Pt–Mg, Pt–Ca, and Pt–Zn Lantern Complexes and Metal-Only Donor

Article pubs.acs.org/IC

Pt−Mg, Pt−Ca, and Pt−Zn Lantern Complexes and Metal-Only Donor−Acceptor Interactions Frederick G. Baddour,† Ariel S. Hyre,† Jesse L. Guillet,† David Pascual,‡ José Maria Lopez-de-Luzuriaga,‡ Todd M. Alam,§ Jeffrey W. Bacon,† and Linda H. Doerrer*,† †

Department of Chemistry, Boston University, Boston, Massachusetts 02215, United States Departamento de Química, Centro de Investigación en Síntesis Química, Universidad de La Rioja, Madre de Dios, 51, 26004 Logroño, Spain § Department of Organic Material Science, Sandia National Laboratories, Albuquerque, New Mexico 87185-0886, United States ‡

S Supporting Information *

ABSTRACT: Pt-based heterobimetallic lantern complexes of the form [PtM(SOCR)4(L)] have been shown previously to form intermolecular metallophilic interactions and engage in antiferromagnetic coupling between lanterns having M atoms with open shell configurations. In order to understand better the influence of the carboxylate bridge and terminal ligand on the electronic structure, as well as the metal−metal interactions within each lantern unit, a series of diamagnetic lantern complexes, [PtMg(SAc)4(OH2)] (1), [PtMg(tba)4(OH2)] (2), [PtCa(tba)4(OH2)] (3), [PtZn(tba)4(OH2)] (4), and a mononuclear control (Ph4P)2[Pt(SAc)4] (5) have been synthesized. Crystallographic data show close Pt−M contacts enforced by the lantern structure in each dinuclear case. 195Pt-NMR spectroscopy of 1− 4, (Ph4P)2[Pt(SAc)4] (5), and several previously reported lanterns revealed a strong chemical shift dependence on the identity of the second metal (M), mild influence by the thiocarboxylate ligand (SOCR; R = CH3 (thioacetate, SAc), C6H5 (thiobenzoate, tba)), and modest influence from the terminal ligand (L). Fluorescence spectroscopy has provided evidence for a Pt···Zn metallophilic interaction in [PtZn(SAc)4(OH2)], and computational studies demonstrate significant dative character. In all of 1− 4, the short Pt−M distances suggest that metal-only Lewis donor (Pt)−Lewis acceptor (M) interactions could be present. DFT and NBO calculations, however, show that only the Zn examples have appreciable covalent character, whereas the Mg and Ca complexes are much more ionic.

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INTRODUCTION Metal−metal bonding is a central concept in chemistry that is evolving as new compounds with formerly unusual atomic arrangements are prepared. Metal centers have demonstrated all extremes of structure and reactivity, from being extraordinarily electrophilic or nucleophilic to a variety of singleelectron radical processes. Recently, donor−acceptor interactions have also been recognized between metals, including s-,1,2 d-,3−7 p-,8−13 and, to some extent, f-block14 metal atoms, to give a selection of recent examples. These metal−metal, donor−acceptor interactions have been recently termed metalonly Lewis pairs (MOLPs),15 and they were earlier called dative metal−metal bonds.16−26 The combination of two disparate elements, one having an electron pair and the other needing an electron pair, was classified by G. N. Lewis in the now eponymous electrondonating bases and electron-accepting acids. Typically taught to young chemists with examples from the p-block nonmetals, such donor−acceptor interactions are now well-established in metals across the periodic table. In d-block chemistry specifically, square-planar d8 complexes have considerable © XXXX American Chemical Society

precedent for their nucleophilic, Lewis-basic donor character,27,28 while closed-shell metal cations typify electronaccepting Lewis acids. The metal Lewis acids have included K,29 Ca,30 Mn,31 Pd,32 Pt,33 Cu,34,35 Ag,35−38 Au,39,40 Zn,41−43 Cd,44 Hg,45,46 Tl,47−51 and Pb,50,52 and have been reviewed.27 In this work, the possible donation of a square-planar Pt(II) dz2 lone pair to the empty valence orbital(s) of Zn, Mg, and Ca is investigated. Our group uses thiocarboxylate lantern complexes as asymmetric building blocks in quasi-1D systems. Previous work53−56 has demonstrated the facile synthesis of heterobimetallic lantern complexes with thiocarboxylate ligands. These compounds constitute an interesting class, because when all S atoms are ligated only to Pt, the lantern complexes can form extended contacts through Pt···Pt metallophilic interactions. Depending on the 3d metal used, they can also exhibit antiferromagnetic coupling between two metal centers that are more than 8 Å apart,54,55 or can ferromagnetically Received: September 28, 2016

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DOI: 10.1021/acs.inorgchem.6b02372 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry couple Cr(III) centers spaced 7.8 Å along a 1D chain.56 The formation of short Pt···Pt metallophilic contacts has been observed with both thiobenzoate (tba)53,54 and thioacetate (SAc)54,55 bridging ligands, and can be achieved with Co, Ni, or Zn as the second metal, as well as either axial H2O or 3nitropyridine (3-NO2py) ligands. Herein we demonstrate the flexibility of the previously reported synthesis and present a series of diamagnetic heterobimetallic lantern compounds of the form [PtM(SOCR)4(OH2)] (R = CH3, M = Mg (1); R = C6H5, M = Mg (2), Ca (3), Zn (4)). These materials have enabled an assessment of the electronic influence of the diamagnetic metals on Pt, including the use of 195Pt NMR spectroscopy, without any potentially obscuring paramagnetism. Heterobimetallic complexes with thiocarboxylates also form nonlantern structures, and have been investigated as precursors to metal sulfides.57 Bimetallic compounds include [(Ph3P)Cu{In(tba)4}],58 (Me4N)[Na{Cd(tba)3}2],59 (Me4N)[Na{Hg(tba) 3 } 2 ], 60 (Me 4 N)[K{Hg(tba) 3 } 2 ], 60 (Me 4 N)[K{Cd(tba) 3 } 2 ], 60 and [M{In(tba) 4 } 2 ] for M = Mg, Ca. 61 Monometallic thioacetate and thiopivalate [M(SOCR)2(L)2] species have also been prepared with M = Cd, Zn, and L = lutidine.62 Very little structural data on thiocarboxylate-bridged lantern complexes existed in the literature prior to our recent reports on heterobimetallic compounds prepared with thiobenzoate53 and thioacetate,54,55 with only a dinuclear rhodium63 and dinuclear nickel species64−67 being structurally characterized. Initially, the [Ni2(SOCR)4(EtOH)] structure was shown to be dimeric, in which the {NiO4} unit was solvated with EtOH, and two {NiS4} units were bridged through Ni···S interactions.67 Subsequently, derivatives were prepared by substituting EtOH in [Ni2(SOCR)4(EtOH)] (R = CH3, C6H5) with a variety of nitrogen donor ligands, including pyridine and α-, β-, and γpicoline, but crystal structures were not obtained for any of the prepared complexes.68 Instead, these moieties were characterized by IR spectroscopy, electronic absorption spectroscopy, and elemental analysis. The authors hypothesized that the reaction of the thiocarboxylate lantern species with these nitrogen donor ligands breaks up the lantern complexes to generate mononuclear octahedral complexes with the stoichiometry [Ni(SOCR)2L2].68 Recently the [Ni2(tba)4(NCCH3)] structure was shown to be quite similar to that of the EtOH adduct.65 Both complexes have Ni···S interactions in the solid state to form a {Ni2S2} square configuration (Scheme 1)56 and an S = 1 ground state due to partial antiferromagnetic coupling between the two Ni(II) centers.65 The thioacetate analog has now been structurally

characterized as well, with an analogous {Ni2S2} square configuration.64 As discussed further below, such {M2S2} interactions have also been seen with Pt(II)55,56 and Au(I).69 General Information. Potassium tetrachloroplatinate (K2PtCl4) was prepared using a combination of literature preparations: hexachloroplatinic acid (H2PtCl6) was prepared70 from commercially obtained platinum metal and was converted to K2PtCl6,71 and K2PtCl4 was synthesized from the prepared K2PtCl6.72 The compounds [PtZn(SAc)4(OH2)],54 [(py)PtZn(SAc) 4 (py)], 5 5 [PtZn(SAc) 4 (py)], 5 5 [PtZn(SAc) 4 (4NH2py)],55 and [PtZn(SAc)4(3-NO2py)]54 were prepared as previously reported. All other reagents were obtained commercially and used without further purification. Elemental analyses were performed by Atlantic Microlab Inc. (Norcross, GA, 30071). 1H NMR and 13C{1H} spectra were recorded on a Varian 500 MHz spectrometer or Varian 400 MHz spectrometer. The 1H and 13C chemical shifts were referenced to the shift of the residual protio material in the deuterated solvent. All 1D and 2D 195Pt NMR spectroscopy experiments were performed on a Bruker Avance III 500 MHz NMR instrument operating at 107.06 and 500.18 MHz for 195Pt and 1H, respectively, using a 5 mm broadband NMR probe. All experiments were performed at 298 K. The direct 1D 195Pt experiments were performed using a standard single pulse sequence, 1H Waltz-16 decoupling, 9.5 μs π/2 pulse, 1s recycle delay, and scan averages between 128 and 2048 depending on concentration. The 2D 195Pt−1H NMR HMBC experiments were obtained using gradient selected zero and double quantum heteronuclear coherences, and were optimized for long-range 1 H−195Pt couplings (∼25 Hz). The 1H chemical shifts were referenced to the residual proton sequence of the deuterated solvent, while the 195Pt chemical shifts were referenced to the secondary external standard of 1 N Na2PtCl6 in D2O, δ = 0 ppm. The 195Pt NMR chemical shift is strongly temperatureand solvent-dependent;73 all spectra were recorded at 298 K in CD2Cl2. All excitation and emission spectra were recorded with a Jobin-Yvon Horiba Fluorolog 3-22-Tau-3 spectrofluorimeter at 77 K on solid samples using an Oxford Cryostat Optistat DN with an accessory for solid samples. Excitation wavelengths were selected to correspond with strong absorption features determined by solution phase UV−vis−NIR spectroscopy. X-ray Crystallography. A summary of crystal data collection and refinement parameters for all compounds is found in Table 1. Crystals of 1 were obtained, but the diffraction results were not of publication quality. At the limits of their reliability, the data are all consistent with the connectivity in the monomeric structure depicted in Scheme 2, and with an eclipsed interaction in the solid state. Preliminary interatomic metrics involving metal atoms are reported in Tables 2 and 3. Crystals of 2−4 were mounted on a Cryoloop with Paratone-N oil, and data were collected at 100 K (2−3) or 300 K (4) on a Bruker Proteum-R with a CCD detector using Cu Kα radiation. Some cracking of these crystals was observed in attempts to collect the structure of 4 at lower temperatures. Data were corrected for absorption with SADABS, and structures were solved by direct methods. All non-hydrogen atoms were refined anisotropically by full matrix least-squares on F2. The oxygen and sulfur atoms on the thiobenzoate groups of 4 were found to be disordered over two positions, such that the two independent ligands are twisted like propeller blades in both directions. Refinement of the

