Volatile Iridium and Platinum MOCVD Precursors: Chemistry, Thermal Properties, Materials and Prospects for Their Application in Medicine

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Review Volatile Iridium and Platinum MOCVD Precursors: Chemistry, Thermal Properties, Materials and Prospects for Their Application in Medicine

Ksenya I. Karakovskaya, Svetlana I. Dorovskikh, Evgeniia S. Vikulova, Igor Yu. Ilyin, Kseniya V. Zherikova , Tamara V. Basova and Natalya B. Morozova *

Nikolaev Institute of Inorganic Chemistry SB RAS, Lavrentiev Pr. 3, 630090 Novosibirsk, Russia; [email protected] (K.I.K.); [email protected] (S.I.D.); [email protected] (E.S.V.); [email protected] (I.Y.I.); [email protected] (K.V.Z.); [email protected] (T.V.B.) * Correspondence: [email protected]; Tel.: +7-383-330-9596

Abstract: Interest in iridium and platinum has been steadily encouraged due to such unique properties as exceptional chemical inertia and corrosion resistance, high biological compatibility, and mechanical strength, which are the basis for their application in medical practice. Metal-organic chemical vapor deposition (MOCVD) is a promising method to fabricate Ir and Pt nanomaterials, multilayers, and heterostructures. Its advantages include precise control of the material composition and microstructure in deposition processes at relatively low temperatures onto non-planar substrates. The development of MOCVD processes is inextricably linked with the development of the chemistry of volatile precursors, viz., specially designed coordination and organometallic compounds. This

review describes the synthesis methods of various iridium and platinum precursors, their thermal properties, and examples of the use of MOCVD, including formation of films for medical application

Citation: Karakovskaya, K.I.; and bimetallics. Although metal acetylacetonates are currently the most widely used precursors, the Dorovskikh, S.I.; Vikulova, E.S.; recently developed heteroligand Ir(I) and Pt(IV) complexes appear to be more promising in both syn- Ilyin, I.Y.; Zherikova, K.V.; Basova, thetic and thermochemical aspects. Their main advantage is their ability to control thermal properties T.V.; Morozova, N.B. Volatile Iridium by modifying several types of ligands, making them tunable to deposit films onto different types of and Platinum MOCVD Precursors: materials and to select a combination of compatible compounds for obtaining the bimetallic materials. Chemistry, Thermal Properties, Materials and Prospects for Their Keywords: noble metals; platinum; iridium; volatile precursors; thermal properties; MOCVD; ﬁlm; Application in Medicine. Coatings nanoparticle; medical electrodes; medical implants 2021, 11, 78. https://doi.org/10. 3390/coatings11010078

Received: 10 December 2020 1. Introduction Accepted: 2 January 2021 Published: 11 January 2021 Interest in platinum group metals has not waned for a long time due to their unique properties and the possibility of application of their films as protective, anticorrosive, buffer, Publisher’s Note: MDPI stays neu- optical, sensor, or electrocatalytic layers in various fields, such as aerospace engineering, tral with regard to jurisdictional clai- catalysis, hydrogen energy, fuel cells, and microelectronics [1–5]. In recent years, the ms in published maps and institutio- number of publications devoted to the medical application of platinum group metals, in- nal affiliations. cluding platinum and iridium, has increased. These metals are distinguished by exceptional chemical inertness and corrosion resistance in biological environments, high biological compatibility, and mechanical strength. Traditionally, platinum- and iridium-containing film materials have been used in medicine to cover the contact poles of electrodes for Copyright: © 2021 by the authors. Li- cardio- and neurostimulation and diagnostics in order to improve their electrochemical censee MDPI, Basel, Switzerland. characteristics [6–8]. In recent works [9–13], the prospect of using platinum and iridium in This article is an open access article composite coatings and film heterostructures to modify the surface of permanent implants, distributed under the terms and con- which are in demand in oncological, orthopedic, and reconstructive surgery, has also been ditions of the Creative Commons At- tribution (CC BY) license (https:// shown. In particular, new bioactive materials consisting of platinum metal coatings with creativecommons.org/licenses/by/ micron dots of silver or gold demonstrate an enhanced antibacterial effect compared to the 4.0/). traditionally used silver [12,13]. A winning feature of such heterostructures is also their abil-

Coatings 2021, 11, 78. https://doi.org/10.3390/coatings11010078 https://www.mdpi.com/journal/coatings Coatings 2021, 11, 78 2 of 36

ity to configure time-limited activity involving low amounts of Ag or Au, which prevents cytotoxicity and undesirable tissue reactions. Moreover, platinum and iridium coatings can improve the biocompatibility and osseointegration of implants [9–11]. The principle of action of such composites is currently under discussion, and apparently includes not only the electrochemical, but also the synergistic aspect [13]. The dependence of the observed effects on the combination of noble metals and the composition and morphology of coatings has been noted in a number of studies [9–13]. The full potential of the application of functional noble metal coatings in medical practice can only be realized if the problem of deposition of the layers with a given composition and structure on materials of different natures and non-planar geometry can be successfully solved. For this reason, the development of universal and precision methods for the deposition of platinum and iridium coatings is one of the most important steps. Metal-organic chemical vapor deposition (MOCVD) fully meets the specified criteria, since it allows obtaining coatings and nanoparticles, multilayers and heterostructures with controlled characteristics (composition, morphology, particle size, etc.) on objects of complex shapes and does not impose special requirements upon the coated material, except for its stability. In addition, the MOCVD principle allows avoiding the difficulties associated with the refractoriness of platinum and iridium, since the use of volatile compounds (precursors) of these metals makes it possible to obtain the required materials at relatively low temperatures (200–600 ◦C). The MOCVD process consists of several main steps (Figure1): heating of the volatile compound source to transfer this precursor into the gas phase, transport of the precursor vapors to the substrate, activated precursor decomposition on the substrate surface to form the target material (coating or nanoparticles), and gaseous by-products that are removed from the reactor chamber. The decomposition reaction could be activated by high temperatures (classical approach), plasma, irradiation, etc. Regarding the thermodynamic and kinetic aspects, the deposition process in MOCVD reactors is a complicated non-equilibrium dynamic system that includes a sequence of equilibrium or partially non-equilibrium chemical reactions of the precursor decomposition. Therefore, the thermochemical properties of precursors play a key role in the reactor construction and experimental parameters necessary for the preparation of the materials with the required properties. The quantitative data on thermal characteristics of volatile compounds are essential to precisely control the deposition process. Specifically, the information on the phase transitions (melting, sublimation, vaporization) and mass transfer gives the opportunity to manage the amount of substance supplied into the reaction zone. The characteristics of precursor vapor thermal destruction processes allows for determining the effective deposition temperature range and the effect of the introduction of various reagent gases or activation forces. Knowl- edge of thermal behavior is the perfect tool for tailoring the precursor structure to specific technological tasks on deposition of the target materials onto the substrates of various natures. For medicine, this approach means the ability to effectively coat a wide range of non-planar materials, including metals, polymers, carbon composites, etc., which differ dramatically in the thermochemical characteristics such as thermal stability, oxidation, or embrittlement resistance. Thus, the development of MOCVD processes is inextricably linked with the development of the chemistry of iridium and platinum precursors, viz., specially designed coordination and organometallic compounds. Another important issue is the expansion of the collection of compounds for the effective selection of compatible combinations of a precursor in order to obtain bimetallic systems based on platinum and iridium, which are of particular interest due to the possibility of a synergistic effect. Coatings 2021, 11, 78 3 of 36 Coatings 2021, 11, x FOR PEER REVIEW 3 of 39

FigureFigure 1. 1. SchemeScheme for for the the main main stages stages of of the the MOCVD MOCVD process. process.

