General cooperative effects of single atom ligands on a metal: a ¹⁹⁵Pt NMR chemical shift as a function of coordinated halido ligands’ ionic radii overall sum

Abstract

An inverse linear relationship between the experimentally observed ¹⁹⁵Pt NMR signals and the overall sum of coordinated halido ligands’ ionic radii was discovered in Pt(II) and Pt(IV) complexes. The reduction of ¹⁹⁵Pt NMR frequencies parallels the increase of coordinated halido ligands’ ionic radii sum. This suggests that each halido ligand may act as a conducting ring whose induced electric current shields the ¹⁹⁵Pt NMR signals proportionally to the ionic radius of the coordinated halido ligand.

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In previous studies we analyzed the NMR properties of pentacoordinate complexes, formed by the interaction of Zeise's anion, [PtCl₃(η²-CH₂=CH₂)]⁻, with N,N-chelate ligands.^1–14 These specific complexes are generally considered to be interesting since platinum bonded unsaturated ligands are useful models in the study of the interaction of alkenes and alkynes with metals.^14–22 In particular, in the study of the single crystal X-ray structures and NMR signals of the symmetric pentacoordinate complexes [PtX₂(η²-CH₂=CH₂)(Me₂phen)] (X = Cl, Br, I; Me₂phen = 2,9-dimethyl-1,10-phenanthroline), we described the evidence of pseudo-ring currents circulating around the Pt–X axes, Fig. 1 and 2. These currents seemed to be modulated by the ionic radii of the halido ligands coordinated at trans positions, above and below the trigonal equatorial plane. Indeed, in these complexes, characterized by negligible Pt–η²-ethene and Pt–N bond length variations in the trigonal plane, the two trans-axial halido ligands clearly influence the observed ¹H, ¹³C, ¹⁵N and ¹⁹⁵Pt NMR chemical shifts of neighbour atoms.²⁰ Platinum chemical shift correlations with other parameters such as coordinated halido ligands electronegativity were previously reported.^23,24 Nevertheless recently we could evidence the existence of a linear relationship between the ionic radii of coordinated halido ligands and the ¹⁹⁵Pt NMR chemical shifts.²⁰

Fig. 1 Schematic representation of the structure of pentacoordinate complexes of the type [PtXY(η²-CH₂=CH₂)(Me₂phen)], (X,Y = Cl, Br, I; Me₂phen = 2,9-dimethyl-1,10-phenanthroline).

Fig. 2 Schematic representation of hypothesized pseudoring currents induced by an applied external magnetic field (B°) on a platinum coordinated X⁻ halido ligand, explaining the reduction of ¹⁹⁵Pt NMR frequency, observed in platinum complexes, which parallels the increase of ionic radius of a coordinated halido ligand.

The problem of understanding the phenomena influencing the NMR signal frequencies, which can also be related to the physical properties of a molecule, has deserved particular attention in the recent literature.^23–28 In this work, we evaluated the NMR shielding properties of coordinated halido ligands in trigonal–bipyramidal Pt(II) and octahedral Pt(IV) complexes (Fig. 1 and 3). ¹⁹⁵Pt NMR data for pentacoordinate Pt(II) complexes of the type [PtXY(η²-CH₂=CH₂)(Me₂phen)] (X≠Y = Cl, Br, I), where two non-equivalent axial halido ligands are bonded to the metal (Fig. 1), have been collected together with those of the previously reported symmetrical analogues (X = Y = Cl, Br, I), Table 1.²⁰ In a further step, octahedral Pt(IV) complexes of the type [PtX_nY_6−n]²⁻ (1 ≤ n ≤ 6; X,Y = F, Cl, Br, I) were also studied by ¹⁹⁵Pt NMR spectroscopy, in order to analyse the effect of changing the oxidation state and the coordination number and geometry of the metal which also includes cis, trans, fac and mer isomers, Table 2.

Fig. 3 Representation of the general structures of octahedral complexes [PtX_nY_6−n]²⁻ (1 ≤ n ≤ 6; X,Y = F, Cl, Br, I).

