Heterotridentate Organodiphosphines in Pt(η³–P¹ P²X¹)(Y) (X¹ = N¹, C¹, Si¹) Derivatives - Structural Aspects

This review covers monomeric Pt(II) complexes of the compositions Pt(η³–P¹ P²N¹)(C^l), Pt(η³–P¹P²C¹)(C^l), and Pt(η³–P¹P²Si¹) (CH₃). The structural parameters of the complexes (Pt-L, L-Pt-L) are analysed, compared with Pt(η³–P¹X¹P²)(Y) and discussed with an attention to the distortion of a square-planar geometry about the Pt(II) atoms as well as of trans-influence. The sums of Pt-L (x⁴) bind distances growing with covalent radius of the X1 atom. Generally, the inner coordination sphere about the Pt(II) atoms in Pt(η³– P¹P²X¹)(Y) derivatives is somewhat less crowded than in the analogous Pt(η³–P¹X¹P²)(Y) derivatives. There is a cooperative effect between a degree of distortion and trans-influence of X1 atom and its position in the metallocyclic rings.

Keywords: Structure; Heterotridentate; Organodiphosphines; Pt(Ii); Trans-Influence; Distortion

Abbreviations: m monoclinic (But)(mes)P(CH2)2P(But)(η3–C9H10) 2-(t-butyl{2-[t-butyl(2,4,6-trimethylphenyl)- phosphanyl]ethyl}phosphanyl)-3,5-dimethyl-phenyl)methyl}triclinic
tr (Ph)(C6H4NH2)P1(CH2)3P2. (3,7-(1,5-bis(3,5-dicarboxyphenyl)-3mesityl-7-(2- (C6H4N1H2)(Ph)(mes)P(η4– methylene-4,6-dimethylphenyl)-1,5-diazo-3,7- C10H9.NO4)2P(η2–C9H10) diphospho-cyclooctane))
(Me)(C9H4)(SiMe3)7P1. methyl-P,P-dimethyl-P,P´-bis(4- (CH2)2P2(Me)(η2–C9H4). tris(trimethylsilyl)methyl-2,6-bis(bis(trimethylsilyl)-
(SiMe3)6Si1(Me)2 (η1– methyl)phenyl)ethane-1,2-diphosphino) C10H13O)2P(C15H12O)P. (η1– 4,5-bis(bis(2-t-butylphenoxy)phosphino)-9,9- dimethylxanthene)(1-(dimethylsilyl))-2-(diphenyl- C10H13O)(η2–C10H12O) phosphino)-1,2-dicarbadodicarboranehydrido-(1- (diphenylphosphino)-1,2-dicarbododicarborane

Platinum exists in a wide range of oxidation states from zero to +6 including non-integral Pt(2.25), Pt(2.5), Pt(2.87), Pt(3.25), and Pt(3.5). Of these, particularly in four and six-coordinate examples +2 and +4 oxidation states are the most common. The huge area of platinum coordination chemistry has been surveyed [1-3] with over 2000 examples.

Platinum (II) has a strong preference for square-planar coordination geometry. The high affinity of the platinum(II) ion for phos- phorus enables it to bind effectively to organophosphines. Organophosphines as a soft P-donor ligand are very useful for building wide variety of platinum complexes. Recently, we classified and analysed structural data of over thirty monomeric platinum (II) complexes with heterotridentate organodiphosphines of composition Pt(η³–P¹X¹P²)(Y), (X¹ = O¹, N¹) [4]. Another reviews covers structural data for Pt(η³–P¹X¹P²)(Y), (X¹ = C¹) [5]. Structural data for another over thirty examples of Pt(η³–P¹X¹P²)(Y), (X¹ = B¹, S¹, Si¹) types were also analyzed [6].

The aim of this survey is to analyse structural parameters for Pt(η³–P¹X¹P²)(Y), (X¹)(Y), (XX¹ = N¹, c¹, Si¹) derivatives. The data are correlated with those of analogous Pt(η3–P1X1P2)(Y), (X1 = N¹, c¹, Si¹) derivatives.