Thus,The growing the development interest in of the MOCVD chemistry processes of volatile is platinuminextricably and linked iridium with compounds the devel- is opmentconfirmed of the by thechemistry regular of appearance iridium and of thematicplatinum reviewsprecursors, [14– 18viz.,]. Whilespecially the designed latest data co- on ordinationnew iridium and precursors organometallic were collected compounds. in 2015 An [18other], the important latest review issue of platinumis the expansion precursors of thewas collection published of in compounds 2008 [15], and for furtherthe effective reviews selection have only of coveredcompatible certain combinations aspects of theirof a precursorapplication in [order17]. It to is obtain also worth bimetallic mentioning systems that based the recent on platinum reviews and covered iridium, various which classes are ofof particular precursors, interest including due specificto the possibility and general of a synthesis synergistic concepts, effect. and then focused mainly on applicationsThe growing in interest MOCVD in processes, the chemistry while of the volatile thermal platinum properties and of iridium the compounds compounds have isreceived confirmed much by lessthe regular attention. appearance of thematic reviews [14–18]. While the latest data on newIn iridium this regard, precursors this review were presents collected the in main 2015 classes [18], the of latest volatile review precursors of platinum of platinum pre- cursorsand iridium, was published including in compounds 2008 [15], and synthesized further reviews in recent have years, only in covered order to certain provide aspects a basis offor their the application further development [17]. It is also of MOCVDworth mentioning processes, that primarily the recent in reviews the aspect covered of medical vari- ousapplications. classes of precursors, Thus, the thermal including properties specific ofand compounds general synthesis will constitute concepts, the and mainstream then fo- cusedof this mainly review, on since applications it is this knowledgein MOCVD thatproc contributesesses, while to the the thermal most efficient properties use of of the the compoundscompounds have as volatile received MOCVD much less precursors. attention. This information will be supplemented with examplesIn this ofregard, specific this deposition review presents conditions the main and classes characteristics of volatile of precursors the resulting of platinum materials. andIn addition, iridium, including the role of compounds the precursor synthesized properties in in recent the preparation years, in order of bimetallic to provide coatings a basis forwill the be further specially development emphasized. of Finally,MOCVD the processes, latest information primarilyon in the successaspect of of medical MOCVD ap- of plications.platinum andThus, iridium the thermal for coatings properties used inof medicinecompounds will will be presented, constitute and the themainstream future scope of thisof their review, application since it is will this be knowledge outlined. that contributes to the most efficient use of the compounds as volatile MOCVD precursors. This information will be supplemented with ex- 2. Volatile Precursors amples of specific deposition conditions and characteristics of the resulting materials. In addition,The the main role requirementsof the precursor for properties effective in MOCVD the preparation precursors of bimetallic are substance coatings purity will ≥ be( specially98.0 mass.%), emphasized. relatively Finally, high vapor the latest pressure information (volatility), on stabilitythe success (both of inMOCVD condensed of plat- and vapor phases during volatilization), the presence of a “window” between the temperatures inum and iridium for coatings used in medicine will be presented, and the future scope of volatilization and vapor decomposition, and a high degree of conversion into the target of their application will be outlined. material. To minimize contamination of the formed material, the decomposition of the 2.precursor Volatile shouldPrecursors occur through the formation of only gaseous by-products. In the case of noble metals, another important aspect is the possibility of synthesizing a precursor with a highThe yield main from requirements available reagents. for effective MOCVD precursors are substance purity (≥98.0 mass.%),As notedrelatively above, high iridium vapor and pressure platinum (volatility), have a high stability chemical (both inertness, in condensed which and is one va- of porthe phases reasons during for the volatilization), limited range the of volatile presence precursors of a “window” of these between metals the used temperatures in MOCVD ofexperiments. volatilization Various and vapor classes decomposition, of volatile and compounds a high degree of iridium of conversion and platinum into the will target be material.considered To separatelyminimize contamination below. The structure of the formed and abbreviations material, the of decomposition ligands are shown of the in precursorTable1. should occur through the formation of only gaseous by-products. In the case of noble metals, another important aspect is the possibility of synthesizing a precursor with a high yield from available reagents. As noted above, iridium and platinum have a high chemical inertness, which is one of the reasons for the limited range of volatile precursors of these metals used in MOCVD experiments. Various classes of volatile compounds of iridium and platinum will be considered separately below. The structure and abbreviations of ligands are shown in Table 1.

