Distortion isomers of PtM, Pt₂M, PtM₂ and PtMM′ (M = non-transition metals) derivatives-structural aspects

Introduction

Systematic studies on the stereoselectivity of coordination compounds have gained increasing interest over the last 50 years. The stereoselectivity in coordination compounds is very often related to the important stereospecifity of biological systems, catalysis and stereochemical effects in the technical processes. The isomers can be broadly classified into two major categories, namely, structural and stereoisomers. Stereosisomers, which dominate the chemistry of platinum complexes, can be divided into geometric (cis-trans, fac-mer), optical, ligand and distortion isomerism. A structural analysis of over 200 isomers of mononuclear platinum coordination compounds (Melník and Holloway, 2006) has identified the presence of distortion (65%), cis-trans (30%), mixed isomers and ligand isomerism. The stereoisomers of almost 50 platinum di- and oligomeric coordination complexes have been summarized in a previous (Melník and Mikuš, 2015a), including the distortion (87%) and cis-trans (13%) isomers. The analysis and classification of over 100 isomers of organoplatinum complexes indicate only two types of stereoisomers: distortion (96%) and cis-trans (4%) isomers (Melník and Mikuš, 2015b).

In this brief review, we discuss and classify the structural parameters of heterobi-(PtM) and heterotrinuclear-Pt₂M, PtM₂, PtMM′ (M = non-transition metal) isomers. The aims of this presentation are to discuss the factors that could lead to a better understanding of the stereochemical interaction within the coordination spheres of platinum and non-transition metal as well as to examine other cooperative effects between isomeric types.

Distortion isomerism

There are 12 examples of such isomers, for which structural data are available.

Heterobinuclear PtM complexes

There are four such complexes and each contains two crystallographically independent molecules within the same crystal. Their structural parameters are presented in Table 1. Two yellow monoclinic complexes [(η²-dppc)(Cl)Pt^IISn^IICl₃] (Dahlenburg and Mertel, 2001) and [(η²-dppp)(I)Pt^IISn^IICl₃] (Farkas et al., 1996) are isostructural. In each pair of fragment Pt(η²-P₂L)(X) (X=Cl or I) and SnCl₃ are connected by the Pt-Sn bonds with the values of (molecule 1 vs. molecule 2) of 2.532(2) vs. 2.572(2) Å when X=Cl and 2.611(1) vs. 2.615(1) Å when X=I. Each Pt(II) atom has a square-planar geometry (PtP₂XSn) with a different degree of distortion (Table 1). Each Sn(II) atom is tetrahedrally coordinated (SnCl₃Pt).

Table 1:

The structural data for the heterobinuclear (PtM) (M = non-transition metal) complexes-distortion isomers.^a

