer from 200-800 nm at room temperature in acetonitrile.Infrared spectra (KBr pellets; 400-4000 cm À1 ) were recorded on a Perkin-Elmer 983 spectrophotometer.

Structure determination by X-ray crystallography
For growing single crystals, we employed a solvent diffusion method by layering solution of the compound in acetone with a fourfold excess of hexane and allowing it to stand undisturbed for several days.Suitable single crystals were selected and glued onto the tip of a glass fiber, which was centered in the X-ray beam.Single crystal data for 2 were collected with an Oxford Diffraction Xcalibur Eos Gemini diffractometer using graphite monochromated Mo-Ka radiation (k ¼ 0.71073 Å).The strategy for the data collection was evaluated using the CrysAlisPro CCD software.Crystal data were collected by standard "phi-omega scan" techniques and were scaled and reduced using CrysAlisPro RED software.The structures were solved by direct methods using SHELXS-97 and refined by full-matrix least squares with SHELXL-97 refining on F 2 [19,20].The positions of all the atoms were obtained by direct methods.Metal ions in 2 were located from the E-maps and all non-hydrogen atoms were refined anisotropically by full-matrix least-squares fitting.Hydrogen atoms were placed in geometrically idealized positions and constrained to ride on their parent atoms with C-H distances of 0.95-1.00Å. Isotropic thermal parameters U eq were fixed such that they were 1.2 U eq of their parent atom U eq for CHs and 1.5 U eq of their parent atom U eq for methyl groups.The crystallographic and structure refinement parameters for 2 are summarized in Table 1. Figure 1 was drawn with the ORTEP3 program [21].The crystal structure contains some disorder in Cp Ã and SbF 6 moieties, which were modeled using EADP, DFIX, and DELU commands.The Check CIF file contains some alerts which are generated because there is a large amount of disorder in the structure, along with weak and relatively imprecise diffraction data, however the connectivity is clear and enables conclusions about crystal packing and H-bonding.

Hirshfeld surfaces
Using Crystal Explorer [22], the Hirshfeld surface [23] of the Rh complex was identified and mapped across d norm , shape index, and curvedness.The shorter contacts are shown in red, longer contacts are highlighted in blue, and the contacts near the vdW separation are specified in white.Longer and weaker contacts are indicated by smaller, light-colored spots.

Computational methodology
DFT-based calculations have been carried out using B3LYP [24] functional to estimate the hydrogen bonding energies (E HB ).The supramolecular calculations and the estimation of energies were done using the crystallographic coordinates obtained from the single crystal XRD structure.Two different basis sets have been used i.e. 6-31 þ g(d,p) for C, H, N, F and Cl atoms and LANL2DZ for heavy atoms like Rh and Sb.The hydrogen bonding energies were computed employing the V(r) values at the BCP obtained from Atoms-In-Molecules (AIM) analysis [25], performed at the same level of theory, using the Multiwfn program [26].Natural Bond Orbital (NBO) analysis [27] was carried out to have a better understanding of the extent of orbital interactions involved in the hydrogen bond formation.