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Table 1. Structure and abbreviation of the ligands of various classes employed to volatile iridium and/or platinum MOCVD precursors a. Table 1. Structure andTable abbreviation 1. Structure of and the ligandsabbreviation of various of the classes ligands em ofployed various to classes volatile em iridiumployed and/or to volatile platinum iridium MOCVD and/or precursorsplatinum MOCVD a. precursors a. Table 1. StructureCoatingsCoatings and 2021 2021 abbreviation, 11, ,11 x ,FOR x FOR PEER PEERofCoatings the REVIEW REVIEW ligands 2021 , 11 of, x various FOR PEER classes REVIEW employed to volatile iridium and/or platinum MOCVD precursors a. 4 of4 of39 39 4 of 39 Coatings 2021, 11, x FOR PEER REVIEW 4 of 39 a a Coatings 2021, 11, x FORTable PEER 1.REVIEW Structure and abbreviationTable 1.of Structurethe ligands and of abbreviationvarious classes of emtheployed ligands to of volatile variousNEUTRAL iridium classesNEUTRAL and/orLIGANDSemployed platinumNEUTRAL to LIGANDSvolatile MOCVD LIGANDS iridium precursors and/or platinum . 4a MOCVDof 39 precursors a. a Table 1. Structure and abbreviationTable 1. ofStructure the ligandsTable and 1. abbreviation of Structure variousNEUTRAL classesand of LIGANDS the abbreviation ligandsemployed of various ofto thevolatile ligandsclasses iridium em of ployedvarious and/or to classes volatileplatinum em iridiumployed MOCVD and/or to volatileprecursors platinum iridium MOCVD. and/or precursors platinum . MOCVD precursors . hexadiens hexadiens hexadiens norbornadiensnorbornadiens norbornadiens cyclooctadiens cyclooctadienscyclooctadiens hexadiens NEUTRAL LIGANDSnorbornadiens cyclooctadiens 1 2 NEUTRAL LIGANDS Structure 1 StructureR 2R RNEUTRAL1 Abbreviation LIGANDSR2 AbbreviationStructure NEUTRALStructure LIGANDS R Abbreviation R AbbreviationStructure a a R StructureAbbreviation R Abbreviation StructureStructure R1hexadiens RR2 TableTable 1.Abbreviation 1.Structure Structure and andR abbreviation Tableabbreviation 1. StructureStructureAbbreviation of ofthe the ligands and ligands abbreviationnorbornadiens of ofvarious various R ofclassesStructure classes the ligands em Abbreviationemployedployed of various to tovolatile volatile classesR iridiumStructure iridium employed and/or and/orcyclooctadiens to platinum Abbreviationvolatile Rplatinum Abbreviation iridium MOCVD MOCVD and/or precursors precursors platinum . . MOCVDStructure precursors a. R Abbreviation hexadiens Table 1. Structure and abbreviation of the ligandsnorbornadiens of various classes employed to volatile iridium cyclooctadiensand/or platinum MOCVD precursors a. hexadiens1 2 H H hexadiensH hd H norbornadiens hd H norbornadiensa cyclooctadiensnbdH nbd H codcyclooctadiens H cod StructureTable 1. StructureH R and abbreviationStructureHHH R of theAbbreviation ligands hd R1 of various R classes2 Structure employedAbbreviationhd to volatile iridiumH R Structureand/or platinumAbbreviation nbd MOCVD H Rprecursors Structure Abbreviation. H R nbdAbbreviation cod Structure R Abbreviation H cod 1 Structure2 Structure R R StructureAbbreviationH R 1 Me R2 RH1 StructureAbbreviation hd-MeMeR 2 AbbreviationNEUTRALNEUTRAL hd-MeStructure R LIGANDS LIGANDS Abbreviation NEUTRALStructure RMe LIGANDSStructureAbbreviation cod-Me Me R R AbbreviationStructureAbbreviation cod-Me R EtAbbreviation Structurecod-Et R Et Abbreviationcod-Et H H HMe H hd-Me hdH Me H hd-Me hd MeH cod-Menbd MeH NEUTRAL LIGANDSnbd Et Hcod-Me cod-Etcod H cod Et cod-Et H H hexadienshexadiens hd NEUTRALH LIGANDShexadiensH hd H norbornadiensnorbornadiensnbd Et norbornadiensnbd-EtHEt H codnbd-Etcyclooctadiensnbdcyclooctadiens Bu cod-Bucyclooctadiens BuH cod-Bucod H Me hd-MeH MeH hd-Me hd hexadiens EtMe nbd-Et cod-Me Et MeH cod-Menbd Bu Et nbd-Etnorbornadiens cod-Bucod-Et H Et cod-Etcod cyclooctadiens Bu cod-Bu Structure 1 R1 2 R2 Abbreviation1 2 Structure R Abbreviation Structure R Abbreviation hexadiensH Structure MeMe Me R hd-MeStructure Me Me R AbbreviationR hd-Me2 norbornadiens R1 Abbreviation2Structure Me cod-MeStructure R cyclooctadiens Abbreviation R Et StructureAbbreviationcod-Et R AbbreviationStructure R Abbreviation Me Me hd-Me2 StructureMeH hd-Me2MeMe R Et hd-Me2hd-MeR Abbreviationnbd-Et i i Structure Bu Me i icod-Bu R cod-MeAbbreviation i Structurei Et R cod-EtAbbreviationi i 1 2 H Me hd-Me i i MeEtPr cod-Menbd-Etinbd-i Pr i i BuEt Bu cod-Bucod-Etcod- Bu i i Structure R R AbbreviationH H H H StructureH hd hd H R Pr hdAbbreviation nbd- Pr StructureHPr H R nbdBunbdAbbreviation nbd- PrH cod- PrBu nbd-nbdPr H H codcod HBu codcod- BuBu cod- Bu Me Me hd-Me2Me Me hd-Me2 H H Et hd nbd-Et Et nbd-EtEt Bu H cod-Bunbd-Et nbd Bu cod-Bu Bu H cod-Bucod H H hd H H MeMe H hd-Me hd-Me Me H iPr hd-MeANIONIC nbdnbd- iPr LIGANDS Mei Me H cod-Me cod-MeiBui cod Me cod-iBu cod-Me Eti Et cod-Etcod-Eti Et cod-Et Me Me hd-Me2 ANIONICMe LIGANDSMe H ANIONIC hd-Me2Me LIGANDS hd-Me Pr nbd- Pr Me cod-MeBu cod- Bu Et cod-Et i ANIONICi LIGANDS i i i i i i H βMe-diketonates hd-MeMe Me hd-Me2 Me Pr cod-Me nbd- PrEt Et Etnbd-Et nbd-Et cod-EtPr Et Bu cyclopentadienylsEt cod-nbd-nbd-EtBuPr nbd-Et Bu Bu cod-Bucod-Bu BuBu Bucod-Bu cod-cod-BuBu β-diketonates β-diketonates ANIONIC LIGANDS cyclopentadienyls i cyclopentadienylsi i i MeMe MeMe β -diketonates hd-Me2 hd-Me2 Et ANIONIC LIGANDSnbd-Et Pr Bunbd- Prcod-Bu cyclopentadienyls Bu cod- Bu 1 2 Me MeMe Me hd-Me2 hd-Me2 i1 i 2 i 3 i 4 5 StructureStructure R1 R2 Abbreviation1 2 StructureAbbreviation1 2 R1 Structure R2 iPrR3 Pr 1 R4 nbd-nbd-RiPr5 Pr2Abbreviation i1 i 3 2i 4 iBuBui 