Compound (colour)	Crys.cl Sp.Gr.Z	a [Ǻ] b [Ǻ] c [Ǻ]	α [°] β [°] γ [°]	Chromophore	M-L [Ǻ]	M-M [Å]	L-M-L [°]	Reference
[(η2-dppc)(Cl)PtII·SnII(Cl)3] (yellow)	m P21/c8	22.019(1) 14.794(1) 22.210(1)	119.36(1)	PtP2ClSn SnCl3Pt PtP2ClSn SnCl3Pt	η2Pb 2.285(6,2) Cl 2.289(4) Cl 2.37(2,3) η2P 2.278(5,3) Cl 2.283(9) Cl 2.35(2,3)	Sn 2.562(2) Sn 2.572(2)	P,Pb 86.9(2)c P,Cl 90.9(3) 177.2(3) P,Sn 97.2(1) 175.8(1) Cl,Sn 85.0(3) Cl,Cl 98 (1,6) Cl,Pt 119.4(1,7) P,P 87.0(2)c P,Cl 90.6(3) 176.9(3) P,Sn 97.3(2) 173.5(1) Cl,Sn 85.2(3) Cl,Cl 96.7(3,3.4) Cl,Pt 120.3(2,2.8)	Dahlenburg and Mertel, 2001
[η2-dppp)(I)PtIISnIICl3] (yellow)	m P21 4	15.488(2) 14.999(2) 16.303(2)	98.67(2)	PtP2ISn SnCl3Pt PtP2ISn SnCl3Pt	η2P 2.248(4) 2.303(5) I 2.6565(1) Cl not given η2P 2.253(4) 2.297(4) I 2.642(1) Cl not given	Sn 2.6113(1) Sn 2.615(1)	P,P 91.7(2)d P,I 90.7(1) 172.50(1) P,Sn 90.6(1) 173.7(1) I,Sn 82.9(1) Cl,Cl not given P,P 91.0(2)d P,I 91.0(1) 170.8(1) P,Sn 91.9(1) 174.7(1) I,Sn 86.7(1) Cl,Cl not given	Farkas et al., 1996
[(NC)2PtIIμ-η2:η6-crownP2}Tl], (NO3)·1.5H2O.0,5CH2Cl2 (colorless) (at 130 K)	m P21/c 8	21.476/7) 14.274(4) 32.660(9)	104.36(2)	PtC2P2Tl TlO4N2Pt PtC2P2Tl TlO4N2Pt	NC 2.01(3,2) η2P 2.336(8,7) O 2.76(2,1) N 3.08(1,7) NC 1.94((3,1) η2P 2.323(1,11) O 2.83(2,8) N 3.02(2,4)	Tl 2.911(2) Tl 2.958(1)	C,C 174.4(4) P,P 171.0(3) C,P 89.8(8,5.6) O,O 60.1(4)e O,N 63.5(1,2.4)e C,C 175.9(2) P,P 172.0(3) C,P 89.9(1,10.6) O,O 63.0(2,4)e O,N 62.2(2,2.7)e	Balch and Rowley, 1990
[(η2-4,4I-But2bpy)(Me)2, (I)PtIISnIV(Me)3]2· [SnIV(Me)3(I)] (yellow)	m P21/m 2	11.204(2) 13.926(2) 22.485(3)	96.401(1)	PtN2C2ISn SnC3Pt PtN2C2ISn SnC3Pt SnC3I (mononuclear)	η2N 2.140(18) MeC 2.063(16) I 2.881(4) MeC 2.176(36,0) 2.220(47) η2N 2.115(21) MeC 2.048(24) I 2.859(4) MeC 2.128(31,0) 2.289(56) MeC 2.106(49,0) I 2.809(5)	Sn 2.547(5) Sn 2.547(4)	N,N 77.0(10)c C,C 88.1(14) N,C 97.5(9) I,Sn 179.6(1) C,C 107.9(18,2) C,Pt 110.9(17,5.9) N,N 74.7(11)c C,C 93.6(14) N,C 95.8(9) I,Sn 176.9(1) C,C 110.5(17,1.4) C,Pt 108.5(8,7) C,C 118.1(32,7.5) C,I 95.0(24,5.2)	Levy et al., 1996

1. ^aWhere more than one chemically equivalent distance or angle is present, the mean value is tabulated. The first number in the parenthesis is the e.s.d. and the second is the maximum deviation from the mean.

2. ^bThe chemical identity of the coordinated atom or ligand is specified in these columns.

3. ^cThe five-membered metallocyclic ring.

4. ^dThe six-membered metallocyclic ring.

5. ^eThe four-membered metallocyclic ring.

The structure of a colorless monoclinic [(NC)₂Pt^II(μ-η²:η⁶-crownP₂)Tl′]⁺ (Balch and Rowley, 1990) is shown in Figure 1. The crown P₂ ligand serves as the bridge via two P donor atoms coordinated to the Pt(II) atom and via four Oand two N atoms to the Tl(II) atom. The respective Pt-Tl bond distances are 2.911(2) Å (molecule 1) and 2.958(1) Å (molecule 2). The inner coordinated spheres around the central atoms are PtC₂P₂ and TlO₄N₂, respectively (Table 1).

Figure 1:

The structure of [(NC)₂Pt^II(μ-η²:η⁶-crownP₂)Tl′]⁺ (Balch and Rowley, 1990).