Scheme 1. Square-Type {M2S2} Interaction of Two Bimetallic Thiocarboxylate Lantern Units

B

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Inorganic Chemistry Table 1. X-ray Crystallographic Data Collection Parameters Compound

2

3

4

5

Formula Fw cryst syst space group a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 Z ρ(calcd), g cm−3 μ, mm−1 Temp, K R(F), %a R(ωF2), %b

C28H22MgO5PtS4·0.04(CH2Cl2) 789.62 monoclinic Pc 11.1776(2) 12.1319(2) 22.0941(4) 90.00 90.903(1) 90.00 2995.71(9) 4 1.751 11.95 (Cu Kα) 100 1.75 4.39

C56H44Ca2O10Pt2S8·2(CH2Cl2)·2(O) 1805.58 monoclinic P21/n 14.7530(4) 11.3221(3) 19.1906(5) 90.00 97.481(1) 90.00 3178.22(15) 2 1.887 14.03 (Cu Kα) 100 2.47 7.89

C28H21O5PtS4Zn·0.74(C4H8O) 878.95 tetragonal I¯4c2 19.0932(18) 22.727(2) 90 90 90 8285.2(14) 8 1.409 9.09 300 3.95 12.28

C56H61O8.50P2PtS4 1255.39 triclinic P1̅ 11.0404(8) 12.0309(9) 12.5730(9) 114.998(2) 104.532(2) 102.470(2) 1362.93(18) 1 1.529 2.839 122 1.70 4.15

a R = ∑∥Fo| − |Fc∥/∑|Fo|. bR(ωF2) = {∑[ω(Fo2 − Fc2)2]}/{∑[ω(Fo2)2]}1/2; ω = 1/[σ2(Fo2) + (aP)2 + bP] with a and b given in CIF, P = [2Fc2 + max(Fo,0)]/3.

solvent loss (see below). The smeared density arising from the variety of torsions was modeled using just the two disorder components at a fixed 0.5:0.5 ratio. The THF molecule of 4 is highly disordered and was modeled isotropically as a single component with an occupancy refined to 0.37; this partial occupancy could indicate that the solvent molecule is partially absent or that all components of the disorder were not found. Multicomponent disorder refinements were unsuccessful. If the THF is partially absent, then the THF oxygen position would be expected to be occupied by water. A crystal of 5 was covered in minimal mineral oil to prevent solvent loss and mounted on a loop made of Kapton. Data were collected at 122 K on a Bruker D8 VENTURE diffractometer equipped with Mo Kα high-brilliance IμS radiation (λ = 0.71073 Å), a multilayer X-ray mirror, a PHOTON 100 CMOS detector, and an Oxford Cryosystems low temperature device. Intensity data were corrected for absorption using the multiscan method implemented in SADABS. The structure was solved using Olex2 by means of the olex2.structure solution program74 using a quasi-E charge flipping algorithm, and refined with the olex2.refine refinement package using Gauss−Newton minimization.75 All non-hydrogen atoms were refined anisotropically, and all hydrogens except the ones belonging to the disordered water molecule (see below) were placed at calculated positions and refined with isotropic displacement parameters using a riding model. In the structure, a water molecule is disordered over two positions around an inversion center. Refinement of the occupancy of the O5 oxygen atom resulted in a value of ∼0.25, and consequently, the occupancy was fixed at 0.25. A Q-peak close to O5 was assigned to the hydrogen atom, H5, and the occupancy of the position was fixed at 0.5, meaning that the hydrogen atoms were modeled as common to the two disordered fragments. The O5−H5 distance was restrained at 0.87(2) Å using the DFIX command. This resulted in 4.5 solvent water molecules per formula unit. The disorder gives rise to two level B alerts in the checkCIF report: the alerts relating to short O2···O5 contacts. However, the obvious hydrogen bonding between the thiocarboxyl oxygen atom and the water molecule, along with the positional

Scheme 2. Synthesis of New Diamagnetic Heterobimetallic Lantern Complexes 1−4

occupancies of the oxygen and sulfur atoms yielded a 0.75:0.25 ratio for both ligands independently, with the twist in the same direction in the two major and two minor components, so the two ligands’ occupancies were combined to a single free variable, with the ratio remaining at 0.75:0.25. The two independent phenyl groups of 4 were also disordered, and two discrete torsions were refined for each, with each ring constrained to a hexagon using SHELX AFIX 66. The occupancies of these rotamers refined to approximately 0.5, in contrast to the oxygen and sulfur atoms, which suggests that the phenyl rings are not rigidly rotationally fixed by intermolecular interactions and can assume a greater variety of torsion angles. This torsional spread may arise during THF C

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Inorganic Chemistry Table 2. Important Structural Parameters for Diamagnetic Lanterns Formula

Pt−M (Å)

Pt−L (Å, atom)

[PtMg(SAc)4(OH2)], 1 [PtMg(tba)4(OH2)], 2 [PtCa(tba)4(OH2)], 3 [PtZn(tba)4(OH2)], 4 [PtZn(SAc)4(OH2)] [PtZn(SAc)4(py)2] [PtZn(SAc)4(py)] [PtZn(SAc)4(3-NO2py)] [PtZn(SAc)4(4-NH2py)]

2.78† 2.687(16)* 3.0637(7) 2.6342(15) 2.6477(7) 2.5313(7) 2.6180(5) 2.6282(3) 2.6617(6)

3.34† 3.23(19)*, S 3.3924(9), S 3.1203(9), Pt 3.1246(4), Pt 2.476(4), N 3.038(3), S 3.44(2), Pt 3.256(1), S

M−L (Å, atom) 2.00† 2.01(2)*, 2.376(3), 2.016(8), 2.053(4) 2.085(4), 2.084(3), 2.096(2) 2.047(3),

MPtPt Angle (deg)

O O O N N N

180† 144(4)* 147.90(2) 180.00(1) 179.38(2) NA 133.35(1) 159.36(1) 142.10(1)

Reference this this this this 54 55 55 54 55

work work work work

*

Average value. †Preliminary value.

Table 3. Formal Shortness Ratio (FSR) Values for Pt···M Distances in Lantern Complexes Compound

Experimental M−M distance (Å)

Sum of covalent radii (Å)

FSR

Reference

[PtMg(tba)4(OH2)], 2 [PtCa(tba)4(OH2)]2, 3 [PtMg(SAc)4(OH2)], 1 [PtZn(SAc)4(py)] [PtZn(SAc)4(3-NO2py)] [PtZn(tba)4(OH2)], 4 [PtZn(SAc)4(OH2)] [PtZn(SAc)4(4-NH2py)]

2.698 3.0637(7) 2.77(1) 2.6180(5) 2.6282(3) 2.6342(15) 2.6477(7) 2.6617(6)

