Five Coordinate Platinum(ii) in [pt(bpy)(cod)(me)][sbf 6 ]: a Structural and Spectroscopic Study

The five coordinate organoplatinum complex [Pt(bpy)(cod)(Me)][SbF6] (cod = 1,5-cyclooctadiene, bpy = 2,2'-bipyridine) was obtained reacting [Pt(cod)(Me)Cl] with Ag[SbF6] and bpy and characterized by multiple spectroscopy (IR and NMR) and single crystal XRD. Although the application of the τ values for the discrimination between trigonal bipyramidal vs. square pyramidal coordination fails, the molecular structure can be unequivocally described as basally-distorted trigonal bipyramidal. Detailed multinuclear NMR spectroscopy in solution at ambient temperature gives strong evidence for the same structure; corresponding low-temperature measurements down to −70 °C revealed no marked dynamic processes.


Introduction
From general considerations, five coordinate Pt(II) complexes are a bit unusual.Due to its d 8 configuration and a very strong ligand field (5d element), platinum in the oxidation state +2 has a strong tendency to form square planar coordinated [PtL4] n (n = charge) complexes.The non-occupation of the high-energy dx 2 -y 2 orbital and the stabilization of the dz 2 orbital are energetically the main reasons for this [1].The highest occupied metal orbital is thus dxy, and this is considered to be the reason that binding to further ligands in the axial positions is not favored.However, this only rules out stable five coordinate Pt(II) complexes with a square-pyramidal configuration.Thus, it is generally assumed that five coordinate Pt(II) complexes with a trigonal bipyramidal geometry might be stable or intermediate species, while square pyramidal species rather have the character of a transition state [2][3][4].Indeed, most of the structurally-characterized five coordinate Pt(II) complexes exhibit clearly a trigonal bipyramidal geometry [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20].Interest in five coordinate Pt(II) species and their Pd(II) analogues comes from their involvement as intermediates in important Pt(II) or Pd(II) catalyzed organometallic transformations [3,[21][22][23][24][25][26][27][28].
The reason why the Pt derivative of the recently reported binuclear Pd complex [(μ-bpy){Pd(bpy)(Me)}2][SbF6]2 [87] could not be observed at all under the chosen reaction conditions lies very probably in the far slower kinetics of Pt(II) compared with Pd(II).

Crystal and Molecular Structures
From a saturated acetone solution, single crystals of [Pt(bpy)(cod)(Me)][SbF6] were obtained.The structure was solved and refined in the monoclinic space group P21/c with the results depicted in Figure 1 (data in Table 1).
142.9(3)°, 97.9(3)° Sum of angles around Pt b 359.4(2) a X 1 = centroid of the equatorial C35-C36 bond; X 2 = centroid of the axial C31-C32 bond.b As defined by N10, N20 and X 1 .A number of weak intermolecular contacts were observed; amongst them are rather weak H … F hydrogen bridges (shortest H … F distance of 2.407(10) Å and the graphite-type π-π-stacking of the bpy ligands (interplanar distance of 3.8015(3) Å).They both contribute probably only weakly to the crystal structure and have no impact on the molecular structure (for detailed information, see the Figures and Tables in the Supplementary Material).
The molecular structure shows a five coordinate complex with both the bpy and the cod ligand in chelate binding.A closer look reveals that one of the olefin groups shows a tighter bonding to Pt with a Pt-X (X = centroid of the olefin bond) of 1.962(10) Å (X1), while the other reveals a distance of 2.215(10) Å (X2).The same huge differences were found for the much related complex [Pt(N^N')(cod)(Me)](BF4) with N^N' = N-((6-chloropyridine-2-yl)methylene)-4-methoxyaniline) [72].The overall geometry for five coordinate complexes can be drawn from the so-called tau-value τ = (β -α)/60 [88,89] with α and β being the largest basal angles.τ is one for an ideal trigonal bipyramid and zero for a square-pyramidal coordination (Scheme 1).Scheme 1.The square pyramidal and trigonal bipyramidal geometries with the basal angles α and β.Note that α = β is 180° for the square pyramid only in case that the metal atom lies within the basal plane.