The yellow monoclinic [(η²-4,4′-Bu^t₂bpy)(Me)₂(I)Pt^IISn^IV(Me)₃]₂[Sn^IV(Me)₃I] (Levy et al., 1996) contains well separated binuclear and mononuclear units. Each binuclear unit contains two crystallographically independent molecules. The Pt-Sn bond distances in the heterobinuclear molecules are 2.547(5) and 2.507(4) Å, respectively.

An inspection of the data reveals that, in each complex, the molecules differ from each other depending on the M-L and Pt-M bond distances as well as on the L-M-L bond angles that are typical for distortion isomers.

Heterotrinuclear Pt₂M, PtM₂ and PtMM′ complexes

There are eight examples containing two crystallographically independent molecules. Their structural parameters are summarized in Table 2.

Table 2:

Structural data for the heterotrinuclear (Pt₂M, PtM₂, PtMM′) (M=non-ranstition metal) complexes-distortion isomers.^a

Complex (color)	Crys.cl Sp.Gr.Z	a [Ǻ] b [Ǻ] c [Ǻ]	α [°] β [°] γ [°]	Chromophore	M-L [Ǻ]	M-M [Å] M-L-M [Å]	L-M-L [°]	Reference
A: Pt2M – type {[(Cl)(μ-η2-dppm)· PtI}2HgIICl2] (orange)	tr P1̅ 2	11.587(3) 22.156(5) 23.036(5)	65.53(2) 76.52(2) 84.30(2)	PtP2ClHgPt HgCl2Pt2 PtP2ClHgPt HgCl2Pt2	Pb 2.300(4,11) Cl 2.343(4,7) Cl 2.447(5,2) P 2.301(4,11) Cl 2.334(4,4) Cl 2.429(4,11)	Hg 2.699(1) 2.710(1) Pt 2.736(1) Hg 2.712(4) 2.713(4) Pt 2.711(1)	P,Pb17 4.3(1,1) P,Cl 87.4(1,1.7) Hg,Pt 59.7(1,3) Cl,Cl 103,7(1) Pt,Pt 60.78(2) P,P 175.2(1,8) P,Cl 88,6(1,2.4) Hg,Pt 60.0(1,2) Cl,Cl 106.0(1) Pt,Pt 59.96(2)	Sharp, 1986
[(NC)2(μ-η2-dppm)2· (μ-I)PtII2Hg(I)] CH2Cl2·1.5H2O (orange) (at 130 K)	tr P1̅ 4	15.311(3) 16.132(5) 24.911(6)	82.99(2) 81.02(2) 89.80(2)	PtP2CI HgPt HgIPt2 PtP2CIHgPt HgIPt2	P 2.308(8,6) NC 1.95(4,5) μI 2.913(3,3) I 2.655(2) P 2.310(7,13) NC 2.00(3,3) μI 2.916(3,9) I 2.656(2)	Hg 2.700(2) 2.725(2) Pt 2.819(2) I not given Hg 2.700(2) 2.729(2) Pt 2.823(2) I not given	P,P 172.2(3,1) C,I 121.3(8,3.2) I,Hg 119.8(1,4) I,Pt 61.2(1,0) Hg,Pt 58.7(1,4) I,Pt 148.6(1,2.8) Pt,Pt 62.6(1) P,P 171.9(3.1) C,I 119.2(9,2.8) I,Hg 119.7(1,8) I,Pt 61.0(1,4) Hg,Pt 58.7(1,5) I,Pt 148.6(1,1.5) Pt,Pt 62.7(1) I,Pt 148.6(1,1.5) Pt,Pt 62.7(1)	Toronto and Balch, 1994