2.77 3.12 2.77 2.58 2.58 2.58 2.58 2.58

0.974 0.982 1.000 1.015 1.019 1.021 1.026 1.032

this this this 55 54 this 54 55

work work work

work

ligands ([Pt(SAc)4]2− or [Pt(tba)4]2−), and the second was made of the acceptor metal ion with H2O ([Zn(OH2)]2+ or [Mg(OH2)]2+). Synthetic Procedures. [PtMg(SAc)4(OH2)] (1). A portion of NaHCO3 (127 mg, 1.52 mmol) was mixed with HSAc (102 μL, 1.45 mmol) in 3 mL of water and allowed to stir for 5 min before a solution of K2PtCl4 (150 mg, 0.361 mmol) in 1 mL of water was added. A solution of MgSO4 (44 mg, 0.36 mmol) in 1 mL of water was added immediately, yielding an orange solution that was stirred for approximately 2.5 h until bright yellow. The aqueous solution was evaporated to dryness with the assistance of an air jet, and the resulting yellow residue was dissolved in 3 mL of acetone and filtered to remove a white powder, presumably a mixture of Na2SO4 and K2SO4 salts. The acetone was removed with the assistance of an air jet to yield a yellow powder. This powder was redissolved in 2 mL of acetone and filtered through a glass fiber disk. A yellow microcrystalline precipitate formed by addition of CH2Cl2 and was dried in vacuo (88 mg, 45% yield). Anal. Calcd for PtMgC8H14O5S4: C, 17.87; H, 2.62; N, 0.00%. Found: C, 17.84; H, 2.51; N, 0.00%. UV−vis-NIR (H2O) (λmax, nm (εM, cm−1 M−1)): 202 (34,500), 259 (25,900), 430 (30). 1H NMR (δ, ppm, {D2O}): 2.35 (s, CH3). 13C{1H} NMR (δ, ppm, {D2O}: 212.36 (s, SO(C)CH3), 34.53 (s, CH3). 13C{1H} chemical shifts referenced to residual acetone from synthesis. 195 Pt (δ,{CD2Cl2}) − 4270 ppm. [PtMg(tba)4(OH2)] (2). A portion of Htba (227 μL, 1.93 mmol) was mixed with 10 mL of an aqueous NaHCO3 solution (171 mg, 2.04 mmol). The reaction mixture was swirled vigorously by hand for 5 min, resulting in a clear yellow solution that was transferred to a clean flask. A solution of K2PtCl4 (200 mg, 0.482 mmol) in 3 mL of water was added to the reaction mixture. Immediately, a solution of MgSO4 (58 mg, 0.48 mmol) in 4 mL of water was added dropwise to the reaction mixture, which became cloudy after 5 min and was stirred overnight. A canary yellow precipitate was filtered from a yellow filtrate, washed with water, and dried in vacuo (195 mg,

uncertainty of the water molecule arising from the disorder, should account for these unusually short contacts. Computational Studies. Density functional theory (DFT) analysis was carried out with the 2014 and 2016 releases of the Amsterdam density functional (ADF) program suite.76,77 The gas-phase geometries of each species were optimized at the PBE level of theory, and relativistic corrections for Pt atoms were made with the scalar ZORA as implemented by ADF.78 The QZ4P basis set was used for Pt atoms; TZ2P for Mg, Ca, and Zn; SZ for H; and DZP for all p-block elements.79 No frozen core approximation was applied to geometry optimizations. These geometries were confirmed as local minima by numerical frequency calculations with the local density approximation, a large frozen core approximation, and TZ2P (Pt, Zn, Ca, Mg), SZ (H), and DZP (E) basis sets.80 Starting coordinates for all complexes were obtained from crystal structures, except for [PtCa(tba) 4 (OH 2 )] and [PtCa(tba)4(OH2)2] (Schemes S1b and S1c), as well as [(bpy)PtAr2] and [(bpy)PtAr2(ZnPhF2)] (Ar = p-C6H4tBu; PhF = C6F5), which were adapted from the published phenanthroline derivatives.42 Final coordinates from geometry optimized structures that were confirmed to be potential energy minima from vibrational calculations are available in the Supporting Information. The Natural Bond Orbital (NBO) 6.081 package and Bader analysis82,83 as implemented by ADF were used to calculate bond orders and occupancies of Lewis valence orbitals. 195Pt NMR spectra were simulated for each complex using the NMR module of ADF.84 Charge transfer complementary occupied-virtual pair analysis85 (COVP) based on absolutely localized molecular orbitals (ALMOs)86 was performed in the Q-Chem 4.4 package87 using the M06-L functional,88 CRENBL89 effective core potential and basis set for heavy atoms, and 6-311G* Pople basis for hydrogen atoms. Starting coordinates were obtained from optimized geometries calculated by DFT as described above. For the COVP analysis, each system was separated into two fragments: the first comprised the donor Pt metal atom and D

DOI: 10.1021/acs.inorgchem.6b02372 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

para-C6H5), 129.07 (s, ortho-C6H5), 128.44, 128.27 (s, metaC6H5). 195Pt (δ,{CH2Cl2}) −4310 ppm. (Ph4P)2[Pt(SAc)4] (5). A sample of NaHCO3 (88.1 mg, 1.02 mmol) was dissolved in 10 mL of H2O, and HSAc (68 μL, 0.96 mmol) was added. The mixture was stirred for 15 min before the addition of a solution of K2PtCl4 (102.8 mg, 0.241 mmol) in 5 mL of H2O. The pink solution was stirred overnight before adding Ph4PCl (184.5 mg, 0.482 mmol). A white solid precipitated immediately and was recovered by filtration after 1 h. Upon drying, the tan solid was recrystallized from CH2Cl2/ hexanes for a recrystallized yield of 198.8 mg (71.0%). Yellow crystals suitable for X-ray crystallography were grown from CH2Cl2/hexanes. Anal. Calcd for C56H52O4P2PtS4·0.5 CH2Cl2: C, 55.77%; H, 4.39%; N, 0.00%. Found: C, 55.80%; H, 4.46%; N, 0.00%. UV−vis-NIR (CH2Cl2) (λmax, nm (εM, cm−1 M−1)): 257(21700). 1H NMR (δ, ppm, {CD2Cl2}): 7.93 (m, 8H, paraP(C6H5)4), 7.79 (m, 16H, meta-P(C6H5)4), 7.65 (m, 16H, ortho-P(C6H5)4), 2.40 (s, 10H, SOCCH3). 13C NMR (δ, ppm, {CD2Cl2}): 207.07 (s, SO(C)CH3), 135.70 (d, 4JPC = 3.8 Hz, para-P(C6H5)4), 134.48 (d, 2JPC= 13.8 Hz, ortho-P(C6H5)4), 130.65 (d, 3JPC = 50 Hz, meta-P(C6H5)4), 117.53 (d, 1JPC = 88.8 Hz, ipso-P(C6H5)4), 36.54 (s, SOC(C)H3). 195Pt (δ, {CD2Cl2}) − 3899 ppm. Note: For synthesizing lantern complexes, the [Pt(SAc)4]2− ion is prepared in situ and reacted with the second metal source. For the spectroscopic and analytical characterization of 5, several recrystallizations are required to separate 5 from the similarly soluble (Ph4P)2[PtCl4].

51% crude yield). This yellow solid was recrystallized three times from DCM/hexanes to yield a powder with the composition [PtMg(tba)4(OH2)]·1/2H2O (70 mg, 18% recrystallized yield). Yellow block crystals were grown from the slow evaporation of a DCM solution for X-ray crystallography. Anal. Calcd for PtMgC28H23O5.5S4: C, 42.30; H 2.92; N 0.00%. Found: C, 42.16; H, 2.89; N 0.00%. UV−vis-NIR (CH2Cl2) (λmax, nm (εM, cm−1 M−1)): 244(49,000), 313(33,800) 1H NMR (Figure S1) (δ, ppm, {acetone-d6}): 8.06 (m, 2H, orthoC6H5) 7.51 (t, J = 7.4 Hz, 1H, para-C6H5), 7.37 (t, J = 7.4 Hz, 2H, meta-C6H5) 13C{1H} NMR (δ, ppm, {THF-d8}): 209.71 (s, SO(C)Ph), 141.31 (s, ipso-C6H5), 133.37 (s, para-C6H5), 129.36 (s, ortho-C6H5), 128.91 (s, meta-C6H5). 195Pt (δ, {CD2Cl2}) − 4276 ppm. [PtCa(tba)4(OH2)] (3). A portion of Htba (113 μL, 0.964 mmol) was mixed with approximately 30 mL of an aqueous NaHCO3 solution (87 mg, 1.036 mmol) and swirled vigorously by hand for about 10 min, resulting in a clear yellow solution that was transferred to a clean flask. A solution of K2PtCl4 (100 mg, 0.241 mmol) in 3 mL of water was added to the reaction mixture. Immediately, a solution of CaCl2 (27 mg, 0.241 mmol) in 2 mL of water was added dropwise to the reaction mixture, which became cloudy after 5 min and was stirred overnight. A beige precipitate was filtered from a light yellow filtrate, washed with three aliquots of 20 mL of water, and dried in vacuo (132 mg, 68% crude yield). The beige solid was dissolved in 75 mL of DCM and filtered over a fine frit, and the yellow solution concentrated to about 4 mL in vacuo. A portion of about 100 mL of hexanes was added to the concentrated solution, resulting in a light yellow precipitate, which was twice recrystallized from DCM/hexanes. Yellow plate crystals were grown from slow evaporation of DCM. Anal. Calcd for PtCaC28H22O5S4: C, 41.94; H 2.77; N 0.00%. Found: C, 41.84; H, 2.95; N 0.00%. UV−vis-NIR (CH2Cl2) (λmax, nm (εM, cm−1 M−1)): 241 (58,600), 292sh (28,800), 314 (34,100). 1H NMR (Figure S2) (δ, ppm, {acetone-d6}): 8.01 (m, 2H, ortho-C6H5) 7.44 (tt, J = 7.5 Hz, J = 1.5 Hz, 1H, para-C6H5), 7.32 (tt, J = 7.5 Hz, J = 1.5 Hz, 2H, meta-C6H5) 13C{1H} NMR (δ, ppm, {THF-d8}): 207.71 (s, SO(C)Ph), 141.97 (s, ipso-C6H5), 131.92 (s, para-C6H5), 128.58 (s, ortho-C6H5), 128.13 (s, metaC6H5). 195Pt (δ,{ CH2Cl2}) − 4101 ppm. [PtZn(tba)4(OH2)] (4). A portion of Htba (226 μL, 1.93 mmol) was mixed with 5 mL of aqueous NaHCO3 solution (170 mg, 2.024 mmol) and stirred for 20 min. The resulting clear yellow solution was filtered through a filter pipet and transferred to a clean flask. A 2 mL aqueous solution of K2PtCl4 (200 mg, 0.482 mmol) was added to the reaction mixture, followed immediately by a 5 mL aqueous solution of ZnCl2 (68 mg, 0.48 mmol). The reaction mixture became cloudy after 5 min, and was stirred for 4 h. An off-white precipitate was filtered from a yellow solution, and the solid was washed with water and dried in vacuo (54% crude yield, 216 mg). This offwhite solid was recrystallized three times from a saturated THF solution to which Et2O and hexanes were added (2% yield, 5 mg). Colorless block crystals were grown from the slow evaporation of a saturated THF solution. Anal. Calcd for PtZnC32H30O6S4: C, 42.74; H 3.36; N 0.00%. Found: C, 42.66; H, 3.24; N 0.00%. UV−vis-NIR (THF) (λmax, nm (εM, cm−1 M−1)): 240 (37,400), 314 (15,900) 1H NMR (Figure S3) (δ, ppm, {acetone-d6}): 8.05 (m, 2H, ortho-C6H5) 7.52 (tt, J = 7.5 Hz, J = 1.3 Hz, 1H, para-C6H5), 7.38 (tt, J = 7.5 Hz, J = 2.0 Hz, 2H, meta-C6H5) 13C{1H} NMR (δ, ppm, {THF-d8}): 208.43 (s, SO(C)Ph), 141.43, 140.45 (s, ipso-C6H5), 132.97, 132.31 (s,