Noticeable, while the former derivatives were studied sporadically, there are only five examples (Table 1) much more attention was paid to the other derivatives with over one hundred examples [4-6], and references therein. As far as we know, there is no paper with analysed structural data of such derivatives, this is the aim of this mini- review.

In monoclinic [Pt{η³–(Ph)(C₆H₄NH₂)P¹(CH₂)₃P²(C₆H₄N₁H₂)(Ph)}(Cl)] (at 103 K) (Figure 1) [7], the tridentate P1P2N1 – donor ligand can derive a six- and five-membered metallocyclic rings with common P² atom of the P¹C₃P²C₂N¹ type with the values of chelate rings 93.0 (2)° (P¹ – Pt – P²) and 84.3(2)° (P² – Pt – N¹). The values of the remaining L–Pt–L bond angles open in the or- der: 88.6(2)° (P¹–Pt–Cl) < 92.4(2)° (N¹–Pt–Cl) < 166.3(2)° (P¹–Pt–N¹) < 174.6(2)° (P²–Pt–Cl). The chloride completed a distorted square -planar geometry about Pt(II) atom. The Pt–L bond distance elongates in the sequence: 2.164(8) Å (Pt–N1, trans to P1) <2.246 (3) Å (Pt– P², trans to Cl) < 2.298(3) Å (Pt–P¹) < 2.393(3) Å (Pt–Cl). This is only example with such tridentate ligand.

There are thirteen examples of Pt(η³–P¹N¹P²)(Cl) derivative types. Their structural data were analysed [5]. The tridentate P¹N¹P² ligands can derive a variety of the metallocyclic rings with common N¹ atom. There are example with a pair of five-membered metallocycles of the types: P¹C₂N¹C₂P², [8-10] P¹OCN¹COP², [11]; P¹NCN¹CNP², [12] five- and six-membered P¹C₂N¹C₃P² [13]; P¹C₂N¹NC₂P² [14], metallocyclic rings with common N¹ atom. The chloride completed a distorted square-planar geometry about each Pt(II) atom. The Pt-L bond distance (mean values) elongates in the order: 2.024 Å (Pt-N¹, trans to Cl) < 2.283 Å (Pt-P, mutually trans) < 2.307 Å (Pt–Cl). The L–Pt–L bond angles (mean values) open in the order: 83.7° (P¹–Pt–N¹/N¹–Pt–P^{2) < 95.1° (P1–Pt–Cl/ Cl–Pt–P2) < 167.0° (P1–Pt– P2) < 176.0° (N1–Pt–Cl).}

There are three examples of Pt(ηη³–P¹P²)(C^l)(Cl) complexes (Table 1). In monoclinic [Pt{η³–(But)(mes)P(CH₂)2P(But) (η²–C₉H₁₀)} (Cl)].0.5CH₂Cl₂ (at 100 K) [15]. Tridentate P¹P2C1 ligand can derived a pair of five-membered metallocyclic rings with common P2 atom of the P1C2P2C2C1 type with the values of the chelate rings of 87.0˚ (P1–Pt–P2) and 82.3˚ (P²–Pt–C¹). The η³–ligand with chlo- ride build up a distorted square-planar geometry about Pt(II) atom. The remaining L–Pt–L bond angles open in the sequence 89.3° (C¹–Pt–Cl) < 101.1° (P¹–Pt–Cl) < 163.2° (P¹–Pt–C¹) < 171.5° (P²–Pt–C^l). The P–L bond distance elongates in the sequence: 2.087 Å (Pt–C¹, trans to P1) < 2.188 Å (Pt–P², trans to Cl) < 2.342 Å (Pt–P¹) < 2.383 Å (Pt–Cl).