(PPh3)4Pt2II{μ3-S)2· ZnII(η2-bpy)(Cl)]·PF6 (yellow) (at 183 K)	tr Pï 4	14.6319(2) 20.832(2) 29.2215(3)	83.434(1) 88.522(1) 83.281(1)	PtS2P2 ZnN2S2Cl PtS2P2	μ3S 2.364(2,14) Ph3P 2.293(2,6) 2.314(2,2) N 2.190(7,19) μ3S 2.491(2,41) Cl 2.299(2) μ3S 2.363(2,4) Ph3P 2.294(2,12) 2.311(2,0)	Zn 3.205(1,5.6) S not given Pt 3.252(2) S not given Zn 3.202(1,64) S not given Pt 3.249(2) S not given	S,S 80.21(6.14) P,P 100.55(8,2.63) S,P 89.70(7,4.92) 169.27(8,5.18) N,N 74.1(3)c S,S 75.35(7) N,Cl 99.5(2,3.9) S,Cl 113.59(8,7.0) S,S 80.40(6,14) P,P 99.76(8,58) S,P 89.93(7,3.53) 169.92(7,3.4)	Li et al., 2000
				ZnN2S2Cl	N 2.169(7,25) μ3S 2.495(2,70) Cl 2.283(2)		N,N 75.6(3)c S,S 75.32(6) N,Cl 99.6(2,3.8) S,Cl 115.4(8,5.37)
[(PPh3)4Pt2II{μ3-S)2.·CdII(η2-bpy)(Cl)]·PF6 (yellow) (at 183 K)	tr Pï 4	14.5256(5) 20.1188(7) 29.3002(11)	83.110(1) 88.390(1) 82.842(1)	PtS2P2 CdN2S2Cl PtS2P2 CdN2S2Cl	μ3S 2.359(2,14) Ph3P 2.287(2,5) 2.301(2,7) N 2.374(6,0) μ3S 2.626(2,1) Cl 2.468(1) μ3S 2.360(2,13) Ph3P 2.283(2,12) 2.301(2,4) N 2.366(6,24) μ3S 2.621(2,3.5) Cl 2.444(2)	Cd 3.270(1,44) S not given Pt 3.274(3) S not given Cd 3.270(2,4) S not given Pt 3.257(3) S not given	S,S 81.7.6(5,15) P,P 100.59(6,2.24) S,P 88.89(6,4.71) 170.13(6,5.09) N,N 68.5(2)c S,S 72.05(5) N,Cl 99.6(1,4.7) P,Cl 97.3(1,1.4) S,Cl115.64(6,6.29) S,S 82.00(5,14) P,P 99.94(7,48) S,P 89.03(6,3.67) 170.61(6,3.79) N,N 69.6(2)c S,S 72.40(5) N,Cl 99.3(1,4.7) S,Cl119.66(7,3.76)	Li et al., 2000
B: PtM2 – type [(η2-dcpe)Pt{Ga(η5-cp*)}2] (yellow) (at 203 K)	m P21/c 8	12.225(9) 19.123(14) 40.55(3)	93.89(1)	PtP2Ga2 GaC5Pt Pt P2Ga2 GaC5Pt	η2P 2.279(2,0) η5C 2.314(7,3) η2P 2.292(1,11) η5C 2.308(7,24)	Ga 2.322(2,5) Ga 3.558 Ga 2.331(2,4) Ga 3.502	P,P 90.63(6)d Ga,Ga100.02(7) not given P,P 90.38(7)d Ga,Ga 97.37(8) not given	Weiss et al., 2000
[(Cl)2Pt{Sb(Ph)3}2] (yellow)	m P21/a 8	20.571(1) 10.194(1) 33.398(1)	94.39(1)	PtCl2Sb2 SbC3Pt PtCl2Sb2 SbC3Pt	Cl 2.325(4,1) PhC not given Cl 2.335(4,2) PhC not given	Sb 2.500(1,10) Sb 3.78(1) Sb 2.505(1,8) Sb 3.82(1)	Cl,Cl not given not given Cl,Cl not given not given	Wendt et al., 1998
[(η2-dhpe)Pt· {In(η5-cp)}2] (ocre)				PtP2In2 InC5In2	η2P 2.317 η5C 2.615(−,7)	In 2.600 In 3.916	P,P 91.0c In,In 91.34(2) not given	Weiss et al., 2002
C: PtMM′ – type [(η4-pp3)PtHgMn(CO)5] F3CSO3·1.5CH2Cl2 (yellow) (at 218 K)	m P21/c 8	9.437(2) 32.241(6) 17.411(4)	94.44(2)	PtP4Hg HgPtMn MnC5Hg PtP4Hg HgPtMn MnC5Hg	η4P 2.271(7,27) 2.315(7,10) OC not given η4P 2.276(7,40) 2.334(7,1) OC not given	Hg 2.590(2) Mn 2.618(4) Hg 2.605(2) Mn 2.647(5)	P,Hg 93.3(2,4.2) 175.1(2) Pt,Mn 176.12(12) C,Hg 82.8(1,3.3) 178.2(1) P,Hg 93.3(2,1.7) 177.7(2) Pt,Mn178.96(14) C,Hg 84(2) 176.4(1)	Schuh et al., 2001