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RESULTS AND DISCUSSION Synthesis. The synthesis of [PtMg(SAc)4(OH2)] (1) employed the same general procedure for the synthesis of heterobimetallic lantern complexes previously developed,53−55 but the 3d metal source was replaced with MgSO4, as shown at the top of Scheme 2. Interestingly, unlike the heterobimetallic complexes prepared with 3d metals,53−55 the Mg-derivative 1 exhibits high solubility in water and does not precipitate from it. The isolation of 1 therefore requires the removal of the reaction solvent, washing, and recrystallization of the resulting residue from acetone. Compounds 2−4 do precipitate from H2O and are therefore prepared using the previously reported method, as shown at the bottom of Scheme 2.53 Compound 5 is prepared by first generating the [Pt(SAc)4]2− species in solution, followed by the addition of Ph4PCl to precipitate 5. (Ph4P)2[PtCl4] is also generated, and can be difficult to separate due to its similarity to 5 in both solubility and appearance. Based on integration of peaks in the 1H NMR, about 20% of the crude material is (Ph4P)2[PtCl4]. Multiple recrystallization cycles yield analytically pure material. Structural Characterization. Compounds 2−5 have been crystallographically characterized, and the data collection parameters are summarized in Table 1. Selected distances and angles are presented in Table S1, and a comparison of the most important lantern core metrical parameters are collected in Table 2. As shown in Figure 1, there are two nonequivalent lantern structures in the unit cell of 2, with distinct distances: Pt(1)··· Mg(1) is 2.6982(17) and Pt(2)···Mg(2) is 2.6754(15) Å, both appreciably shorter than the sum of their covalent radii90 (2.77 Å), while Mg(1)−O(1W) is 2.023(4) and Mg(2)−O(2W) is 1.990(4) Å. The Mg centers are displaced from Pt and from the best {O4} planes by 0.127, Mg(1), and 0.136 Å, Mg(2). No crystal structures of Mg-containing heterobimetallic lantern E

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Figure 1. ORTEP of [PtMg(tba)4(OH2)]2 (2), including short dimeric Pt···S contacts between units. Ellipsoids are drawn at the 50% level. Hydrogen atoms have been removed for clarity.

Figure 2. ORTEP of the Ca-bridged dimer of [PtCa(tba)4(OH2)]2 (3). Ellipsoids are drawn at the 50% level. Hydrogen atoms and lattice solvent have been removed for clarity.

complexes exist in the literature for direct comparison, and only one structurally characterized example of a homometallic Mg lantern-type complex with a carbamate ligand exists: [Mg2(O2CNPh2)4(HMPA)2] (HMPA = hexamethylphosphoramide).91 This structure is a highly distorted lantern, with each Mg center extending well beyond the best plane of the carbamate oxygen atoms by 0.473 Å, a substantially greater displacement than is observed in 2. The {MgO4} faces of [Mg2(O2CNPh2)4(HMPA)2] are canted with respect to the planes of the coordinating carboxylate O atoms, resulting in the nonlinear Mg(1)−Mg(1B)−O(5B) (HMPA) angle91 of 168.58° and a Mg···Mg separation of 3.150(1) Å, such that the Mg centers are farther from each other than the intramolecular Pt···Mg spacing observed in 2.91 In comparison, the {PtS4} faces of 2 dimerize in the solid state via a {Pt2S2} square55,56 from two Pt···S contacts between two adjacent lanterns in a square configuration,55 as shown in Figure 1. Each lantern in the unit cell forms three S···S contacts, between S(2)···S(7), S(4)···S(6), and S(3) ···S(7), that were found to be 3.535(2), 3.230(1), and 3.494(2) Å, respectively. There are two short Pt···S distances of 3.364(2) and 3.095(1) Å between Pt(1)···S(7) and Pt(2)···S(4). The square motif is one of four different geometries observed in the dimerization of [PtM(SOCR)4(L)] lantern complexes.56 A related analysis of Au(I) dimers bridged by dithiocarboxylates, xanthates, and dithiocarbamates has also been reported.69 In contrast to the many bimetallic Pt dimers that are head-tohead, 3 forms a tail-to-tail carboxylate bridged dimer, as shown in Figure 2. The Ca atom forms one contact with an oxygen from an adjacent lantern’s thiobenzoate ligand and binds one terminal water molecule, making the calcium six-coordinate with a trigonal prismatic configuration (Figure 2 and Scheme S1a). Defining one trigonal face with O(1), O(1i), and O(2), and the other with O(3), O(4), and O(1w), the average twist angle, ϕ,92 is 9(6)°. The twist angles were calculated as torsions from, for example, O(3)−Cnt1−Ca(1)−O(1), where Cnt1 is the centroid of O3, O4, and O(1w). To accommodate the adjacent lantern, the axially coordinated water molecule is offset from the central lantern Pt···Ca vector, as illustrated by the

nonlinear Pt(1)−Ca(1)−O(1W) angle of 137.84(9)°. The Ca(1)−O(1W) distance is 2.376(3) Å, consistent with the average nonbridging Ca−OH2 distance of 2.40(6) Å, as determined from a survey of the data in the CSD.93 The Ca(1)···Pt(1) distance is 3.0637(7) Å, and the Ca atom is displaced substantially from the mean {O4} plane by 0.630 Å. This distortion has been seen in other {CaM} lantern-type structures.30,94−97 The geometry about the Pt center in 3 also deviates from square planar, with the Pt center removed from the mean plane of S(1)−S(4) by 0.106 Å toward the Ca center. This displacement may partly be a result of the geometric distortion caused by the size of the Ca center. Ca2+ has an ionic radius of 1.14 Å, which is roughly 68% larger than the largest other divalent cation used to date to prepare a thiocarboxylate lantern complex (Co2+, 0.68 Å).98 This distortion may also result from the geometric adjustment of the oxygen atoms to support the Ca six-coordinate geometry. The platinum centers on both ends of the bridged complex form two short Pt···S contacts in a square56 configuration with an adjacent tetrametallic (bridged lantern pair) unit (Figure S4, bottom), displaying a unique Pt(1)···S(1)i distance of 3.3924(9) Å and a Ca(1)−Pt(1)−Pt(1)i angle of 147.90(2)°. Several examples of Ca-containing bimetallic lantern-type complexes have been structurally characterized, including a {Ca2} complex,96 two examples prepared with Cu,94,95 one with Pd,97 and a fourth with Pt.30 The heterobimetallic {CaCu} lantern complex [CaCu(O2CCH2OC6H4Cl-2)(H2O)4] has a square planar Cu center coordinated to four bridging carboxylates and an axial water molecule, and a Cu···Ca distance of 3.479(3) Å.94 The Ca center is displaced by 1.258 Å from the mean plane defined by the Ca-coordinated carboxylate oxygen atoms, which is substantially more than the 0.629 Å displacement observed in 3. Interestingly, the other reported {CaCu} lantern complex with the stoichiometry [CaCu(Et3NCH2CO2)4(NO3)2(H2O)]24+ forms a carboxylate-bridged dimer much like 3; however, it is the Cu centers that are bridged by two reciprocal Cu−O bonds (Cu−O is 2.301 Å) with carboxylate oxygens of the adjacent lantern unit.95 The Ca center of this complex is similarly displaced from the mean F

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between atoms A and B, and RA and RB are the covalent radii of each atom, respectively. The lower the number, the shorter the bond, and a value of unity indicates an unexceptional covalent separation. FSRs lower than unity typically indicate multiple bonds between the atoms. Table 3 ranks the intramolecular Pt···M distances in the {PtM} thiocarboxylate complexes by increasing FSR. The ordering suggests more interaction between Pt and the Group 2 atoms than between Pt and Zn: Pt···Zn FSR values are slightly greater than 1.0, whereas the Pt···Mg and Pt···Ca ones are a bit less than 1. However, geometric propinquity is not the only requirement for interaction between two atoms. As described in more detail below, there is actually greater interaction in the Zn case than there is with Mg or Ca. The mononuclear (Ph4P)2[Pt(SAc)4] (5) is structurally unremarkable, as shown in Figure 4. The two phosphonium