Results and discussion
3.1.Synthesis of [Rh III (g 5 -C 5 Me 5 )(L)r 2  (N,N') Cl]SbF 6 (1) The Cp Ã Rh(III) complex was synthesized by reaction of dimeric [Cp Ã RhCl 2 ] 2 precursor with L in methanol.This complex was isolated as yellow cationic salt with SbF 6 counterions and is stable and non-hygroscopic in the solid state. 1 was characterized by spectroscopic methods, which confirmed its formation.A vapor diffusion method using acetone and hexane was used to isolate a single crystal for crystallography.
3.2.Spectroscopic characterization of [Rh III (g 5 -C 5 Me 5 )(L)r 2  (N,N') Cl]SbF 6 (1) Preliminary confirmation of the formation of 1 was evidenced by its IR spectrum (Figure S1), which exhibited characteristic bands corresponding to C¼N, C¼C and NH 2 stretching frequencies.The NH 2 frequencies were observed at 3115-3132 cm À1 and were almost unaltered as compared to the free ligand, suggesting that it is not involved in coordination.The C¼N stretch was observed at slightly higher frequencies as compared to the free ligand, around 1501-1585 cm À1 , indicating coordination of the azine ligand to rhodium.In addition, a strong band at 665 cm À1 for the Sb-F bond was observed corresponding to the stretching frequency of the counter ion SbF 6 , thus confirming the formation of a cationic complex.
To further confirm the coordination of pyridyl azine ligand (L) to the rhodium ion, NMR spectra were recorded.Proton signals corresponding to pyridyl ring and Cp Ã fragment confirmed attachment of the ligand to the rhodium ion.The signals associated with the ligand appeared as doublets and triplets in the downfield region around 7.01-8.96ppm.The NH 2 protons were observed as a multiplet at 7.01-7.06ppm.In addition to these proton signals a sharp singlet for the methyl protons of Cp Ã fragment was observed at 1.59 ppm (Figure S2).
The 13 Cf 1 Hg NMR spectra also supported formation of the Cp Ã Rh complex, which displayed signals associated with the carbons from ligand, methyl carbon of Cp Ã fragment and ring carbon of Cp Ã .The carbon resonances for the ligands were observed at 132.83-160.61ppm.Also, a peak at 98.68 ppm was observed for the ispo (ring) carbons of Cp Ã and the methyl carbon resonance of Cp Ã was observed at 8.51 ppm (Figure S3).Overall, the results from spectroscopic studies strongly supports the formation of the Cp Ã Rh complex containing the azine ligand.(N,N') Cl]SbF 6 (2) In an attempt to crystallize the rhodium complex with two free NH 2 groups, we found that a condensation reaction between acetone molecules and the azine -NH 2 occurred during crystallization, thus resulting in the formation of an imine -C¼Nbond as observed from the molecular structure.This phenomenon has also been observed for a Cp Ã Ir complex by Rao et al.where they observed condensation of an amine with acetone during workup [28].The ORTEP plot of 2 is shown in Figure 1.In its asymmetric unit there is a cationic fragment of Cp Ã Rh and an antimony hexafluorophosphate anion.The crystallographic details and structure refinement parameters are summarized in Table 1.The complex crystallized in the orthorhombic crystal system with space group P2 1 2 1 2 1 .Single crystal X-ray diffraction was carried out to confirm the coordination of the ligand and to understand the geometry of the complex.The coordination geometry is best described as pseudo-tetrahedral wherein the centroid of the Cp Ã ring, the pyridine-N, the imine-N and Cl occupy the four vertices of the tetrahedron around Rh.In this three-legged "piano stool" type geometry, the coordination sites are occupied by two nitrogen donor atoms from the chelating azine ligand in a bidentate j 2 NN 0 manner through pyridyl and the azine nitrogen, leading to formation of a five-membered metallacycle, one chloride at the base of the stool and the Cp Ã ring in a g 5 manner forming the apex of the stool.The Rh-Centroid distance of the Cp Ã fragment is 1.789 Å whereas the Rh(1)-N(1), Rh(2)-N(3) and Rh(1)-Cl(1) bond distances are 2.126(9), 2.123(8) and 2.436(2) Å, respectively.The Rh-N (pyridine, azine) bond lengths of 1 are comparable to those reported for similar rhodium complexes containing azine donor ligands [17,29].The N(3)-N(4) bond length is 1.42(1) Å and can be defined as a N-N single bond which is comparable to the N-N bond distance in hydrazine (1.47 Å).The N-Rh-N bond angle is 75.0(3) and N-Rh-Cl bond angles are 84.2(2) and 92.3 (2) .These values are comparable to similar reported values for Cp Ã rhodium complexes bearing azine ligands [17,29].

Crystal packing and intermolecular interactions
In the crystal packing of 2 several types of weak intermolecular interactions and hydrogen bonding were present.The chloride attached to the rhodium center was involved in C-HÁ Á ÁCl (2.802 Å) and N-HÁ Á ÁCl (2.524 Å) interactions between the hydrogens from the pyridine ring and amino hydrogens as shown in Figure 2. Also, the hydrogens of the cationic fragment were involved in C-HÁ Á ÁF interactions between the hydrogens from the amine, Cp Ã (methyl) and methyl hydrogens with fluorine of PF 6 as shown in Figure 3.

Hirshfeld surface analysis and supramolecular interactions
From the SC-XRD structure, 2 possesses significant hydrogen bonding between the repeating monomer unit, which forms the three-dimensional network of the supramolecular architecture.Also, the same hydrogen bonded contacts such as C-HÁ Á ÁF, C-HÁ Á ÁCl, N-HÁ Á ÁCl and C-HÁ Á ÁN have been observed from the 2D fingerprint plot as well.The 2D fingerprint plots recognized from d e and d i explain the observed contacts/interactions in the complex.The FÁ Á ÁH contacts are observed in the Hirshfeld surface analysis of 2 which constitutes about 38.8% of the total interactions.The ClÁ Á ÁH contacts constitute up to 6.2% whereas the C-HÁ Á ÁN contributes up to 3.1% of the total interactions.The intermolecular interactions appearing as distinct spikes in the 2D fingerprint plot are shown in Figure 4.The Hirshfeld surface with a red/white/blue color scheme is displayed with the d norm parameter.The bright red spots represent shorter connections, the white regions represent contacts around the van der Waals separation, whereas the blue regions indicate the absence of any close contacts.