Table 1 Experimental ¹⁹⁵Pt NMR chemical shifts of pentacoordinate platinum(II) complexes of the type [PtXY(Me₂phen)(η²-ethene)] (X,Y = Cl, Br, I)

Complex	^195Pt δ (ppm)	Ref.
a This work.
[PtCl2(η^2-CH2=CH2)(Me2phen)]	−2169	20
[PtBrCl(η^2-CH2=CH2)(Me2phen)]	−2456
[PtBr2(η^2-CH2=CH2)(Me2phen)]	−2886	20
[PtClI(η^2-CH2=CH2)(Me2phen)]	−3087
[PtBrI(η^2-CH2=CH2)(Me2phen)]	−3426
[PtI2(η^2-CH2=CH2)(Me2phen)]	−4041	20

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Table 2 Experimental ¹⁹⁵Pt NMR chemical shifts of octahedral platinum(IV) complexes of the type [PtX_nY_6−n]²⁻ (1 ≤ n ≤ 6; X,Y = F, Cl, Br, I)

Complex	^195Pt δ (ppm)	Ref.
[PtF6]^2−	7090	25
[PtClF5]^2−	5856	25
[PtBrF5]^2−	5669	25
trans-[PtCl2F4]^2−	4887	25
trans-[PtBr2F4]^2−	4551	25
cis-[PtCl2F4]^2−	4547	25
cis-[PtBr2F4]^2−	4136	25
mer-[PtCl3F3]^2−	3551	25
fac-[PtCl3F3]^2−	3191	25
mer-[PtBr3F3]^2−	2907	25
trans-[PtCl4F2]^2−	2534	25
fac-[PtBr3F3]^2−	2487	25
cis-[PtCl4F2]^2−	2171	25
trans-[PtBr4F2]^2−	1157	25
cis-[PtBr4F2]^2−	1142	25
[PtCl5F]^2−	1111	25
[PtCl6]^2−	0	23
[PtCl5Br]^2−	−286	23
[PtBr5F]^2−	−311	25
cis-[PtCl4Br2]^2−	−583	23
trans-[PtCl4Br2]^2−	−585	23
[PtCl5I]^2−	−854	23
fac-[PtCl3Br3]^2−	−892	23
mer-[PtCl3Br3]^2−	−894	23
cis-[PtCl2Br4]^2−	−1213	23
trans-[PtCl2Br4]^2−	−1216	23
[PtClBr5]^2−	−1547	23
cis-[PtCl4I2]^2−	−1806	23
trans-[PtCl4I2]^2−	−1884	23
[PtBr6]^2−	−1891	23
[PtBr5I]^2−	−2545	23
fac-[PtCl3I3]^2−	−2849	23
mer-[PtCl3I3]^2−	−2849	23
trans-[PtBr4I2]^2−	−3132	23
cis-[PtBr4I2]^2−	−3252	23
mer-[PtBr3I3]^2−	−3898	23
cis-[PtCl2I4]^2−	−3932	23
trans-[PtCl2I4]^2−	−3932	23
fac-[PtBr3I3]^2−	−4014	23
trans-[PtBr2I4]^2−	−4603	23
cis-[PtBr2I4]^2−	−4719	23
[PtClI5]^2−	−5081	23
[PtBrI5]^2−	−5483	23
[PtI6]^2−	−6293	23

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The general structure of the considered pentacoordinate complexes is reported in Fig. 1. The ¹⁹⁵Pt NMR chemical shifts of these pentacoordinate species are reported in Table 1. A clear inverse linear relationship between ¹⁹⁵Pt NMR frequencies and the overall sum of coordinated halido ligands’ ionic radii (R² = 0.998) can be observed in the plot of ¹⁹⁵Pt NMR data (Table 1) as a function of coordinated halido ligands’ ionic radii sum (Fig. 4). This means that in [PtXY(η²-CH₂=CH₂)(Me₂phen)] complexes the shielding produced by each halido ligand on the observed ¹⁹⁵Pt NMR signal frequency, depends in first approximation only on its ionic radius, as previously stated for the symmetric species [PtX₂(η²-CH₂=CH₂)(Me₂phen)].²⁰ On the other hand, it appears that the ¹⁹⁵Pt NMR shielding is not directly related to the mutual trans influence of the axial X⁻ ligands, potentially able to affect the Pt–X(Y) bond lengths in [PtXY(η²-CH₂=CH₂)(Me₂phen)] complexes.