1. ^aWhere more than one chemically equivalent distance or angle is present, the mean value is tabulated. The first number in parenthesis is the e.s.d. and the second is the maximum deviation from the mean.

2. ^bThe chemical identity of the coordinated atom or ligand is specified in these columns.

3. ^cThe five-membered metallocyclic ring.

4. ^dThe six-membered metallocyclic ring.

Pt₂M type

The Orange triclinic [{(Cl)(μ-η²-dppm)Pt}₂Hg(Cl)₂] (Sharp, 1986) contains an ideal Pt-Hg-Pt triangle with the values of 60.78(2)° (molecule 1) and 59.96(2)° (molecule 2). The Pt-Pt and the mean Pt-Hg bond distances are 2.736(1) and 2.704 Å (molecule 1) and 2.711(1) and 2.712(1) Å (molecule 2), respectively (Table 2A). The homobidentate -P,P′-dppm ligand serves as a bridge between the Pt atoms with the mean values of P-Pt-P bond angles of 174.3(1) and 175.2°, respectively. The chlorine atoms complete an inner coordination sphere around the metal atoms (PtP₂ClHgPt and HgCl₂Pt₂).

The structure of the orange triclinic [(NC)₂(μ-η²-dppm)₂(μ-I)Pt₂Hg(I)] (Toronto and Balch, 1994) is shown in Figure 2. The metal atoms form a Pt-Hg-Pt triangle with the mean values of 62.6°, which is somewhat larger than those of Pt-Pt-Hg with the mean values of 58.7°. The Pt-Pt bond distances of 2.819(2) Å (molecule 1) and 2.823(2) Å (molecule 2) are bridges with an iodine atom as well as a homobidentate -P,P′-dppm ligand. The inner coordination spheres around the metal atom are PtP₂CIHgPt′ and HgIPt₂ (Table 2A).

Figure 2:

The structure of [(NC)₂(μ-η²-dppm)₂(μ-I)Pt₂Hg(I)] (Toronto and Balch, 1994).

The two remaining yellow triclinic [(PPh₃)₄Pt₂(μ₃-S)₂M^II(η²-bpy)(Cl)]PF₆ (M=Zn or Cd) are isostructural (Li et al., 2000). These complexes contain a Pt(μ₃-S)₂M central core as an S-bicapped MPt₂ triangle with negligible metal-metal interactions. Each Pt(II) atom has asquare-planar geometry with varying degrees of distortion (PtS₂P₂) and each M(II) is five- coordinated (MN₂S₂Cl) (Table 2A).

PtM₂ type

There are three complexes of such type, and each contains two crystallographically independent molecules within the same crystal (Table 2B). The structure of the yellow monoclinic [(η²-dcpe)Pt{Ga(η⁵-cp^*)}₂] (Weiss et al., 2000) is shown in Figure 3. The metal atoms form an asymmetric triangular core with two Pt-Ga bond distances of 2.317(2) and 2.327(2) Å in molecule 1 and 2.327(2) and 2.336(1) Å in molecule 2. The Ga···Ga separations are 3.558 and 3.502 Å, respectively. The homobidentate -P,P donor ligand (dcpe) is coordinated to the Pt atom and η⁵-cp^* to the Ga atom (Table 2B).

Figure 3:

The structure of [(η²-dcpe)Pt{Ga(η⁵-cp^*)}₂] (Weiss et al., 2000).