NMR Spectroscopy
The title complex [Pt(bpy)(cod)(Me)] + was studied in detail by multinuclear NMR spectroscopy, as shown in Figures 2-4, allowing the complete assignment of all C and H atoms (Scheme 3).At the low field of the 1 H NMR spectrum (Figures 2 and 3), four signals represent the four magnetically-independent protons of the bpy ligand in [Pt(bpy)(cod)(Me)] + .Thus, both N atoms bind to the Pt atom in a local Cs symmetry, in line with the molecular structure in the crystal (Figure 1) and the Structures (a), (e) and (f) in Scheme 2. The two olefin protons at 3.5 and 5.5 ppm and the Me coligand (Table 2), respectively, show satellites due to 2 JPt-H coupling ( 195 Pt with I = ½ and 33.7% nat.abundance).While the signal at 5.5 ppm of one of the olefin groups lies in the normal range for olefin Pt(II) complexes [53][54][55][94][95][96][97][98][99][100][101][102][103][104], an upfield shift to 3.5 ppm is unusual and has been assigned previously to correspond to an axial olefin ligand in a trigonal bipyramidal configured five coordinate Pt(II) complex [53][54][55].In our previous work, we could show that the 2 JPt-H coupling constants have been found to be very indicative of the local symmetry around Pt and the trans influence of the translocated coligand [94][95][96].The signal at 5.5 ppm exhibits a coupling of 31 Hz, indicative of a very strong trans-oriented ligand, very probably the Me coligand, e.g., in [Pt(cod)(Me)Cl], the olefin protons trans to Me show a 2 JPt-H coupling of 36 Hz, while those trans to Cl exhibit 75 Hz [95].The signal at 5.5 ppm shows satellites with a 2 JPt-H coupling of 74 Hz, and a weak ligand must be in the trans position; presumably, this is bpy.For the pyridine complex [Pt(cod)(Me)(Py)] + , a value of 72 Hz was recorded in the same solvent [44].The Me signal exhibits a 2 JPt-H coupling of 66 Hz.This clearly indicates a trans-oriented ligand, thus ruling out Structures (c) and (f) in Scheme 2. In square planar codPt(Me) complexes, this value ranges from 65 to 83 Hz [44,[94][95][96][101][102][103], putting the observed value at the lower end, which is in line with one of the olefin groups (X2) being this trans ligand.Furthermore, at 600 MHz, the cod CH2 groups show four distinct multiplets from 2.2 to 2.9 ppm (Figures 2 and 3).Detailed 2D NMR experiments reveal that the four signals correspond to each proton of two sets of magnetically rather equivalent C atoms (Figure 2 insert).One of these groups shows a marked 3 JPt-H coupling of about 60 Hz.This signal corresponds to two protons on two different CH2 groups having a perfect orientation for coupling to Pt.We name these two protons E (Scheme 3), using the E,Z nomenclature for olefins, since the Pt-C-C-H dihedral angles of these two protons lie about 20° (from the crystal structure), thus close to the 0° for two E oriented groups in an unsaturated system.The two other protons on these CH2 groups, the Z protons, exhibit angles of about 80° and no detectable 3 JPt-H coupling.The 1 JPt-C coupling to the two olefin groups of 31 Hz for the axial and 334 Hz for the equatorial position, which were comparable to related five coordinate Pt(II) olefin complexes [15,16,[48][49][50][51][52][53][54][55]71,72,79,105], also reflects the very different binding strength of the two groups and, thus, strongly suggests a trigonal bipyramidal coordination [15,[53][54][55].Thus, the NMR data are completely in line with the molecular structure from the XRD and are represented by Structure (a) in Scheme 2. As has been pointed out before, relatively high 1 JPt-C for a Pt bound olefin group points to a more pronounced description of the Pt-olefin binding as a metallacyclopropane unit [15].Very similar values (332-340 Hz) have been reported for the above-mentioned derivatives [Pt(N^N')(cod)(Me)] + , whereas for the very frequent monoolefin complexes [Pt(N^N)(η 2 -C2H4)Cl2], the 1 JPt-C can reach similar values for very bulky N^N diimine ligands [15].For less bulky diimines and diamines, markedly lower values (<230 Hz) were observed.
Interestingly, in the mixtures of [Pt(bpy)(cod)(Me)] + and [Pt(cod)(Me)(OH2)] + (Table 2, Figure S4), as obtained from initial synthesis, the NMR signals of the five coordinate complex are identical to a pure sample.Thus, the aqua complex seems not to be involved in dynamic processes, such as scrambling with the latter.The 2 JPt-H coupling of [Pt(cod)(Me)(OH2)] + of the olefin protons trans to the H2O ligand is 88 Hz and smaller than the 91 Hz found for the acetone complex [Pt(cod)(Me)(acetone)] + [44], which was not observed in the NMR spectra.In terms of bond strength, this indicates that water is a slightly better ligand to Pt than acetone in such complexes.This and the fact that the aqua complex seems to be quite stable in solution might be of importance in view of the ongoing investigations of the thriving, but yet not understood cytotoxicity of such organoplatinum(II) complexes with olefin ligands [94][95][96]106].The other olefin protons in [Pt(cod)(Me)(OH2)] + , located trans to the Me coligand, show a 2 JPt-H coupling of 35 Hz.The signal for the Me coligand exhibits a 2 JPt-H coupling of 66 Hz.An identical value was found for [Pt(bpy)(cod)(Me)] + , which supports the assumption that in the five coordinate complex, the Me coligand faces an olefinic trans ligand.As pointed out above, at ambient temperature in acetone-d 6 solution, the title complex exhibits two signals of each of two protons for the olefinic groups, indicating the equivalence of the two protons on the axial and the equatorial olefin group, respectively.This has been observed for most of the so far reported square planar codPt complexes and is due to a facile motion of the cod -CH2-CH2-group, rendering the two protons on a -HC=CH-group equivalent.In the solid, this motion is frozen, and consequently, four 1 H signals and also four 13 C signals were observed [105].Furthermore, for most of the above-mentioned derivatives [Pt(N^N')(cod)(Me)] + with various pyridine-2-iminomethyl-aniline ligands (N^N'), four distinct signals were reported [72].This can be explained by the unsymmetrical nature of the here used diimine ligands and, thus, supports once more the assumed trigonal bipyramidal structure of these complexes in solution.Interestingly, for the derivative carrying the least bulky N^N' ligand, the olefin signals are merged into two signals at ambient temperature, and the axial signal splits into two at −213 K, while the signal for the equatorial protons remains unchanged.This has been assigned to a dynamic processes involving partial splitting of the N^N' ligand [72].In contrast to this, the corresponding chloride complexes [Pt(N^N')(cod)Cl] + seem to exhibit fluxional behavior in corresponding NMR experiments due to rearrangements, including partial splitting of the cod ligand [72].
We have studied the NMR behavior of the title complex at low temperatures and 600 MHz (Figure 4).In our experiments, two signals for the olefin protons and four distinct signals for the cod -CH2-groups were detected at temperatures from 303 to 203 K.No merging or splitting of signals occurs within this temperature range; only the 2 JPt-H coupling constant of the equatorial olefin protons trans to bpy decreases from about 73 Hz to 63 Hz, while the value for the axial olefin protons remains largely constant.This obvious difference between our results and those obtained for the [Pt(N^N')(cod)(Me)] + derivatives with unsymmetrical pyridine-2-iminomethyl-aniline ligands [72] might lie in the lower symmetry of these complexes and the bulkiness of those ligands.In future work we will thus synthesize further derivatives of the title complex with more bulky diimine ligands, but also use unsymmetrical cod ligands, such as 1-methyl-2,5-cylooctadiene [106].