Fig. 4 ¹⁹⁵Pt NMR chemical shifts (δ) of pentacoordinate complexes [PtXY(η²-CH₂=CH₂)(Me₂phen)] (Me₂phen = 2,9-dimethyl-1,10-phenanthroline; X,Y = Cl, Br, I), as a function of the sum of coordinated halido ligands’ ionic radii, Σ(r). The interpolating line is also shown in red.

The correlation of ¹⁹⁵Pt NMR frequencies, with the overall sum of coordinated halido ligands’ ionic radii, also in platinum complexes with a different oxidation state, coordination number and geometry was therefore investigated. We studied with a similar approach the ¹⁹⁵Pt NMR frequency variation observed in octahedral Pt(IV) complexes of the type [PtX_nY_6−n]²⁻ (1 ≤ n ≤ 6; X,Y = F, Cl, Br, I), Fig. 3. The ¹⁹⁵Pt NMR signals of the investigated Pt(IV) model complexes are reported in Table 2. Also for octahedral Pt(IV) complexes [PtX_nY_6−n]²⁻ a clear inverse linear relationship between ¹⁹⁵Pt NMR frequencies and the overall sum of coordinated halido ligands’ ionic radii was found. This is evident in the linear plot (R² = 0.996) of ¹⁹⁵Pt NMR data, as a function of coordinated halido ligands’ ionic radii sum, reported in Fig. 5. The pure additional effect of halido ligands’ ionic radii on ¹⁹⁵Pt NMR frequencies, in the studied Pt(IV) species, is further evidenced in Fig. 6, where the ¹⁹⁵Pt chemical shifts are reported as a function of the X⁻ halido ligand ionic radius, for six different groups of complexes of the type [PtCl_nX_6−n]²⁻. In each group, the platinum complexes bear a specific number (n; 0 ≤ n ≤ 5) of chlorido ligands and a variable X⁻ halido ligand for the remaining 6 − n coordination sites. Six different lines interpolating the ¹⁹⁵Pt NMR chemical shifts, as a function of the remaining specific X⁻ ionic radius, in [PtCl_nX_6−n]²⁻ complexes, are reported in Fig. 6A. Interestingly, each unit increase of the chlorido ligand number (n), which results in a nearly constant increase of the slope of the specific interpolating line. The linear relationship between the slopes of the six lines in Fig. 6A and the chlorido ligand number (n) (R² = 0.999) is reported in Fig. 6B. It is also noteworthy that all the six interpolating lines of Fig. 6A pass exactly through the data point representing in the graph the ¹⁹⁵Pt NMR chemical shift of the [PtCl₆]²⁻ complex, the only one that belongs to all the six groups of model complexes. These results further confirm the regular trend of the observed correlation. Consistently, similar trends are observed in the analogue [PtF_nX_6−n]²⁻, [PtBr_nX_6−n]²⁻ and [PtI_nX_6−n]²⁻ series of complexes.

Fig. 5 ¹⁹⁵Pt NMR chemical shifts (δ) of octahedral complexes [PtX_nY_6−n]²⁻ (1 ≤ n ≤ 6; X,Y = F, Cl, Br, I), as a function of the sum of coordinated halido ligands’ ionic radii, Σ(r). The interpolating line is also shown in red.

Fig. 6 (A) ¹⁹⁵Pt NMR chemical shifts (δ) of octahedral complexes [PtX_nY_6−n]²⁻ (0 ≤ n ≤ 5; X,Y = F, Cl, Br, I), as a function of the variable ionic radius of the coordinated X⁻ halido ligands, r. (B) Slopes (s) of the interpolating lines reported in (A), represented as a function of the number of variable X⁻ halido ligands, n. In (A) and (B) the interpolating lines are also shown in red.