5cod-3cod-iBuBu 4 i 5 i i i Abbreviation Meβ-diketonates StructureRMe Structure hd-Me2R β -diketonates R R RANIONIC Abbreviation LIGANDSR AbbreviationStructurecyclopentadienyls ANIONIC StructureR LIGANDS R cyclopentadienyls R RPrR Pr R nbd-R Pr R nbd- RPr RR AbbreviationR R BuR BuAbbreviationcod- Bucod- RBu i i i i 1 Me 2 acac 1 2 Pr H 1 nbd- PrH 2 H 1 3 H 4Bu H 2 cod-5 BuCp 3 4 5 Structure β-diketonatesR Structure R MeAbbreviationR Me Rβ -diketonatesStructureMeAbbreviation acacacac ANIONIC R acac Structure LIGANDS cyclopentadienylsR H RHR R RR H H Abbreviation HcyclopentadienylsR H HR H R H HAbbreviation H Cp H HH Cp H Cp t ANIONICANIONIC LIGANDS LIGANDS ANIONICANIONIC LIGANDS LIGANDS Me 1 BuMe 2 t thdacac t MeH 1 H H 2 H H H3 H H H4 Cp Cp5 Me Me Structure R R BuStructureAbbreviationβ-diketonates Bu Me ANIONICR1 LIGANDSStructuretButhdthd acac R 2 AbbreviationthdR RStructure MeH Me cyclopentadienylsR HR H MeR 1 H R HAbbreviation H RHH2 H H H HR H3 Cp Cp RH 4 HRH5 AbbreviationCpMe H Cp t t β-diketonatesβ-diketonates β-diketonatesβ-diketonates cyclopentadienylscyclopentadienyls Et Mecyclopentadienylscyclopentadienyls Bu/CF3 C(OMe)MeBu 2 t zis/zifthd 1 t 2t Et Me H H H 1H H H H2 H CpCp 3 4 5 Me Et Et t MeStructure Bu/CF3R1acac 1 C(OMe)Me Bu 2R 2 Bu/CF2 3 Abbreviation zis/zifC(OMe)Methd 2 zis/zifStructureH H MeR1 Et 1 H R2H 2H Et H R3H 3 H Cp HR4H 4 H R5H 5 HAbbreviation H Cp Cp H H Cp Et OOβ-diketonatesStructure Structure R R R R AbbreviationMeAbbreviation1 1 2 acac2Structurecyclopentadienyls Structure R R R R H 1 R R1 RH R2 R2 R AbbreviationAbbreviationH3 3 H4 4 5H 5 Cp tt Bu/CF3 C(OMe)MeStructure Structure2 zis/zifR RR RAbbreviationAbbreviation StructureEt Structure R H Et R R R HR R R HRR AbbreviationR Abbreviation H Cp BuBu/CFOO 3 C(OMe)MeCF 3 2 OOtptac zis/zift 3 2 allyl Et H H H H H H H H Cp-allyl Cp Et Structure R1 tBuR 2 AbbreviationBuBu/CF thd MeC(OMe)MeCF 3 StructuretBu tBu ptac zis/zifacacCF 3 R1 thdptac Me R2 HR 3 allylH Et R4 H R5 HAbbreviation H allylMe H H H CpMeH H H H H HH H CpCpCp-allyl HH HH Cp-allylCpMe OO t MeMe acacacacMe Men acac acac H H H H H HH H HH H HH H H CpCp H H HH H Cp Cp Met OOCF3 3 tfac t CF ptac Pr H allylH H H Cp-propenylHHHH Cp-allyl t Bu BuCF ptacBu tBu3 CFt3 ptacthd allyl H allyl Men H H HH n Cp-allylH Et H H Cp-allylCpMe Et Bu/CFMe3 C(OMe)Me2 acacMe zis/ziftBu tBu CF 3 Bu/CFMe 3 C(OMe)MetfacthdthdtCF 3 2 tBuH zis/ziftfac Et H thd H H Me Me Pr H H H H H H EtPrCp H H Me H H CpH HH H H HH H H H HCp CpCp-propenylMe MeH HH H HH HCp-propenyl CpMe Cp Me 3 Bu i n thd n Mei H H H H Cp Me tCF CF3 hfact tfac 3 Pr Pr H H Hn H H H H H Cp-Cp-propenylMepropenyl Et OO t Bu MeOOthdt Bu/CF t Me 3 3CFC(OMe)Me3 CF t 2 tfac zis/ziftfac t Me H H Pr EtPri H H H iCpHHHH H H H Cp-propenyl CpEt Et i i Et Cp-propenyl Bu CF3 Bu/CFBu/CFptacCF3 3 C(OMe)MeC(OMe)MeBu2 2 tBu/CFCF3hfac zis/zif 3 zis/zifCF C(OMe)Me3Bu/CF 3 2C(OMe)Me n ptachfac allyl zis/zif 2 zis/zif H Et EtPr H H HH allyl Pr Et HBu H Et H HCp-allyl H HH H H H H H H H HH H Cp Cp- Cp Hpropenyl HHH H H H H HCp- CpCp-allylpropenylEt Cp 1 2 CF3 3 Ph btfac Bui H Hi H H Cpi i t CF hfact CF3 OO3 hfac Pr H i PrH H HH Cp-Et propenylH H H Cp- propenyl i R R Bu/CF3 OOOOC(OMe)Me2 CF zis/zifOOt But CF ptac t Et 3 n H H allyln H H H nn CpHHHH H H H Cp-allyl Bu Bu 1 Me CF13 2 3 CF3Bu tfacBu Ph2CF CF3 3 MeCF3 t hfac btfacptacptacCFPh 3 Bu 3 CF btfactfac Pr ptac H allylPr allylBu H H HH PrBu Me3H H allyl HCp-propenyl H HHH H H H H H HCp-allylH H Cp-allyl H Cp HH H HH HCp-propenyl CpCp-allyl Cp- propenyl R 3 Ph R dbac Bu CF Men ptac H Men Me H allylCp Bu H H Bu H H Cp-allyl 1 OO2 t CF1 PhR 2 btfacCF3 R Ph3 btfac Bu H nBuH H HH Cp H H H Cp R R Bu CF3 ptac Me CF tfac allyl 3 i H Hn n Prn H H H Cp-allyl nH i H H Cp-propenylMe3 Me3 Bu R CF3 R MeMehfac Ph CFCF3 3 3dbactfactfac Me 3 CF Pr tfac H Pr Me Pr H H HH i n Me4H HPr HCp- Me propenylH H Me HH H Cp-propenylHCp-propenyl HCp H H i Cp-propenyl Me CFPh3 bac PhCFMePh btfac CF Me hfacdbac tfac Me MeBu Me H MePrPr HHHH Cp Me3 HHH MeH Me3 HMeH HH H Cp-propenylCp- propenylCp Cp Me Ph CF3 tfacdbac CFPh3 dbachfac nPr Me H H H MeiPrMe H MeH HCp-propenyl H Cp MeiH MeH H Cp-iCppropenyl i 3 CF3 n hfac i i n i Pr Bu H i i H Me4 H H i Cp-Me4Bupropenyl 1 2 CF3 Ph1 Me btfacCF CF3 Ph 2 CFMe3 bachfachfacCF PhPh 3 btfac bacBu hfac H Pr Me Pr H H H MeH MeBu Pr H HMe4 H Me CpH MeHH H Me H H Cp-HCp- MeH propenyl propenylCp HMeH HH H Cp- propenylCp Me3 R R CFMe3 C4HPh4RS Phttfac bac R dbac i Me Me Me Me MeMen Me MeMe MeH i HCp*Cp MeMe4Bu Me H CF3 hfac CFMe3 1 Ph btfac2 bac CF3 Pr Ph H btfac H n MeBun H H MeHCp- propenyl n MeHBu MeH HH CpBu BuH H H CpBu Cp 1 1 2 12 CF3CF CF3 3 R C4HPh4SPh 2 CF3ttfac btfacR btfac Ph btfac Bu MeBu H H Me nBu H HMe Me3HH H Me H H MeH Cp Cp Cp*H H CpBu Me3 β-iminoketonatesR R 3 Ph 4 4 R R dbac CF3 Ph C 4βH-iminoalcoholates4S n dbacttfac Me H Meω-Alkenyls Me Me Me Bu H Cp MeH MeMe MeMe MeH CpCp* 1 2 CFCF3 PhC H S R btfac ttfac CF 3 Ph C4H4SR dbacttfac Bu Me H Me H MeMe H Me H MeHMe Cp Cp*Me Me MeH CpCp*Me3 Me3 Me4 R R Me Ph bac Ph dbac Me Me Me H MeMe3Me3Me MeMe H Me3 Cp H 1 2 3 PhPh dbacdbacPh dbac Me Me H H Me MeMe Me4MeHMe H H MeCp Cp Me H Cp Me4 Cp Structure Rβ-iminoketonates MeR R Ph βAbbreviation-iminoketonates bac β-iminoketonatesMe Structure Ph β-iminoalcoholates R bac Me Abbreviationβ-iminoalcoholates Me Structure Meβ-iminoalcoholates ω-Alkenyls Me MeAbbreviation Me3 H Cp Me ω-Alkenyls Me Me4 Meω-Alkenyls H Cp Me4 Ph dbacβ-iminoketonates Me Ph bac Me Me Ph H bacβ-iminoalcoholates Me Me Me H Me Cp Me Meω-Alkenyls MeH CpMe4Me4 Me Me H Cp CF 1 MeMe 2 C H S3PhPh Mettfac