DFT studies
Quantum mechanical calculations were carried out to understand the role of hydrogen bonding in the crystal packing.Numerous theoretical studies, adapting Espinosa's work [30], utilize the inter-relationship between the topological parameters of the bond critical points (BCP) and the interaction energy of the hydrogen bonded complexes.Atoms-In-Molecule (AIM) is a useful method to determine the BCPs (in nonbonded interactions) and their various properties such as electron density q(r), their Laplacian r 2 q(r), the local kinetic energy (G(r)), potential energy (V(r)), and total local energy H(r) [31].Espinosa proposed a relationship which estimates the hydrogen bond energy from the values of V(r), given as D int (HB) ¼ 1/2 V(r) (where D int is the interaction energy).This approach has been used effectively in hydrogen bonding studies especially for systems having anion-cation interactions [32], as the Coulombic interactions between the cation and the anion counterparts sometimes project abnormally high hydrogen bond energies [33].As shown in Figure 5, the asymmetric unit of the Rh complex possesses two weak hydrogen bonds i.e. pyridyl C-HÁ Á ÁF and methyl C-HÁ Á ÁF interactions having HÁ Á ÁF distances of 2.674 and 2.554 Å, respectively.AIM analysis also confirms a BCP in these  hydrogen bonded contacts which further validates the existence of hydrogen bonding between the H atoms of the ligand and the F atom of the counter anion.We have calculated the hydrogen bond energy of these two contacts utilizing the values of V(r) at the BCP.The pyridyl C-HÁ Á ÁF and methyl C-HÁ Á ÁF interactions are characterized with a V(r) value of À0.0029 and À0.0047 a.u., respectively.Employing Espinosa's relation, the hydrogen bond energies of these contacts are À3.84 and À6.17 kJ/mol.The sum of the hydrogen bond energy of these two contacts is about À10 kJ/mol meaning these hydrogen bond interactions provide a substantial stability to the whole supramolecular architecture.The formation of these two hydrogen bonded pyridyl C-HÁ Á ÁF and methyl C-HÁ Á ÁF interactions account for second order hyperconjugation energies of 3.2 and 4.9 kJ/mol, resulting from the orbital interactions of the lone pair of F and the antibonding orbital of the C-H bond.
Beyond the asymmetric unit, hydrogen bonding plays a vital role in the 3D arrangement of the supramolecular packing as well.As shown in Figure 6, the dimeric units are connected to each other through hydrogen bonded contacts.There are four hydrogen bond contacts, i.e.C-HÁ Á ÁF, N-HÁ Á ÁF, N-HÁ Á ÁCl and C-HÁ Á ÁCl (BCP1, BCP2, BCP3 and BCP4) which hold the dimeric structure together.The intermolecular hydrogen bond distances were measured as 2.502, 2.323, 2.524 and 2.802 Å, respectively.These contacts were validated via AIM analysis, which showed bond critical points between the two hydrogen bonded atoms.The V(r) values for these contacts are À0.0053,À0.0084, À0.0076 and À0.0040 a.u.The estimated hydrogen bond energies for these contacts are À6.96,À11.03, À9.98 and À5.25 kJ/mol.The sum of these four energies (-33.02kJ/mol) roughly estimates the total hydrogen bond energy that forms between two monomeric units.

Conclusion
We report the synthesis of a Cp Ã Rh(III) complex that was supported by a pyridyl azine ligand and was characterized by spectral studies.Crystallographic analysis of this complex confirmed the coordination of the ligand to the rhodium ion.Subsequent condensation of an acetone molecule with the azine NH 2 group was observed, yielding an imine -C¼Nbond.The geometry of this complex was pseudo-tetrahedral, with the pyridyl azine ligand coordinated to rhodium in a bidentate chelating fashion through its pyridyl and azine nitrogen atoms.DFT studies (AIM and NBO) demonstrated the importance of hydrogen bonds in the overall crystal packing and stability of the supramolecular assembly.

Figure 3 .
Figure 3.A view of the C-HÁ Á ÁF interactions between the hydrogens of cationic fragment with PF 6 -.

Figure 5 .
Figure 5. (a) AIM picture of 2 showing bond critical points (BCP); (b) hydrogen bond contacts in the asymmetric unit of 2.

Figure 6 .
Figure 6.(a) AIM picture of the dimeric assembly of 2 showing bond critical points (BCP); (b) hydrogen bonded contacts in the dimeric unit of the Cp Ã Rh complex.