Structure of monoclinic [Pt{η³–(η¹–C₁₀H₁₃O)₂P(C₁₅H₁₂O).P(η¹–C₁₆H₁₃O)(η²–C₁₀H₁₂O)}](C^l) (at 150 K) is shown in Figure 2 [16]. As can be seen the tridentate P¹P²C¹ ligand forms eight- and five-membered metallocyclic rings with common P2 atom of the P1C2OC2P2OCC1 type, with the values of the chelate rings of 103.5˚ (P¹–Pt–P²) and 79.8˚ (P²–Pt–C¹). The η³–ligand with chloride build up a distorted square-planar geometry about Pt(II) atom. The remaining L–Pt–L bond angles open in the sequence 86.6° (P¹- Pt-Cl) < 89.9° (C1-Pt-Cl) < 168.9° (P²-Pt-Cl) < 176.1° (P¹-Pt-C1). The Pt-L bond distance elongates in the sequence: 2.073 Å (Pt–C1, trans to P¹) < 2.192 Å (Pt–P², trans to Cl) < 2.341 Å (Pt–Cl) < 2.368 Å (Pt–P¹).

In triclinic [Pt{η³–(mes)P(η⁴–C₁₀H₉NO₄)₂P(η²–C₉H₁₀)}(Cl)].4.dmf (at 220 K) [17] (Figure 3) the tridentate P¹P²C1 ligand can derive a “double” six- and five-membered metallocyclic rings with common P2 atom of the P¹(CNC)₂P²C₂C¹ type, with almost equal value of the chelate rings: 84.3° (P¹–Pt–P²) and 84.0° (P²–Pt–C1). The remaining L–Pt–L bond angles open in the order: 89.2° (C¹–Pt–Cl)< 102.3° (P¹–Pt–Cl) < 168.3° (P¹–Pt–C1) < 172.9° (P²–Pt–Cl). The Pt–L bond distance elongates in the order 2.096 Å (Pt–C¹, trans to P¹) < 2.162 Å (Pt–P², trans to Cl) < 2.327 Å (Pt–P¹) < 2.366 Å (Pt–Cl).

Much more attention was paid to Pt(η³–P¹C¹P²)(Cl) derivatives. The tridentate P¹C¹P² ligands can derive a variable combination of metallocycles with common C¹ atom: P¹C₂C¹C₂P² [18-20]; P¹OCC¹COP² [21,22]; P¹CPC¹PCP² [23]; P¹C₂C¹NCP² [24], P¹C₂NC-¹NC₂P² [25], and P¹NC₂C¹C₂NP² [26]. The tridentate P1C1P2 ligands with chloride build by a distorted square-planar geometryabout Pt(II) atoms. The structural parameters of the complexes were analysed [6]. The L–Pt–L bond angles (mean values) open in the sequence 83.0° (P¹–Pt–C¹/C¹–Pt–P²) < 95.0° (P¹–Pt–C^l/C^l–Pt–P²) < 165.0° (P¹–Pt–P²) < 177.5° (C¹–Pt–C^l). The Pt–L bond distance (mean values) elongates in the order 2.020 Å (Pt–C¹, trans to Cl) < 2.027 Å (Pt–P, mutually trans) < 2.388 Å (Pt–Cl, trans to C¹).

Triclinic [Pt{η³–(Me)(C₉H₄)(SiMe₃)₇P¹(CH₂)P²(Me)(η²–C₉H₄)(SiMe₃)₆)Si¹(Me₂)} (CH₃)] (at 93 K) (Figure 4) [27] is the only exam- ple in which tridentate P¹p²Si¹ ligand with methyl group build up a distorted square-planar geometry about Pt(II) atom (Table 1). The tridentate ligand forms five- and six-membered metallocyclic rings with common P2 atom of the P¹C₂P²C₃Si¹ type, with the values of the respective angles of 88.4° (P¹–Pt–P²) and 93.6° (P²–Pt–Si¹). The remaining L–Pt–L bond angles open in the sequence: 83.6° (Si¹–Pt–C) < 92.6° (P¹–Pt–C) < 168.5° (P^{2,/sup>–Pt–C) < 170.2° (P1–Pt–Si1). The Pt–L bond distance elongates in the sequence: 2.117 Å (Pt–C, trans to P2) < 2.275 Å (Pt–P2) < 2.321 Å (Pt–Si1.) < 2.419 Å (Pt–P1).}