General
All preparations were carried out in a dry argon atmosphere using Schlenk techniques.Additionally, the preparation of the title complex was carried out in the dark.Solvents (CH2Cl2, THF, toluene, diethyl ether and MeCN) were dried using a MBRAUN MB SPS-800 solvent purification system.

Instruments
The NMR spectra were recorded on a Bruker Avance II 300-MHz ( 1 H: 300.13 MHz, 13 C: 75.47 MHz) double resonance (BBFO) 5-mm observation probe head with a z-gradient coil, a Bruker Avance 400 spectrometer ( 1 H: 400.13 MHz, 13 C: 100.61MHz, 195 Pt: 86.01 MHz) triple resonance (TBI) 5-mm inverse probe head with a z-gradient coil and a Bruker Avance II 600 spectrometer ( 1 H: 600.13 MHz, 13 C: 150.93 MHz, 15 N: 60.83 MHz) triple resonance (TBI) 50-mm inverse probe head with z-gradient coil.The broadband coil was tuned to either the carbon or the platinum frequency and the detection coil to the proton frequency, resulting in 90° pulses of 11.9 µs for 13 C, 12.5 µs for 195 Pt and 12.4 µs for 1 H.The unambiguous assignment of the 1 H, 13 C and 195 Pt resonances was obtained from 1 H TOCSY, 1 H COSY, 1 H NOESY, gradient-selected 1 H, 13 C HSQC and HMBC and gradient-selected 1 H, 195 Pt HMBC experiments.All 2D NMR experiments were performed using standard pulse sequences from the Bruker pulse program library.Chemical shifts were relative to TMS for 1 H and 13 C, NH3 for 15 N and Na2[PtCl6] in D2O for 195 Pt.The spectra analyses were performed by the Bruker TopSpin 3.2 software.EI-MS spectra were measured using a Finnigan MAT 95.Elemental analyses were carried out on Hekatech CHNS EuroEA 3000 Analyzer.IR spectra were measured on a Bruker IFS66νS.

Reagents
The complex [(COD)Pt(Me)Cl] was prepared according to published procedures [94][95][96].All other chemicals were purchased by commercial suppliers and were used without further purification.

Conclusions
The five coordinate organoplatinum complex [Pt(bpy)(cod)(Me)][SbF6] (cod = 1,5-cyclooctadiene, bpy = 2,2'-bipyridine) was obtained reacting [Pt(cod)(Me)Cl] with Ag[SbF6] and bpy and characterized by multiple spectroscopy (IR and NMR) and single-crystal XRD.The molecular structure clearly shows the five coordinate binding.Application of the so-called trigonality index, the τ value, for the discrimination between trigonal bipyramidal vs. square pyramidal coordination gave a value of 0.528, thus largely in-between the two geometries.However, a closer look revealed that chelate bite angles for the bpy ligand (73°) largely distort the basal trigonal plane (X1-Pt-N angles of about 143°), and the cod bite angle (86°) leads to a distortion of the "axial" Me-Pt-X2 angle; both devaluate the τ value.Nevertheless, the molecular structure can be unequivocally described as basallydistorted trigonal bipyramidal.Based on multinuclear and 2D NMR experiments, a complete picture of the molecular structure in solution could be drawn giving strong evidence for the same structure as found in the crystal.

Table 2 .
Selected1 H and 195Pt NMR data of cod platinum complexes a .