bac bac Ph bac Me Me Me Me Me Me Me Me Me MeMe H MeH MeCp Cp Me MeH CpMe4 Me Cp* 1 CFStructure3 2 C343H4S R Rttfac 4 4R 1CF3 Abbreviation2 C4H34S ttfac Structure Me Me R Me Abbreviation Me Me Me4 Me Cp*Structure Me Me AbbreviationMe Me Cp* Structure MeR R StructurePhEt R StructureAbbreviationEt baci-hfacR 1CF 3 R2 C4RH3R4 S StructureR Abbreviation ttfacR CF 3 MeAbbreviation C R4H4S Structure MeAbbreviation ttfac Structure Me Me R MeStructure AbbreviationH - Me Cp R Abbreviation Me Abbreviation StructureMe MeMe AbbreviationCp*StructureMe Me MeAbbreviation Cp* CF3 CFCF3 3 C4HC44HS 4S CF3 ttfacttfac C4H4S Me ttfac Mei-hfda Me Me Me Me MeС 5Н9Me Me Me MeMe MeMe MeCp* Cp* Me Me Cp* β-iminoketonatesn n Et Eti-hfac β-iminoalcoholates ω-Alkenyls βCF-iminoketonates3 C4HPr4SEt PrttfacβEti-iminoketonates-hfaci -hfac β Et-iminoketonates Eti-hfac Etβ-iminoketonates β-iminoalcoholates Me Et i-hfac Me β-iminoalcoholatesMe Meβ -iminoalcoholatesMe - βω-iminoalcoholates-AlkenylsCp* ω-Alkenyls- - ω-Alkenylsω-Alkenyls- CF31 2 βCF-iminoketonatesβ-iminoketonates3 3 3β-iminoketonates Me Meβ-iminoalcoholatesi-βhfda-iminoalcoholates Me β-iminoalcoholatesMe i-hfda С 5Н9 ω-Alkenylsω-Alkenyls С5Нω9-Alkenyls 5 9 1 2 n 3 n CF3 n 1 CF 2 n 3 Me Mei-hfdaMe Mei-hfda С5Н9 С Н StructureStructure β-iminoketonatesCF3R RMe R Structure Me PrR R StructureMeAbbreviationPri-tfacRi1-hfac R2R Pr nRStructure3R AbbreviationR StructureAbbreviationPrni-hfacβ-iminoalcoholatesRnPrR 1 R2 AbbreviationnPr StructureRi3-hfac Structure R Abbreviation Abbreviation StructureR RStructure ω Structure-Alkenyls AbbreviationStructure Abbreviation n RAbbreviation RAbbreviation AbbreviationStructureAbbreviation Abbreviation StructureStructure Structure AbbreviationAbbreviation Abbreviation Structure 1R1 2R2 3PrR3 1 AbbreviationPr2i-hfac 3 Structure R Abbreviation Structure Abbreviation 1 2 Structure3 R StructureR R RAbbreviation R R Et Abbreviation Structure Et i-hfda Structure R Abbreviation R StructureAbbreviation AbbreviationStructure Abbreviation Structure R 3 R R2 Et3 Abbreviation- Eti-hfac Structure Et R EtEti-hfac AbbreviationEti-hfac Structure Abbreviation 6 11 CFCF 3 HMe CH CF Me CFTFB3 MeCFTFEAiMe3-tfac Me Et Me MeEtCF 3 MeEt Mei-hfacMeEti-tfaci-hfaci Me-tfac Mei-tfac Structure 0 n AbbreviationС-Н Structure Structure - n nAbbreviationStructureAbbreviation - n - Abbreviation 3 Et Et EtiEt-hfaci-hfac CF Et3 Eti-hfac Me 5 9 Me- i--hfda С5Н9 CF CF3 CF3 n CF3 n Etn Me nEti-hfdaMe i-hfdaMeMe EtEt MeEtEti-i-hfdahfdai-MehfdaMe i -hfda MeС Нi-hfda С5Н9 - С5Н9 C5H9 3 EtH 2n 3 Etii-hfac-acacn- CF 3 3 n n- -n n Pr Pri-hfac Me - Me1i- hfdaEt С7Н613 11 Eti-hfda 5С59Н9 CF H CH CFPr CF3TFB CFCFPrTFEA3H i -hfacH CHPr 2 CHCFPrn23CF 3 CFTFBPr3TFBi Prn-hfacTFEAi-hfacTFEAPr2 3 Pri--hfac Me Mei-hfda0 Me С Н Mei-hfda 0 0 ССН6Н 11С 6Н11 С5Н69 11 CFMe3 nPr PrCF H nPriPr CH-hfaci-hfacCF nPr Men PrTFB TFEAnPri-hfacMe ni-Prhfdai-hfda С5Н9 0 С Н nPr nPri-hfac MeH Mei-acaci-acac CF 3 Me Me Mei-tfac 2 1 С8НС715Н 13 7 13 Structure7 13 n Abbreviation CFCF3 3Me Me MeCFMe3 i-tfacMe Me H MeHCF 3 MeMe i-acacMei-tfaci-acaci Me-tfacH Men i-acac i-tfac n Structure n AbbreviationStructure n1 Abbreviation1 StructureStructureС Н С Н n1 AbbreviationС7Н13n Abbreviation Me CFCF3 3 MeMe Me Me 3 MeMei-tfaci-tfac Pr Pri-hfda nn nnn EtStructure Structuren Et i-nhfdan Abbreviation Abbreviation 3 Me Me Me CF -Me 3 Me 2 Me3Et i-tfac - Et i-hfdaEt EtPrPr Et Pri-Pri-hfdahfdai-EthfdaEt Pri- hfda EtPri-i-hfdahfda Structure n Abbreviation6 11 α-aminoalcoholates CF Me MeMe Me iMe-tfaci-acac3 -β-heteroarylketonate 2 3 - CF H CH -βCF-alkenols TFB TFEA Structuredithio/diselenoimidodiphosphinatoEt Et Etn i-Ethfdai-Abbreviationhfda2 Et С8Н 15 Eti-hfda 68 1115 0 С Н CFCF3 H CH2CF3H CFTFB CHTFEAH CFMe CH MeCFCF 3 TFBH MeTFBMeiTFEA CH-acac- iTFEA-acac-Me2CF 3 Et TFBMeiTFEA-acac- Eti- hfda 0 С6Н11 02 2 С Н С 8Н15 02 СС68НН1115 0 C H 3 CF- CF3 3 H H 2 CH CH32CF2CF3 3 CF3 TFBTFBHTFEA TFEA CH2CF3 TFB TFEA 0 0 С6НС611Н 11 0 С6Н11 6 11 CF3 H CH2CF3 TFB TFEA H i-acac 0 С6Н11 1 С7Н13 Structure α-aminoalcoholatesR Abbreviation Structure β-heteroarylketonateH Abbreviation i -acac Structure β-alkenolsR Abbreviation dithio/diselenoimidodiphosphinatoStructure Hal Abbreviationn n1 С7Н13 α-aminoalcoholatesHα -aminoalcoholates i-acac H i-acacHβ -heteroarylketonateβMe-heteroarylketonate i-acac n ββ-alkenols-alkenolsn 1 dithio/diselenoimidodiphosphinatoPrdithio/diselenoimidodiphosphinatoС7Н13 Pri-hfda 7 713 13 1 С7Н 13 1 C H Meα-aminoalcoholates H H i-acaci-acac H n i-acacβ -heteroarylketonate nn Pr Pri-hfdan n β-alkenolsn 1 1 dithio/diselenoimidodiphosphinatoС НС Н 1 С7Н13 7 13 Me H 2 i-acac Me Me Pr Mei-acac Pr i-hfdan n n1 n СPr 7n Н13 Prni-hfda 2 С8Н15 Structure MeR amN(Me)AbbreviationMe O StructureMe NMe AbbreviationMe n Structure MenR Abbreviationalk(Me) PrPrPrStructure Pri-PrhfdaSi- HalhfdaPrPri-S/S-idppAbbreviationhfda Pri-hfda 8 15 Me StructureMe RMei-acacAbbreviation Me iMeStructurei-acac Pr AbbreviationPri-hfda Structure R Abbreviation 2 Structure С8Н15 Hal2 Abbreviation С Н 8 15 Structure StructureR Abbreviation Me MeMe R MeAbbreviationStructureMe-acacMei-acacMei-acac Me MeStructure Abbreviationi-acacMei -acac Abbreviation Structure R Abbreviation Structure R StructureAbbreviation 2 2 HalСStructure8НСAbbreviation815Н15 2 Hal 2 AbbreviationС8СН15Н 2 C8H15 Me amN(Me)Me 2 Mei-acac α-aminoalcoholates Me alk(Me)β-heteroarylketonate 2 S С8Н15S/S-idpp β-alkenols dithio/diselenoimidodiphosphinato α-aminoalcoholatesO Me N amN(Me)2 ThTFP O Nβ-heteroarylketonate β-alkenolsMe alk(Me) dithio/diselenoimidodiphosphinatoS