{O4} plane of the carboxylate atoms by 1.237 Å with a Ca···Cu separation of 3.420 Å. 95 The Ca centers of [CaCu(Et3NCH2CO2)4(NO3)2(H2O)]24+ are eight-coordinate, just as in the case of [CaCu(O2CCH2OC6H4Cl-2)(H2O)4], except three aquo ligands have been replaced with two nitrate moieties: one that binds in a bidentate fashion, and another that behaves as a monodentate ligand.94,95 A recently reported trinuclear paddlewheel-type complex with four monothiosuccinimide ligands contains a {Mo2} lantern unit with {MoS4}, {MoN4}, and {CaO4} coordination from the succinimide lignads and a long Mo···Ca separation of 3.699(1) Å and a much greater distance of Ca from the best lantern {O4} plane of 1.123 Å.99 The only reported {PtCa} lantern complex,30 [PtCa(ox)4Cl]− (ox = 2-mercaptobenzoxazoyl), is similar to 3 with a Pt center displaced from the mean {O4} plane by 0.096 Å toward the Ca center. This displacement itself deviates from the carboxylate oxygen plane by 0.488 Å toward the axially bound chloride atom. These displacements and the Pt···Ca distance of 2.960(5) Å are consistent with those observed in 3. Compound 4, [PtZn(tba)4(OH2)]·THF, forms a perfectly staggered56 metallophilic dimer (Figure 3) much like the

Figure 4. ORTEP of (Ph4P)2[Pt(SAc)4] (5). Ellipsoids are drawn at the 50% level. Only a single cation is shown, and hydrogen atoms and lattice H2O have been excluded for clarity.

cations have no close contacts with the anion, and the anion has square-planar geometry about Pt, with two thioacetate ligands above and two below the {PtS4} plane. In the absence of a second metal ion to bind the oxygen atoms, no lantern structure is observed. The average Pt−S distance of 2.32(2) Å with monodentate carboxylates is statistically identical to those in the four lantern complexes described above. 195 Pt NMR Spectroscopy. The 195Pt NMR spectra were recorded for the five published diamagnetic complexes [(py)PtZn(SAc) 4 (py)], 55 [PtZn(SAc) 4 (py)], 55 [PtZn(SAc)4(3-NO2py)],54 [PtZn(SAc)4(4-NH2py)],55 and [PtZn(SAc)4(OH2)],54 as well as 1−4. (Ph4P)2[Pt(SAc)4], 5, was also included as a mononuclear control. The results are collected in Table 4. The 195Pt chemical shift of mononuclear 5 is −3899 ppm (Figures S5 and S6), well downfield of the shifts of all the lantern complexes. The observation of a significant 1 H−195Pt through-bond J-coupling correlation observed in the 2D HMBC experiments (Figure S7) for [PtZn(SAc)4(3NO2py)] confirms that the Pt coordination to the S-ligand is retained in solution, and is not disrupted or displaced by the solvent. Several of the 195Pt spectra are shown in Figure 5. Several patterns emerge from the data plotted in Figure 6. The chemical shifts for all the lantern compounds are downfield from that of the monometallic control, 5, at −3899 ppm. The most notable difference within the group of lantern complexes is between the chemical shifts in the Zn-containing compounds and those of the species that contain either Ca or Mg. A downfield shift of about 30 ppm is observed between the

Figure 3. ORTEP of [PtZn(tba)4(OH2)]·THF (4). Ellipsoids are drawn at the 50% level. Only a single rotamer is shown, and hydrogen atoms have been excluded for clarity.

previously reported Co and Ni thiobenzoate53 and Zn thioacetate54 complexes, with a short Pt···Pt distance of 3.1203(9) Å. The observed Pt···Pt distance in 4 is slightly shorter than the Pt···Pt distance observed in [PtZn(SAc)4(OH2)] (3.1246(3) Å) and significantly shorter than that of the partially eclipsed dimer in [PtZn(SAc)4(3-NO2py)] (3.4453(2) Å).54,56 The {PtS4} faces are staggered, with an average torsion angle (defined by SPt···PtS) of 45(5)°. The observed Pt···Zn distance of 2.6342(15) Å is similar to the average Pt···Zn distance of 2.639(12) Å observed in the thioacetate derivative, [PtZn(SAc)4(OH2)]. One way to analyze the degree of metal−metal interaction is with Cotton’s formal shortness ratio (FSR).100 It is defined as shown below, in which RE is the experimentally determined distance RE FSRAB = (RA + RB) G

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Inorganic Chemistry Table 4.

195

Pt Chemical Shifts and Luminescence Signals of Diamagnetic Lantern Complexes

Compound [PtMg(SAc)4(OH2)], 1 [PtMg(tba)4(OH2)], 2 [PtCa(tba)4(OH2)], 3 [PtZn(tba)4(OH2)], 4 [PtZn(SAc)4(OH2)] [PtZn(SAc)4(py)2] [PtZn(SAc)4(py)] [PtZn(SAc)4(3-NO2py)] [PtZn(SAc)4(4-NH2py)] (Ph4P)2[Pt(SAc)4] *

Pt···Pt (Å) 3.37(1)

†

3.1203(9) 3.1246(4)

3.4453(2)

δ

Pt···M (Å) †

195

Pt (ppm)

−4270 −4276 −4101 −4310 −4300 −4305 −4303 −4304 −4311 −3899

2.77(1) 2.687(16)* 3.0637(7) 2.6342(15) 2.6477(7) 2.5313(7) 2.6180(5) 2.6282(3) 2.6617(6)

Emission (nm) None 580/695/702

664 None None 625

Reference this this this this 54 55 55 54 55 this

work work work work

work

†

Average value. Preliminary value.

Figure 6. Comparison of calculated and experimental 195Pt NMR shifts. Green: 5; orange: 3; red: 1 and 2; blue: 4 and previously reported [PtZn(SAc)4(L)] lanterns. Red line is guide to the eye indicating exact agreement.

Figure 5.

There is also a modest difference in the chemical shifts between thioacetate- and thiobenzoate-containing lanterns. The observed signal for [PtMg(SAc)4(OH2)] (1), −4270 ppm, is 6 ppm downfield from the signal of [PtMg(tba)4(OH2)] (2), at −4276 ppm. The same pattern is observed for the Zncontaining species: [PtZn(SAc)4(OH2)] shows a 195Pt NMR peak at −4300 ppm, slightly downfield of the resonance of [PtZn(tba)4(OH2)] (4) at −4310 ppm. The platinum coordination environments are all pseudosquare-pyramidal with in-plane {PtS4} coordination and an axial M ligand, except for [(py)PtZn(SAc)4(py)], which has a {PtS4N1M} coordination environment with pseudo-octahedral geometry. Changes in coordination number are typically accompanied by substantial changes in chemical shift on the order of thousands of ppm.101 This effect has been clearly shown with some Pt(0) species; for example, [Pt(PEt3)3] has an observed δ 195Pt of −4526 ppm,102 and upon increasing the coordination number by one to [Pt(PEt3)4], the observed resonance shifts to −5262 ppm.101 The change from an environment of {PtS4} in [PtZn(SAc)4(py)] to {PtS4N1} in [(py)PtZn(SAc)4(py)] is marked by a shift of only 2 ppm, consistent with the long Pt−N distance (2.476(4) Å) in [(py)PtZn(SAc)4(py)].55 There is very little change in the 195Pt chemical shift observed when pyridine has a para-amino (NH2) or meta nitro (NO2) substituent, consistent with lack of an aromatic resonance effect on the 195Pt chemical shift.73 The

195

Pt NMR spectra recorded in CD2Cl2.

spectra of the Zn family and those of the Mg-containing 1 and 2. A further downfield shift of 130 ppm is seen when Mg is substituted with Ca. These data indicate that the electronic environment at the Pt atom in 5 changes significantly upon coordination of Zn, Mg, or Ca, and that the effects of each M2+ are distinct from the others to a non-negligible degree. As noted above, [PtCa(tba)4(OH2)]2 (3) has a substantially different geometry than 1. In the solid state, the trigonal prismatic, six-coordinate Ca atom forms contacts with an oxygen from the thiobenzoate of an adjacent lantern unit to form a tail-to-tail dimer. The larger change in the δ 195Pt of the Ca thiobenzoate lantern could be the result of the distorted square planar geometry of Pt in 3, in which the Pt is displaced inward by 0.106 Å. In contrast, the Pt center of 2 is more strictly square planar, with an average displacement of only 0.01(1) Å from the plane defined by the thiocarboxylate S atoms. This geometrical difference, the origin of which we attribute to the large ionic radius of Ca2+, may also be partly responsible for the difference in δ 195Pt between 3 and the other lantern complexes. H

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Inorganic Chemistry difference in δ 195Pt as a function of para substituents on the axial pyridine ligand has been demonstrated to be less than 5 ppm, even with modified carboxylates that are directly coordinated to the Pt center. Complexes of the form [Pt(PEt3)2HL] also exhibit a small change in observed δ 195Pt values (4.7 ppm) when L is changed from p-NMe2-C6H4CO2 to p-NO2-C6H4CO2.103 This pattern is borne out in the series [PtZn(SAc)4(NC5H4R)] (R = H, NO2, NH2), in which modification of the pyridine ligand results in a modest shift range of −4303 to −4311 ppm. These three compounds also differ in their solid state structures56 as partially eclipsed (R = NO2)54 versus square (R = H, NH2)55 configurations. This measurable degree of pyridine substituent influence over the 195 Pt chemical shift suggests that modification of the pyridine ligand does impact to a minor degree the electronic environment of the platinum two bonds away. The change in 195Pt chemical shift upon coordination of a donor Pt atom to a Lewis acid metal center has been observed previously, and a collection of these data from the literature is given in Figure 7 and Table S2. These patterns are discussed further in the context of the DFT calculations below.