Our hypothesis on the ¹⁹⁵Pt NMR chemical shift linear dependence from the overall halido ligands’ ionic radius sum therefore holds not only for Pt(II) trigonal-bipyramidal [PtXY(η²-CH₂=CH₂)(Me₂phen)] but also for octahedral Pt(IV) [PtX_nY_6−n]²⁻ (X,Y = halido ligand) complexes. As a further confirmation of the simple additional effect of the overall halido ligands’ ionic radius sum, only slight variations of the ¹⁹⁵Pt NMR frequency are observed here on passing from cis to trans and from fac to mer geometric isomers. Therefore, at least for the studied model complexes, it is also possible, based on the ¹⁹⁵Pt NMR data for some complexes within a series, to easily predict the ¹⁹⁵Pt NMR shifts for analogous complexes bearing a different combination of coordinated halido ligands.

Experimental

All solvents and reagents, except otherwise stated, were purchased from Aldrich Chemical Company and used as received. The symmetric pentacoordinate complexes [PtX₂(η²-CH₂=CH₂)(Me₂phen)] (X = Cl, Br, I) were prepared with the previously reported methods.²⁹ These complexes [PtXY(η²-CH₂=CH₂)(Me₂phen)] (X≠Y = Cl, Br, I) were obtained in equilibrium mixtures with symmetric species [PtX₂(η²-CH₂=CH₂)(Me₂phen)] (X = Cl, Br, I), following the previously reported procedures.²⁹ Alternatively, similar mixtures of complexes could be also obtained by direct reaction of symmetric [PtX₂(η²-CH₂=CH₂)(Me₂phen)] (X = Cl, Br) derivatives with KBr or KI. The asymmetric [PtXY(η²-CH₂=CH₂)(Me₂phen)] species, with mixed halido ligands, could be identified by ¹H and ¹⁹⁵Pt NMR spectroscopy. NMR (CDCl₃, 600 MHz (¹H), 130 MHz (¹⁹⁵Pt), 300 K): [PtClBr(η²-CH₂=CH₂)(Me₂phen)]. δ(¹H) 3.48 (s, 6H, CH₃); 3.68 (m, 2H, CH, η²-ethene, ²J_Pt–H = 70 Hz); 3.78 (m, 2H, CH, η²-ethene, ²J_Pt–H = 70 Hz); 7.79 (d, 2H, CH, ³J_H–H = 8 Hz); 7.85 ppm (s, 2H, CH); 8.31 (d, 2H, CH, ³J_H–H = 8 Hz). δ (¹⁹⁵Pt) −2456 ppm. [PtClI(η²-CH₂=CH₂)(Me₂phen)]. δ (¹H) 3.47 (s, 6H, CH₃); 3.68 (m, 2H, CH, η²-ethene, ²J_Pt–H = 70 Hz); 3.90 (m, 2H, CH, η²-ethene, ²J_Pt–H = 70 Hz); 7.79 (d, 2H, CH, ³J_H–H = 8 Hz); 7.85 ppm (s, 2H, CH); 8.29 (d, 2H, CH, ³J_H–H = 8 Hz). δ (¹⁹⁵Pt) −3087 ppm. [PtBrI(η²-CH₂=CH₂)(Me₂phen)]. δ (¹H) 3.46 (s, 6H, CH₃); 3.68 (m, 2H, CH, η²-ethene, ²J_Pt–H = 70 Hz); 3.90 (m, 2H, CH, η²-ethene, ²J_Pt–H = 70 Hz); 7.79 (d, 2H, CH, ³J_H–H = 8 Hz); 7.85 ppm (s, 2H, CH); 8.27 (d, 2H, CH, ³J_H–H = 8 Hz). δ (¹⁹⁵Pt) −3426 ppm.

The University of Salento (Italy), the PON 254/Ric. Potenziamento del “CENTRO RICERCHE PER LA SALUTE DELL'UOMO E DELL'AMBIENTE” Cod. PONa3_00334, and the Consorzio Interuniversitario di Ricerca in Chimica dei Metalli nei Sistemi Biologici (CIRCMSB), Bari (Italy) are acknowledged for financial support.

Notes and references

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