S/S-idpp α-aminoalcoholatesα-aminoalcoholates α-aminoalcoholatesα-aminoalcoholatesMe α -aminoalcoholatesamN(Me) β-heteroarylketonateα-aminoalcoholates2 Me OamN(Me) β -heteroarylketonateN β2-heteroarylketonate β-heteroarylketonate β-heteroarylketonate β-alkenolsβ-heteroarylketonate β-alkenolsβ-alkenolsdithio/diselenoimidodiphosphinatoβ-alkenolsMe alk(Me) β-alkenolsβdithio/diselenoimidodiphosphinato-alkenolsdithio/diselenoimidodiphosphinatoMe alk(Me) dithio/diselenoimidodiphosphinatoS dithio/diselenoimidodiphosphinatoS/S-idpp S S/S-idpp α-aminoalcoholatesCF3 amakN(Me)2 β-heteroarylketonateS Structure R AbbreviationO β-alkenols N CF 3 Structurealk(CF3) dithio/diselenoimidodiphosphinato Abbreviation Se Se/Se-idpp Structure R Abbreviation Structure Hal Abbreviation Structure R Abbreviation Structure Abbreviation Structure R Abbreviation Structure Hal Abbreviation Structure R AbbreviationStructureStructure F 3C StructureRStructureR AbbreviationAbbreviation R ThTFP AbbreviationAbbreviation StructureStructure Structure Abbreviation Structure AbbreviationThTFP R Abbreviation Abbreviation Structure Structure RStructureRAbbreviationAbbreviation Structure Hal StructureAbbreviationRStructure Abbreviation Hal Hal AbbreviationAbbreviationStructure Hal Abbreviation StructureStructure RCF3 RAbbreviationamakN(Me) 2 AbbreviationStructureStructure AbbreviationR Structure AbbreviationMe Structure amN(Me) Abbreviation 2Structure R AbbreviationCF 3 alk(CF 3)Structure AbbreviationStructure Hal R AbbreviationSe Structure Se/Se-idppAbbreviation R AbbreviationMe alk(Me) StructureStructure Hal AbbreviationS S/S-idpp Hal Abbreviation CFMe3 amakN(Me)SamN(Me)2 2 ThTFP O N ThTFP MeCF3 alk(Me)alk(CF3) SeS Se/Se-idppS/S-idpp F C 3 2 2 O N S 3 3 a Me amN(Me)t 2 2 3 CF Me Me amakN(Me)amN(Me)amN(Me)Me2CF 3 FMeCamakN(Me)amN(Me) amN(Me)2N 2 S 2 n Me alk(Me)i MeMeCF alk(Me)alk(Me)alk(CF n ) S MeCFMeS/S-idpp3 alk(Me)alk(CFalk(Me) 3) S S SeS/S-idpp S/S-idppSe/Se-idpp S SeS S/S-idppSe/Se-idppS/S-idpp group symbols: Me = CHMe3 (methyl),MeamN(Me) Bu = C(CH3)amN(Me)3O (tert-butyl),O N 2 NPh = C6H5 (phenyl),3 O OEt = CN2H5 (ethyl), OPr =O CHMeN 2C 2HNSalk(Me)5 (n-propyl), Pr = CH(CH3)2 (iso-propyl),SMe S/S-idpp Bu alk(Me) = S S/S-idpp F3C F C ThTFP CF3 3amakN(Me)ThTFP2 CF3 alk(CF 3) Se Se/Se-idpp a i a t t n ThTFP S i n n i n CH group2CH2C 2symbols:H5 (n-butyl), Me =Bu CH = 3(C (methyl),2H group5)CH(CH symbols:Bu 3=) (iso-butyl).C(CH Me3) 3= (tert-butyl), CHCF33 (methyl),amakN(Me) Ph =Bu C 6=H C(CH2ThTFP5 (phenyl),3 )3ThTFP (tert-butyl), Et = C2H5 Ph(ethyl), = C6H Pr5 (phenyl), ThTFP= ThTFPCHF 2C C2 HEt5 =(n-propyl), C2H5 (ethyl), PrThTFP =Pr CH(CH = CH2CFC3)23 H (iso-propyl),5 (n-propyl),alk(CF3) BuPr = CH(CH3)2 (iso-propyl),Se Se/Se-idpp Bu = a CFCF3 3 amakN(Me)amakN(Me)t 2 2 S 3 ThTFPn CFCF3 3 alk(CFalk(CF3) 3) i Se Se Se/Se-idppSe/Se-idppn CFi3CF group3 amakN(Me) symbols:amakN(Me)2 Me2a = CH3 (methyl), Bu = CFC(CH33 3F)CF3C amakN(Me)(tert-butyl),3 tamakN(Me)2 SPhS =2 3C36H5 (phenyl),CF3 alk(CF Et S=CF C3)6 32 H55 alk(CF(ethyl),3) Pr = CH2 2C5Se2H 5 (n-propyl),Se/Se-idppn Se Pr2CF CF=Se/Se-idpp23 CH(CH3 5 alk(CFalk(CF3 )323) ) (iso-propyl), i Bu 3=2 SeSe Se/Se-idppSe/Se-idppn CH2CH2C2H5 (n-butyl), Bu = (C2H5)CH(CH3) (iso-butyl).group symbols:i S MeS = CH (methyl),F CF3 C Bu = C(CH ) (tert-butyl),S Ph = C H (phenyl), Et = C H (ethyl), Pr = CH C H (n-propyl), Pr = CH(CH ) (iso-propyl), Bu = CH2CH2C2H5 (n-butyl),F3C F C Bu = (C2H5)CH(CHa 3) (iso-butyl).3 3 F C F3tC n i n CF3 amakN(Me) i3 2 groupi symbols: Me = CH3 (methyl),3 Bu = C(CH3)3 (tert-butyl), Ph = C6H5 (phenyl), CF Et =3 C2H5 (ethyl), alk(CF Pr =3 CH) 2C2H5 (n-propyl), Pr = CH(CH3 )2 (iso-propyl), BuSe = Se/Se-idpp CHa 2CH2C2H5 (n-butyl),CH2 CHBu 2=C (C2H25H (n-butyl),5)CH(CH t 3 Bu) (iso-butyl). = (C2H5)CH(CH3) (iso-butyl). n i n groupa a symbols: Me = CHa3 (methyl), But t = C(CH3)3 (tert-butyl),i t Ph = C6H5 (phenyl), Et = C2H5 (ethyl),n n Pr = CH2C2H5 (n-propyl),n i i Pr = CH(CH3)2 (iso-propyl),i n n Bu = n aa group symbols: Me = CH3 (methyl), group group tBu symbols: symbols:=t C(CHa Me3) 3Me (tert-butyl), = CH= CHgroup3 (methyl),3 (methyl), Phsymbols: =CH CBu6 H2BuCH 5 =Me (phenyl), C(CH=2C C(CH=2H CH53 (n-butyl),)33 (tert-butyl),)Et(methyl),3 (tert-butyl), = tC2H Bu5 (ethyl), Bu= Ph(C =Ph 2 =HC(CH C5n=)CH(CHPr 6CH 6=5H3 )(phenyl),CH35 (tert-butyl),(phenyl),32)Cn (iso-butyl).2H5 (n-propyl),Et Et= PhC= 2CH =25 H C(ethyl), 56iPr H(ethyl),5 =(phenyl), CH(CH Pr Pr = CHi3= )Et 2CH (iso-propyl), 2=C 22CCH22H5H (n-propyl),55 (ethyl),(n-propyl), nBun =Pr Pr =Pr =CH CH(CH= 2CH(CHC2nH5 3(n-propyl),)23 (iso-propyl),)2 (iso-propyl), iPr = BuCH(CH Bu = = 3)2 (iso-propyl), Bun = groupa symbols: Me = CH3 (methyl), Bu = C(CH groupi t 3) 3symbols: (tert-butyl), Me Ph= CH = C3 (methyl),6H5 (phenyl), Bu =Et C(CH = C2H3)53 (ethyl),(tert-butyl), Pr = Ph CH =2 C26H55 (n-propyl),(phenyl), Etn Pr = C= 2CH(CHH5 (ethyl),3)2 (iso-propyl),Pr = CH2C2 HBu5 (n-propyl),i = Pr = CH(CH3)2 (iso-propyl),n Bu = group symbols:i MeCH2 =CH CH2C2H3 5(methyl), (n-butyl), BuBu i= (Ci =2H C(CH5)CH(CH3)33)(tert-butyl), (iso-butyl).i Ph = C6H5 (phenyl), Et = C2H5 (ethyl), Pr = CH2C2H5 (n-propyl), Pr = CH(CH3)2 (iso-propyl), Bu = CH2CH2C2H5 (n-butyl), CH2CH2C2H5 (n-butyl), Bui = (CCH2HCH52)CH(CHCH2CH2C22CH235H) (n-butyl),(iso-butyl).5 (n-butyl), BuCH Bu =2 CH (C= (C22HC25H2)CH(CHH5)CH(CH5 (n-butyl),i 3) (iso-butyl).3) (iso-butyl). Bu = (C2H5)CH(CH3) (iso-butyl). CHiBu2CH =2 (CC2HH5 (n-butyl),)CH(CH Bu ) (iso-butyl).= (C2H5)CH(CHCH3) (iso-butyl).2CH2C2H5 (n-butyl), Bu = (C2H5)CH(CH3) (iso-butyl). 2 5 3