Figure 8. Solid state luminescence of [PtMg(tba)4(OH2)] (2). Excitation at 380 nm (black trace) and emission at 580 and 695 nm (red trace).

Figure 9. Solid state luminescence of [PtMg(tba)4(OH2)] (2). Excitation at 460 nm (black trace) and emission at 702 nm (red trace). Figure 7. Comparison of δ 195Pt values for complexes with and without interactions with Lewis acidic M (Exact values and literature references in Table S4).

to intraligand transitions within thiobenzoate, since it shows an emission at similar wavelength. By contrast, the lower energy emission is likely to arise from a charge transfer transition between a Pt orbital influenced by Mg and a π* orbital of the tba ligand. Interestingly, the Zn-thioacetate derivative, [PtZn(SAc)4(OH2)], was found to exhibit a broad emission at low temperature centered at approximately 664 nm when excited at 383 nm (Figure 10). This emission shows a vibrational structure that may be related to a transition involving the thioacetate ligand. Curiously, although the 3-NO2py derivative also exhibits a short (3.4453(2) Å) Pt···Pt contact, it is not luminescent. These two results suggest that in these compounds a short Pt···Pt contact alone does not result in luminescence, but rather the excitation of electrons from the Pt···Pt σ* orbital into one with Pt···Zn character. Alternatively, the emission could result from excitation of electrons resident in orbitals with Pt···Zn character. The compound [PtZn(SAc)4(py)] was determined to be nonluminescent, whereas the 4-NH2py derivative, [PtZn(SAc)4(4-NH2py)], was found to emit at 625 nm when an excitation wavelength of 360 nm was employed (Figure 11). These two compounds have very similar Pt···Zn distances (Table 2) and no Pt···Pt contacts shorter than

Luminescence Characterization. The solid-state luminescence behavior of four Zn ([PtZn(SAc)4(OH2)],54 [PtZn(SAc)4(3-NO2py)],54 [PtZn(SAc)4(py)],55 [PtZn(SAc)4(4NH2py)]55) and two Mg ([PtMg(SAc)4(OH2)] (1) and [PtMg(tba)4(OH2)] (2)) compounds was investigated as a companion study to the 195Pt NMR experiments, to assist in the assessment of the electronic perturbations felt by the Pt center when in close proximity to Mg and Zn. Preliminary structural data for 1 show connectivity similar to [PtZn(SAc)4(OH2)], with a short Pt···Pt distance of 3.37(1) Å and a Pt···Mg separation of 2.78(1) Å that is considerably less than the sum of their van der Waals radii (3.48 Å). Unsurprisingly, 1 was found to be nonemissive, because of a lack of valence electrons on Mg to be excited and the nonemissive character of the thioacetate ligand at room temperature. However, the thiobenzoate derivative 2 was found to emit at 580 and 695 nm if excited with 380 or 460 nm light (Figures 8 and 9). The emission at higher energy of the latter complex can be tentatively assigned I

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electronic environment in {PtMg} and {PtZn} lantern complexes that are otherwise structurally analogous, support the hypothesis that a Pt(II)···Zn(II) metallophilic interaction exists in the {PtZn} lantern complexes. To date, there are few examples of metallophilic interaction between Pt(II) and Zn(II) centers. In one case, the authors report no evidence to support the claim beyond structural data, merely citing the short Pt···Zn distance of (2.760(1) Å) as evidence of metallophilicity.41 A series of octanuclear {Pt4M4} compounds was prepared, including a Zn derivative with average Pt···Zn of 2.77 Å, in which weak σ-donation from Pt to M was described.105 We note that these vectors are longer than those found for the Pt···Zn distances of [PtZn(SAc)4(OH2)] and 4. In another case, the Pt···Zn distance is 2.593 Å, for which electrochemical and spectroscopic data demonstrated only weak interactions between the metal atoms.106 There are reports107 in the literature that attempt to model the electronic structure of Zn(II)···Zn(II) metallophilic dimers, and a lone recent example43 of a Pt(0)-to-Zn(II) dative bonding interaction. Further analysis of the intermetallic interaction in 4 is below in the computational section. Unsurprisingly, neither the Zn-containing 4, nor the Mgcontaining 1 and 2, nor the Ca-containing 3 exhibit NIR absorbances, and they only have tails of ligand π−π* transitions above 350 nm, further supporting the proposal that the LMCT bands reported earlier53−55 utilize partially occupied orbitals on the 3d metal. Additionally, NIR transitions that result from an intermetallic 3d−Pt transition are present in the previously reported Co- and Ni-derivatives,53−55 but not in the Znderivatives. The one transition found in the diamagnetic thioacetate complexes is an LMCT band that falls between 244 and 263 nm for all diamagnetic thioacetate complexes. The thiobenzoate derivatives 2−4 exhibit two primary transitions in the UV region: one near 241 nm that can be attributed to an LMCT, and another near 314 nm that is suggestive of a π to π* transition within the thiobenzoate phenyl ring. Computational Studies. Crystallographic coordinates were used as starting points to calculate optimized gas-phase geometries for each complex, except for the hypothetical [PtCa(tba)4(OH2)] and [PtCa(tba)4(OH2)2] (Schemes S1b and S1c), and the literature species [(bpy)PtAr2] and [(bpy)PtAr2{Zn(PhF)2}] (Ar = p-C6H4tBu; PhF = C6F5).42 Pt−M distances and selected angles were found to be in good agreement with experiment, with the calculated bond lengths being consistently shorter than those seen in the crystal structures by 0.12 Å or less (Tables 4 and S3). Calculated 195Pt NMR shifts in the gas phase for 1, 2, and 4 were in excellent agreement with experiment (less than 2% difference, +78 ppm).108−111 Shifts calculated for the previously reported [PtZn(SAc)4(L)] complexes55 were even more so, with 0.25% difference or less (+11 ppm or less, Table S4). In 3, in which M = Ca, the difference was distinctly greater (6.2%, +255 ppm). Because it is uncertain whether the dimeric structure observed in the solid-state persists in solution, two monomer analogs (Scheme S1) were also constructed and analyzed in order to determine whether the observed 195Pt shifts were more consistent with a monomer or dimer structure. One monomer was constructed with a single axial H2O coordinated to the Ca, analogous to 1, 2, and 4; the other contained two H2O ligands coordinated to Ca, resulting in a pseudo-6-coordinate environment similar to that observed in the solid-state structure of 3 (Figure 2). All 195Pt NMR shifts calculated for these three {PtCa} complexes were further from

Figure 10. Solid state luminescence of [PtZn(SAc)4(OH2)]. Excitation at 383 nm (black trace) and emission at 664 nm (red trace).

Figure 11. Solid state luminescence of [PtZn(SAc)4(4-NH2py)]. Excitation at 360 nm (black trace) and emission at 625 nm (red trace).

4 Å. The luminescence in [PtZn(SAc)4(4-NH2py)], therefore, does not arise from a Pt···Pt based orbital, and is likely the result of excitation from a Pt···Zn interaction based orbital to the π* orbital of the 4-NH2py or thioacetate ligand. The Zn derivatives that exhibit luminescence behavior, [PtZn(SAc)4(OH2)] and [PtZn(SAc)4(4-NH2py)], have intramolecular Pt···Zn distances of 2.6477(7) and 2.5517(6) Å, respectively, which are substantially shorter than the sum of the van der Waals radii for Pt and Zn (3.15 Å).104 In addition to a short Pt···Zn distance, [PtZn(SAc)4(OH2)] exhibits a Pt···Pt interaction in the solid state, with a short Pt···Pt separation of 3.1246(4) Å. However, the luminescent behavior of [PtZn(SAc)4(OH2)] and [PtZn(SAc)4(4-NH2py)] cannot be attributed to these structural features alone. The unsubstituted pyridine derivative [PtZn(SAc)4(py)] exhibits a short Pt···Zn separation (2.5180(5) Å), and [PtZn(SAc)4(3-NO2py)] exhibits both short Pt···Zn (2.6282(3) Å) and Pt···Pt (3.4453(2) Å) distances, yet both compounds are nonemissive. These data suggest that short Pt···Zn and Pt···Pt contacts are not sufficient to engender luminescent behavior, but that different L donor ligands can have an impact on the observed photophysical properties. These luminescence data, along with the 195Pt NMR data that show that Pt experiences a different J

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Inorganic Chemistry Table 5. Calculated M−M Distances and Bond Orders in Lantern Complexesa M−M distance (Å)

a

M-M Bond Order

Compd

Calcd

Exptl

% Diff

Mayer

Wiberg

[PtMg(SAc)4(OH2)], 1 [PtMg(tba)4(OH2)], 2 [PtZn(SAc)4(OH2)] [PtZn(tba)4(OH2)], 4 [PtZn(SAc)4(py)] [PtZn(SAc)4(4-NH2py)] [PtZn(SAc)4(3-NO2py)] [PtCa(tba)4(OH2)]2, 3 [PtCa(tba)4(OH2)2] [PtCa(tba)4(OH2)]