Structural parameters are available for four Pt(η³–P¹Si¹P^2,)(CL) derivatives [6]. Each tridentate P1Si1P2 ligand can derive a pair of five-membered metallocyclic rings with common Si1 atom of P¹C₂Si¹C₂P² types [28-30] with the mean value of 82.9° (P^{1,/sup>–Pt–Si1/ Si1–Pt–P2). The remaining L–Pt–L bond angles (mean values) open in the sequence: 97.6° (P1–Pt–C/C-Pt-P2) < 160.8° (P1–Pt–P2)< 173.0° (Si1–Pt–C). The Pt–L bond distance (mean values) elongates in the sequence: 2.112 Å (Pt–C, trans to Si1) < 2.286 Å (Pt–P, mutually trans) < 2.339 Å (Pt–Si1).}

In the chemistry of “soft” platinum is found in a wide variety of ligands forming a square-planar geometry about Pt(II) with varying degrees of distortion, and Pt(η³–P¹P²i¹X¹)(Y) and Pt(η³–P¹X¹P²)(Y) derivatives are not an exception. Although platinum should preferentially bind to “soft” donor ligand in complexes studying a distorted square-planar geometries about Pt(II) atoms are build up with combination of “soft” (PL, SiL) and “hard” (NL, Cl) donor ligands. In Pt(η³–P¹P²X¹)(Y) derivative, the trans ef- fect of X¹ on Pt(II)-P1 distance shortness the Pt-P¹ bond: 2.349 Å (Si1–Pt 2.232 Å) < 2.346 Å (C1–Pt 2.065 Å) < 2.298 Å (N¹–Pt2.164 Å). The trans effect of Y on the Pt-P² distance increases the length (weakens), the trans bond: 2.215 Å (Cl–Pt 2.379 Å) <2.275 Å (C–Pt 2.122 Å). This results suggest that in the former case is less transfer of donor electrons from X¹ to Pt(II) than in the latter case. The total mean values of Pt-P bond distances are 2.336 Å (Pt-P¹) and 2.364 Å (Pt-P²).

In Pt(η³–P¹X¹P²))(Y) derivatives the Pt-X¹ bond distance (trans to Y) elongates in the order (mean values): Pt-C¹ 2.020 Å (C^l–Pt2.388 Å) < Pt-N¹ 2.024 Å (C^l–Pt 2.307 Å) < Pt-Si¹ 2.339 Å (C–Pt 2.122 Å). The Pt-P1 (trans to P²) bond distance range from 2.258 to 2.286 Å (average 2.275 Å).

The total sums of Pt-L (x4) bond distances of PtP¹P²X¹Y vs PtP¹X¹P²Y types are:
PtP¹P²N¹C^l, 9.101 Å vs. 8.897 Å, PtP¹P²N¹C^l
PtP¹P²N¹C^l, 8.973 Å vs. 8.953 Å, PtP¹P²N¹C^l
PtP¹P²N¹C^l, 9.132 Å vs. 9.026 Å, PtP¹P²N¹C^l

As can be seen, the total sums of Pt-L (x4) bind distances in the PtP¹P²X¹Y species are somewhat higher than the sums in PtP-1P²X¹Y species. This reflects how isomerism can alter the perception of the trans effect.

In Pt(η³–P¹P²X¹)(Y) derivatives each heterotridentate ligand forms two metallocyclic rings with common P2 atom with vary- ing numbers of atoms in the rings. Correspondingly, there is a variety of metallocyclic rings, and the effects of both steric and electronic factors can be seen from the values of the L–Pt–L bite angles. The angles opening in the sequence: P¹(CNC)₂P²/P²C₂C₁ 84.3°/84.0° (168.3°) < P¹C²P²/P²C²C¹ 87.0°/82.3° (169.3°) < P¹C₃P²/P²C₂C¹ 93.0°/84.3° (177.3°) < P¹C₂P²/P²C₃Si¹ 88.4°/93.6° (182.0°)< P¹C₂OC₂P²/P²OCC¹ 103.5°/79.8° (187.8°).