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2.1. Volatile Iridium Precursors 2.1. Volatile Iridium Precursors As a rule, metal-organic and coordination compounds of iridium in the oxidation As a rule, metal-organic and coordination compounds of iridium in the oxidation state state +3 and +1 are used in the deposition of iridium coatings. +3 and +1 are used in the deposition of iridium coatings.

2.1.1.2.1.1. Iridium(III) Iridium(III) Precursors Precursors Iridium(III)Iridium(III) compounds compounds thatthat cancan bebe used as precursors precursors in in MOCVD MOCVD processes processes are are re- restrictedstricted to to the classes ofof ββ-diketonates-diketonates andand allylsallyls [ 14[14,18].,18]. At At the the same same time, time,β -diketonateβ-diketonate chelateschelates are are synthetically synthetically more more accessible accessible than than tris-allyl tris-allyl iridium; iridium; however, however, their their synthesis synthesis withwith high high yields yields still still present present a significanta significant challenge. challenge. TheThe traditional traditional approach approach toto thethe synthesis of of iridium(III) iridium(III) ββ-diketonates-diketonates consists consists in inthe theinteraction interaction of iridium(III) of iridium(III) chloride chloride with with the corresponding the corresponding β-diketoneβ-diketone or its or salt its in salt water- in water-alcoholalcohol solutions solutions with with controlled controlled pH pH[19,20]. [19,20 However,]. However, the theyields yields of the of the products products do do not notexceed exceed 15%–20%, 15%–20%, since since the the reaction reaction is is comp complicatedlicated by thethe formationformationof of monomeric monomeric and and dimericdimeric complexes complexes of of iridium(III) iridium(III) with withγ C-boundγC-boundβ -diketonesβ-diketones [19 [19],], as as well well as as heteroligand heteroligand complexescomplexes of of polymer polymer structure structure formed formed as as a a result result of of competition competition between between acetylacetonate acetylacetonate andand acido-ligands acido-ligands [21 [21].]. Isakova Isakova et et al. al. [21 [21]] developed developed a a universal universal method method of of the the preparation preparation ofof Ir(L) Ir(L)3 3β β-diketonates-diketonates withwith high yi yieldselds by by the the reaction reaction of of ββ-diketone-diketone with with iridium(III) iridium(III) aq- aquafluorocomplexesuafluorocomplexes formed formed when when a aK K3[IrF3[IrF6] 6solution] solution was was heated heated in hydrofluoric in hydrofluoric acid. acid. The Themethod method was was based based on onthe the lability lability of offluoro- fluoro- and and aqua-ligands, aqua-ligands, which which made made it it possible possible to tosubstitute substitute them them with with ββ-diketonate-diketonate anions anions with with high yieldsyields (up(up toto 90%)90%) and and for for a a wide wide rangerange of ofβ β-diketonates-diketonates (L(L = acac, acac, thd, thd, tfac, tfac, hfac, hfac, ptac). ptac). All All iridium(III) iridium(III) β-diketonatesβ-diketonates were wereshown shown to be to mononuclear be mononuclear mole molecularcular complexes complexes formed formed by chel by chelatingating coordination coordination of all ofthree all three ligands. ligands. In the In case the of case an ofasymmetric an asymmetric tfac-ligand, tfac-ligand, the formation the formation of cis/trans of cis/trans isomers isomersdiffering differing in thermal in thermal properties properties was confirme was confirmed;d; however, however, the crystal the crystal structure structure was deter- was determinedmined only only for fortranstrans-Ir(tfac)-Ir(tfac)3 [22].3 [22 The]. The structure structure of of only only one one complex complex ofof thisthis class, class, viz., viz., Ir(acac)Ir(acac)3,3, was was also also determined determined by by the the single-crystal single-crystal XRD XRD method method [23 [23].]. Crystallographic Crystallographic datadata confirmed confirmed the the bidentatebidentate coordinationcoordination of ββ-diketonate-diketonate ligands ligands and and the the octahedral octahedral co- coordinationordination environment environment of of the the metal metal in in the the molecule molecule (Figure (Figure 22).).