2.658 2.631 2.527 2.514 2.578 2.591 2.578 2.973 2.915 2.832

2.77(1) 2.687(16) 2.6477(7) 2.6342(15) 2.6180(5) 2.6617(6) 2.6282(3) 3.0637(7)) 3.0636(8) 3.0636(8)

4.1 2.5 4.6 4.5 1.5 2.7 1.9 3.0 4.9 7.6

0.02 0.04 0.34 0.35 0.32 0.31 0.31 0.12 0.17 0.21

0.07 0.07 0.14 0.14 0.14 0.14 0.14 0.07 0.08 0.09

Ref this this 54 this 55 55 54 this this this

work work work

work work work

Calculated bond distances for complexes [PtCa(tba)4(OH2)2] and [PtCa(tba)4(OH2)] are compared to the experimental value of 3.

where 195Pt data were available for both Pt-containing species, were chosen. Two examples of Pt(0) functioning as a Lewis base have also been included. First we consider the broad pattern of chemical shift as a function of the ligand donor environment. The experimentally determined 195Pt NMR signal of 5 falls on the most negative end of the range of the Pt-only literature compounds. The compounds with the most positive shifts have the strongest σdonor ligands, including methyl groups and ortho-metalated phenylpyridine. In general, the chemical shifts of the Pt(II) donor species are more upfield when the Pt center is ligated by softer donors, such as thiophene and tertiary phosphines, and more downfield when the Pt atom is coordinated to hard donor ligands. Although neutral, anionic, and cationic species are all represented in this survey, the ligand environment around Pt seems to contribute more to the magnitude of the NMR shift than does the overall charge of the complex. For example, the cation [Pt(2-phenylpyridine)(S3C6H12)]+ has a 195Pt NMR value of −3787 ppm,37 far from the most positive value of −1749 ppm,44 and the shift of the anion 5 is only +15 ppm away from that of the neutral species [Pt(C^N^C)(SC4H8)].49 According to this pattern, phosphine ligands donate the most electron density to the Pt atom and provide the greatest shielding; nitrogen donors are the weakest, and the sulfur groups S3C6H1237 and SC4H849 fall in the middle with anionic [SAc−]. Within this cohort of divalent Pt centers, the relative 195 Pt NMR shift of 5 is consistent with those of other molecules reported in the literature. The seeming outlier among these compounds is [Pt(PCy3)2], with a shift of −6505,43 which is due to the electron-rich Pt(0) center. Upon addition of a Lewis-acidic metal-containing fragment to these precursors, the majority of the changes in 195Pt NMR shifts are downfield, consistent with the donation of electron density from Pt to the heterometal. The only exception among the literature examples is the complex [(dmpe)Pt(μ2-Ar)2(Zn(PhF)2)] (Ar = p-C6H4tBu; PhF = C6F5), which displays a relatively small change in shift of −38 ppm (versus the average change of ∼200 ppm, Table S2) upon coordination of [Zn(PhF)2].42 It is also the only species in the literature survey to have a bridging ligand (μ2-C6F5) between Pt and Zn, as opposed to an unsupported interaction. In both of these ways, [(dmpe)Pt(μ2-Ar)2(Zn(PhF)2)] is similar to the lantern complexes, suggesting that the presence of a bridging ligand affects the way that electron density at Pt is redistributed upon addition of a second metal fragment. Bond order, NBO, and MO analyses were performed in order to characterize the nature of the Pt···M interactions

the experimental value than those obtained for the other lanterns, but the calculated δ 195Pt of the dimer was found to be closer to the experimentally observed shift (6.2% difference) versus that of the doubly hydrated monomer (6.8%). The singly hydrated monomer had the worst correspondence to experiment (9.5%), suggesting that the higher coordination number is retained in solution. The Pt···Ca distance in the simulated dimer is also a closer match to experiment than those found in either of the hypothetical monomers (Tables 4 and S4). Therefore, the dimeric structure was used in subsequent analyses unless otherwise noted. The calculated 195Pt shift for (Ph4P)2[Pt(SAc)4] (5), −3839 ppm, is also in close agreement with the experimental value of δ = −3899 ppm. These measured and calculated 195Pt NMR shifts for the Mg, Ca, and Zn lantern complexes are quite distinct from one another and from the monometallic 5. Formation of the Pt···M contact results in a change of between −371 and −412 ppm from the δ 195Pt of 5 to those in 1−4 and the [PtZn(SAc)4(L)] series of lanterns (Table 4, Figure 6, and Table S4). These changes are reproduced in the calculations. There is also a difference in calculated chemical shift of 70 ppm between [PtMg(SAc)4(OH2)] (1), and [PtZn(SAc)4(OH2)]; and a change of 72 ppm between [PtMg(tba)4(OH2)] (2) and [PtZn(tba)4(OH2)] (4). These values are larger than the experimental differences of 30 and 34 ppm, respectively, but they confirm that substitution of M has a measurable effect on the electronic environment around Pt. These differences in δ 195 Pt from [Pt(SAc)4]2− indicate that the electronic structure of Pt in [PtM(SAc)4(L)]2− is affected not only by the presence of M, but also by its identity. Although the NMR simulations reproduce the effect of the second metal, the experimentally observed differences between complexes with thiobenzoate vs thioacetate ligands were within the uncertainty of the calculations. Differences of 10 ppm or less between the pairs 1/2 and 4/[PtZn(SAc)4(OH2)] were not reproduced by the 195Pt NMR simulations, nor was the sign of the change. Likewise, the experimentally determined chemical shifts of complexes in the [PtZn(SAc)4(L)] series are all within 11 ppm of each other. Because the range of values is so small, all empirically observed changes in 195Pt NMR shifts as a function of L are not exactly reproduced by calculations with the parameters employed in this study (Tables S2 and S4). To gain a better understanding of the effect of a Lewis acidic metal in close proximity to a donor Pt(II) atom, 195Pt NMR shifts of related compounds from the literature were examined (Figure 7, Table S2). Pairings of a Pt-only compound and the product of its reaction with a Lewis-acidic metal fragment, K

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Inorganic Chemistry present in 1−4 and [PtZn(SAc)4(OH2)], and place them in the broader context of literature compounds. Wiberg (WBO)112 and Mayer (MBO)113,114 bond orders along the Pt−M vector were determined for 1−4 and related [PtZn(SAc)4(L)] lanterns, and are shown in Table 5. The clearest correlation is between the bond order and the identity of M: all Zn species display a 2-fold increase in WBO over their Mg counterparts, and that factor rises to ten when comparing MBOs. The values for Ca-containing 3 are intermediate between those of the Zn and Mg species, presenting a qualitative trend of increasing covalency with increasing atomic number. The bond orders found for the M = Zn lanterns are similar to those calculated for several compounds in the literature containing Pt(II)···Zn(II) interactions. MBOs and WBOs were found for [(phen)Pt(Ar)2(Zn(PhF)2)] (0.39, 0.12) and the analogous bpy- (0.39, 0.12) and dmpe-coordinated (0.15, 0.03) species. The Pt(0) complex [Pt(PCy3)2(ZnBr2)]43 (0.39, 0.14) was also studied (Table 6). Three of these species display

Figure 12. Localized, partially filled nonbonding orbitals in 1 (left, occupancy 0.23) and [PtZn(SAc)4(OH2)] (right, occupancy 0.43) from NBO analysis.

tions from both the 5dz2 orbital of Pt and the 4s orbital of Zn, including the LUMO+1, although the contribution is no greater than 4% of one AO or the other in each MO. In addition, the 3dz2 orbital of Zn (6%) and 5dz2 orbital of Pt (36%) both contribute to the HOMO−4. Considering MOs comprising any other combination of Pt and Zn character, 18 are occupied and six are unoccupied. Similar patterns were also observed in 4 (Figure S8) and the other [PtZn(SAc)4(L)] complexes. In all lanterns studied, the primary interactions experienced by either metal are with the ligand donor atoms. In 1 and 2, the lack of covalency between metals is pronounced (Scheme 3). In 1, there are three MOs containing both Pt 5dz2 and Mg 3s character: two occupied, and one unoccupied. Of these, the most significant overlap is observed through the empty Pt 6s and Mg 3s, which contribute in an 11%:8% ratio to an occupied orbital of primarily ligand character. There are 15 MOs with any other Pt/Mg AO mixing present, and the majority of mixing is seen in high-energy, unoccupied MOs. The Mg center has primarily ionic interactions with the O atoms of the thiocarboxylate, consistent with behavior observed in other Mg(II) compounds. Similar behavior is seen in 2 (Figure S9). The Ca species 3 displays one occupied orbital containing 16% Pt 5dz2 and only 1% Ca 3dz2 character (Figure S10, Table S6, and Scheme S2), two unoccupied orbitals with less than 2% contribution of those AOs, and 20 unoccupied MOs with any other type of Pt/Ca AO mixing. Examination of MO contributions calculated when using an ionic-fragment-based approach revealed the same trend that was observed in the results discussed above. In 1, there are one occupied and two unoccupied MOs containing both Pt2+ 5dz2 and Mg2+ 3s character. Twelve MOs contain any other combination of Mg2+ and Pt2+ character. In contrast, among the MOs of [PtZn(SAc)4(OH2)] are five occupied and four unoccupied orbitals containing contributions from both Pt2+ 5dz2 and Zn2+ 4s, plus 20 orbitals with any other combination of Pt2+ and Zn2+ character. For ease of comparison, the hypothetical, monomeric [PtCa(tba)4(OH2)] lantern (Scheme S1b) was also examined by fragment calculation rather than the dimer 3. [PtCa(tba)4(OH2)] was found to have two occupied and two unoccupied MOs with Ca2+ 4s/Pt2+ 5dz2 contributions, and 16 unoccupied MOs with any other Ca2+/Pt2+ mixing, placing it between 1 and [PtZn(SAc)4(OH2)] in terms of degree of covalency. Using the absolutely localized molecular orbital (ALMO) method, complementary occupied-virtual pairs (COVP) were