In another yellow monoclinic PtSb₂ derivative (Wendt et al., 1998) the metal atoms also form an asymmetric triangular core with two Pt-Sb bond distances of 2.490(1) and 2.510(1) Å in molecule 1 and 2.497(1) and 2.513(1) Å in molecule 2, respectively. The values of Sb···Sb separations are 3.78(1) Å and 3.82(1) Å, respectively. Two chlorine atoms complete a square-planar arrangement around the Pt atom (PtCl₂Sb₂), and three molecules of Ph₃ complete a tetrahedral environment around the Sb atom (SbC₃Pt).

PtMM′ type

The yellow monoclinic [(η⁴-pp3)PtHgMn(CO)₅]F₃CSO₃·1.5CH₂Cl₂ (Schuh et al., 2001) is the only example of PtMM′, which contains two crystallographically independent molecules (Table 1). The structure of this complex cation is shown in Figure 4. As can be seen, the mercury atom has an almost linear coordination Pt-Hg-Mn with the values (molecule 1 and molecule 2) of 176.1(1) and 178.9(1)°, respectively. The Hg-Pt and Hg-Mn bond distances are 2.590(2) and 2.618(4) Å in one molecule and 2.605(2) and 2.647(5) Å in another molecule, respectively.

Figure 4:

The structure of [(η⁴-pp3)PtHgMn(CO)₅] (Schuh et al., 2001).

Conclusions

An analysis of the structural parameters of heterobi-(PtM) and heterotri-(Pt₂M, PtM₂, PtMM′) nuclear complexes, in which M are non-transition metals show that these complexes contain two crystallographically independent molecules within the same crystal. These molecules differ mostly by the degree of distortion and exemplify the distortion isomerism. In the heterobinuclear PtSn complexes, the respective fragments are connected only via the Pt-Sn bonds (Table 1). The PtTl complex (NCl₂Pt) units with Tl atoms are held together via the octadentate crown P₂ ligand, which coordinate via two P atoms to the Pt atom and via four Oatoms and two N atoms to the Tl atom. As can be seen (Table 1) in each complex, the respective molecules differ from each other depending on the bond distances and bond angles in the inner coordination spheres.

The heterotrinuclear complexes can be divided into the three sub-groups, Pt₂M (4 examples), PtM₂ (3 examples) and PtMM′ (1 example). In the two Pt₂Hg derivatives (Sharp, 1986; Toronto and Balch, 1994) the three metal atoms form almost regular triangles with the following Pt-Pt and Pt-Hg (mean) bond distances (molecule 1 vs. molecule 2): 2.736(1) and 2.704 Å vs. 2.711(1) and 2.712 Å (Sharp, 1986); 2.819(2) and 2.712 Å vs. 2.823(2) and 2.713 Å (Toronto and Balch, 1994).

The structures of the two remaining Pt₂M complexes (M=Zn or Cd) (Li et al., 2000) are isostructural with the Pt···Pt and Pt···M (mean) separations (molecule 1 vs. molecule 2): 3.252(2) and 3.205 Å vs. 3.249(2) and 3.202 Å (for M=Zn); and 3.274(3) and 3.270 Å vs. 3.257(3) and 3.270 Å (for M=Cd).

In the PtM₂ type derivatives, the metal atoms form an asymmetric triangular core. The mean M-M bond distances as well as the Pt···M separation elongated with the M radius in the following order: 2.316 and 3.530 Å (Ga, 1.35 Å)<2.502 and 3.80 Å (Sb, 1.59 Å)<2.600 and 3.916 Å (In, 1.67 Å).

In PtMM′ derivative, the ‘central’ mercury atom is almost linearly coordinated (Pt-Hg-Mn) with the values of the respective angles of 176.1 Å (molecule 1) and 178.9° (molecule 2). The Pt-Hg and Hg-Mn bond distances (molecule 1 vs. molecule 2) are 2.590(2) and 2.618(9) Å vs. 2.605(2) and 2.647(5) Å, respectively.

Notably, despite the importance of the cis-trans geometry in the chemistry of platinum compared with other transition metal systems, within platinum chemistry (coordination, organoplatinum, heterometallic) distortion isomerization is far more common.