As was already mentioned, there are over 120 examples of Pt(η³–P¹X¹P²)(Y) (X¹ = O¹, N¹, C¹, S¹, B¹, Si¹) derivatives which were analysed (Melník, Mikuš 2021a,b, 2022) Therein, will be briefly outline Pt(η3–P1X1P2)(Y): X1/Y = N1/Cl (12 examples), X¹/Y = C¹/C^l (22 examples); X¹/Y = Si¹/C (4 examples); and X¹/Y = Si¹/H (2 examples) for comparison with analogous Pt(η³–P¹P²X¹)(Y).

In Pt(η³–P¹N¹P²)(Cl) the respective bite angles opening in the sequence: P¹NCN¹/N¹C²P² 81.0°/81.1° (162.1°) < P¹OCN¹/N¹COP² 82.1°/82.3° (164.4°) < P¹C²N¹/N¹C₂P² 83.3°/82.7° (166.0°) < P¹C₂N¹/N¹NC₂P² 80.7°/91.2° (171.9°) < P¹C₂N¹/N¹C₃P² 80.0°/95.1° (175.6°).

In Pt(η³–P¹C¹P²)(Cl) derivatives, the respective bite angles opening in the sequence: P¹OCC¹/C¹COP² 80.0°/81.2° (161.2°) ~ P¹C₂C¹/C¹C₂P² 82.2°/79.0° (161.2°) < P¹NC₂C¹/C¹C₂NP² 84.0°/84.6° (168.6°) < P¹CPC¹PCP² 88.2°/88.6° (176.80°).

In Pt(η³–P¹Si¹P²)(Y) derivatives with only P1C2Si1/Si1C2P2 type, the values are: 82.8°/83.0° (165.8°) for Y = CH₃ and for Y = H, the values are 84.3°/84.9° (169.2°).

In transition metal complexes, the oxidation state of metal plays a leading role in the geometry formed and platinum is no excep- tion. In four-coordinate Pt(II) prefer a square-planar geometry. The utility of a simple metric to assess molecular shape and degree of distortion as well as exemplified best the Ʈ4 parameter for a square-planar geometry by equation [31].

Ʈ4= 360 – ( α + β )/ 360 for square-planar, and
Ʈ4 = 360 – ( α + β )/ 141 for tetrahedral

The values of Ʈ4 range from 0.00 for the perfect square-planar geometry to 1.00 for a perfect tetrahedral geometry, since 360- 2(109.5) = 141.

The total mean values of trans-L-Pt-L bond angles (α and β) as well as parameter Ʈ4 for Pt(η3–P1P2X1)(Y) species are: Pt(η³–P¹P²N¹)(Cl): 166.3° (P¹–Pt–N¹); 174.6° (P^{2–Pt–Cl); 0.030 (Ʈ₄)
Pt(η3–P1P2N1)(Cl): 166.3° (P1–Pt–C1)(Cl): 169.4° (P1–Pt–C1); 170.1° (P2–Pt–Cl); 0.051 (Ʈ₄)
Pt(η3–P1P2N1)(Cl): 166.3° (P1–Pt–Si1)(CH3): 170.2° (P1–Pt–Si1); 168.5° (P2–Pt–C); 0.059 (Ʈ₄)}

The values for Pt(η3–P1X1P2)(Y) species are:
Pt(η³–p¹N¹p²)(Cl): 167.0° (P¹–Pt–P²); 176.4° (N¹–Pt–Cl); 0.046 (Ʈ₄)
Pt(η³–P¹C¹P²)(Cl): 165.4° (P¹–Pt–P²); 177.2° (C¹–Pt–Cl); 0.048 (Ʈ₄)
Pt(η³–P¹Si¹P²)(CL): 160.8° (P¹–Pt–P²); 173.0° (Si¹–Pt–C); 0.072 (Ʈ₄)

The Pt(η³–P¹P²X¹)(Y) complexes are somewhat more distorted than the Pt(η³–P¹X¹P²)(Y) complexes, except PtP2SiC, which is less distorted than PtPSiPC complexes.

We believe that such review as this, can continue to serve as useful function by centralizing valuable material and delineating areas worthy of future investigation.

This work was supported by the projects VEGA 1/0463/18, KEGA 027UK-4/2020, and APVV-15-0585.