Table 6. Calculated Bond Orders of Literature Compounds with Pt → ZnII Interactions (Ar = p-C6H4tBu) Compound

Pt−Zn (Å)

[MBO]

[WBO]

Ref

[Pt(PCy3)2(ZnBr2)] [(phen)PtAr2(ZnPhF2)] [(bpy)PtAr2(ZnPhF2)] [(dpme)PtAr2(ZnPhF2)]

2.472 2.553 2.675 2.822

0.39 0.39 0.32 0.15

0.14 0.12 0.12 0.03

43 42 42 42

bond orders between Pt and Zn that are extremely similar to 4 and the other [PtZn(SAc)4(L)] lantern complexes shown in Table 5. These unbridged Pt···Zn interactions are clearly dative, and therefore, the members of the [PtZn(RCOS)4(L)] series also fall into the broader category of metal-only donor− acceptor complexes. The fourth compound, [(dmpe)Pt(μ2-Ar)2(Zn(PhF)2)], contains a bridging ligand and shows an uncommon upfield shift in the 195Pt spectrum. Its Pt···Zn bond orders (0.15, 0.03) are distinctly lower than those of the other three complexes, and the metal−metal distance is longer than other species listed in Table S2. One possible reason for these differences is the binding mode of the bridging aryl groups. Donation from the aryl π-system into empty p orbitals on Zn, as well as backbonding from the 3d orbitals on Zn, were observed computationally.42 These increased interactions between Zn and the aryl groups may be responsible for the smaller degree of Pt···Zn covalency. NBO analysis further supports a covalent description of the Pt···Zn interaction in 4 and the previously reported [PtZn(SAc)4(L)] lanterns, and a lack of covalent Pt···Mg interaction in 1 and 2. Rather than significant bonding overlap between the metals, there is an unfilled valence nonbonding orbital with fractional occupancy, accounting for electron density that is part of neither the atomic core orbitals nor the bonding/ antibonding orbitals, localized on Zn. A similar localized orbital is seen in the Mg species as well (Figure 12), but the occupancy is much lower, consistent with the hypothesis that the Mg center is accepting less density from the atoms it interacts with. Parsing metal contributions to the calculated MOs for 2, 3, and 4 (Tables S5−S7) reveal that while there is only a small degree of covalent overlap between the occupied 5d orbitals of Pt and the empty s orbitals of M, the extent of mixing is greatest when M = Zn. In the case of [PtZn(SAc)4(OH2)], three occupied and three unoccupied MOs contain contribuL

DOI: 10.1021/acs.inorgchem.6b02372 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 3. Qualitative MO Diagrams for 1 versus 4a

a

The majority of Pt density is shared with {RCOS}− in both cases.

calculated for 1, 2, 4, and [PtZn(SAc)4(OH2)]. A pair of these orbitals describes an intramolecular electron donation from an occupied orbital on the [Pt(RCOS)4]2− fragment of the lantern to an empty acceptor orbital localized on the [M(OH2)]2+ fragment. The orbitals in [PtZn(SAc)4(OH2)] and 4 (Figures 13 and S11) indicate that the donor orbitals in [Pt(SAc)4]2− are

Table 7. ALMO COVP Calculations System

ΔE (kJ/mol)

[Pt(SAc)4]2− → [Mg(OH2)]2+, 1 [Pt(tba)4]2− → [Mg(OH2)]2+, 2 [Pt(SAc)4]2− → [Zn(OH2)]2+ [Pt(tba)4]2− → [Zn(OH2)]2+, 4

−8.93 −8.46 −67.76 −65.98

ΔQ (me)̅ 5.06 4.65 53.61 50.82

% COVP contribution 35 35 69 69

energy change, less distortion of the electron clouds upon formation of intramolecular interactions, and less involvement of ligand orbitals than with Zn (Table 7). These data indicate that a valence electron in either {PtZn} complex can be excited much more easily than an electron in the analogous {PtMg} species, a characteristic that may be partially responsible for the luminescence properties of [PtZn(SAc)4(OH2)] versus Mgcontaining 1 (vide supra). Overall, the data indicate that 1, 2, and 3 have almost exclusively ionic metal−ligand interactions, with no Pt···M interactions, whereas the interactions in 4 and other {PtZn}containing species are more dative and covalent. Scheme 4 depicts four general types of metal−metal pairings between closed-shell metals. A homobimetallic Pt(II)−Pt(II) is shown in Scheme 4(a), including the mixing of the occupied 5d and unoccupied 6s orbitals in a d8−d8 metallophilic interaction. A heterobimetallic d10−d8 interaction is shown in (b) between Au(I) and Pt(II). Another potentially metallophilic d10−d8 interaction is shown in (c), emphasizing the greater energy disparity between the valence shells of Zn and Pt versus Au and Pt. Lastly, in (d) the Mg(II) 3s orbital has no interaction with the Pt 5d. These sketches may help to think about metal−metal

Figure 13. COVP visualizations for [PtZn(SAc)4(OH2)], left, and 1, right. Donor orbitals are shown in solid red and blue; acceptor orbitals are translucent.

comprised of both dz2 character from Pt and p character from electron-rich oxygen atoms on the thiocarboxylate ligand, whereas the acceptor orbitals on [Zn(OH2)]2+ have largely s character from the Zn center. The quantitative COVP analysis in Table 7 includes (i) ΔE, a calculation of charge transfer energy, (ii) ΔQ, a measure of the distortion of the electronic clouds upon formation of the interaction, and (iii) the percentage of the total COVP that consists of orbitals that participate in charge transfer from donor to acceptor.85 In Mgcontaining 1 and 2, Pt still contributes a majority dz2 component (Figures 13 and S11), but there is a less favorable M

DOI: 10.1021/acs.inorgchem.6b02372 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

metal-only donor−acceptor interaction does not necessarily occur.

Scheme 4. Qualitative MO Diagrams Depicting (a) Homonuclear Metallophilic, (b) Heteronuclear Metallophilic, (c) Dative, and (d) Nonexistent Metal−Metal Interactions

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02372. 1 H NMR spectra for 2−4; 195Pt NMR spectra for 5; COVP visualizations for 2 and 4; structures of {PtCa} species; a qualitative MO diagram for 3; quantitative MO diagrams for 2−4; tables of structural, 195Pt NMR, and MO contribution data; and DFT-optimized Cartesian coordinates for 1−5 and others (PDF) CIFs for 2−5 (CIF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Linda H. Doerrer: 0000-0002-2437-6374 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank NSF-CCF 0829890 (L.H.D.), NSF-CHE 0619339 (NMR spectrometer at Boston University), and NSF-DGE 1247312 (A.S.H.) for funding. J.M.L.-de-L. and D.P. thank DGI MINECO/FEDER (CTQ2016-75816-C2-2-P) for financial support. The 195Pt NMR spectroscopy (T.M.A.) was performed at Sandia National Laboratories, which is a multiprogram laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the U.S. Department of Energy’s National Nuclear Security Administration. We thank Mikkel Agerbaek for the X-ray crystal structure of 5, and Ruslan Tazhigulov for assistance with the COVP calculations, and James McNeely for helpful discussions.

interactions on a continuum of differences in valence orbital energies. As these computational data have shown, the Pt···Mg compound has no significant metal−metal interaction in spite of the short M···M distance, because the Mg center’s Lewis acidity is satisfied by the ionic interactions with the ligand O donors. The Pt···Zn interactions do exhibit demonstrable dative character in the presence of the same ligand donor set. The Pt···Ca case is an in-between one based on the 195Pt NMR data and calculated bond orders.

■

CONCLUSIONS Four new heterobimetallic thiocarboxylate lantern complexes with {PtMg} (1 and 2), {PtCa} (3), and {PtZn} (4) pairings have been prepared, as well as the mononuclear control species (Ph4P)2[Pt(SAc)4] (5). A cooperative study employing 195Pt NMR, structural analysis, solid-state fluorescence spectroscopy, and electronic structure calculations was conducted to elucidate and characterize the Pt···M interactions present in each lantern complex, as well as the structural factors that influence those interactions. Trends in the 195Pt data for complexes 1−5 and comparable species in the literature suggest a qualitative correlation between the type of Lewis-acidic metal atom interacting with Pt and the change in 195Pt chemical shift of the donor Pt(II) atom. The nature of the ligands bound to Pt, the identity of the Lewis acidic metal, and the presence or absence of a bridging ligand all influence the chemical shift values. Additionally, MO, fragment MO, NBO, and COVP analyses have provided evidence for a covalent Pt···Zn interaction with metal-only donor−acceptor character in 4 and other {PtZn} lantern species, which need to be thought of simultaneously with the already demonstrated existence of metallophilic interactions in the d10−d8 pairings. This analysis has also demonstrated that the Pt···Mg and Pt···Ca compounds lack significant M···M interactions. Together, the experimental and computational techniques presented here demonstrate that, even in the quite close contact of a bimetallic lantern complex, a

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REFERENCES

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DOI: 10.1021/acs.inorgchem.6b02372 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b02372 Inorg. Chem. XXXX, XXX, XXX−XXX