Figure 2. Molecular structure of trans-Ir(tfac)3. Figure 2. Molecular structure of trans-Ir(tfac)3.

ThermalThermal properties properties of of a a series series of of iridium(III) iridium(III)β -diketonatesβ-diketonates were were described described in in [ 21[21].]. ◦ WithinWithin the the investigated investigated series, series, Ir(acac) Ir(acac)3 has3 has the the highest highest melting melting point point (269–270 (269–270 C).°C). A A de- de- t t creasecrease in in the the melting melting point point is is observed observed when when both both CF CF3 and3 andBu Bu substituents substituents are are introduced introduced ◦ intointo the theβ -diketonateβ-diketonate ligand, ligand, so so that that the the most most fusible fusible complex complex is is Ir(ptac) Ir(ptac)3 (83–843 (83–84C). °C). The The vaporvapor pressure pressure increases increases with with the the introduction introduction of of these these terminal terminal groups, groups, and and the the inﬂuence influence ofof the the CF CF3 group3 group is is more more pronounced pronounced (Figure (Figure3). 3). The Th generale general range range of of variation variation of of vapor vapor pressure values from the least volatile Ir(acac)3 complex to the most volatile Ir(hfac)3 ex-

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pressure values from the least volatile Ir(acac)3 complex to the most volatile Ir(hfac)3 ex- ceedsceeds two two orders orders of of magnitude. magnitude. The The equations equations of of temperature temperature dependences dependences of of saturated saturated vaporvapor pressure pressure and and thermodynamic thermodynamic parameters parameters of sublimation of sublimation and vaporizationand vaporization processes pro- are given in Table2. It is interesting to note that the cis-isomer of Ir(tfac) is characterized cesses are given in Table 2. It is interesting to note that the cis-isomer of3 Ir(tfac)3 is charac- byterized the lower by the values lower of values enthalpy of enthalpy and entropy and entropy of sublimation of sublimation and the and lower the melting lower melting point trans- ◦ ◦ comparedpoint compared to the to theisomer trans-isomer (153–155 (153–155C vs. 186–187°C vs. 186–187C). °C).

Figure 3. Temperature dependences of the saturated vapor pressure over the solid (sublimation Figure 3. Temperature dependences of the saturated vapor pressure over the solid (sublimation process) and liquid (vaporization process) Ir(III) β-dikenonates. process) and liquid (vaporization process) Ir(III) β-dikenonates. Table 2. Equations of the temperature dependences of saturated vapor pressure over the solid (sublimation, subl.) and liquid (vaporization, vap.)Table Ir(III) 2.β-diketonates.Equations of The the sublimation temperature and dependences vaporization of enthalpies saturated ( vaporΔHT*) and pressure entropies over (Δ theS0T* solid) are referred to the average(sublimation, temperature subl.)(T*) of and the liquidinterval (vaporization, measured (ΔT vap.)), p0 = Ir(III) 760 Torrβ-diketonates. = 1 atm = 10 The5 Pa. sublimation The ligand and 0 abbreviations correspond tovaporization Table 1. enthalpies (∆HT*) and entropies (∆S T*) are referred to the average temperature (T*) of 5 the interval measured (∆T), p0 = 760 Torr = 1 atm = 10 Pa. The ligand abbreviations correspond to ln(p/p0) = A − B/(T/K) ΔHT*, ΔS0T*, Ref. Complex Table Process1. ΔT, K A B kJ·mol−1 J·(mol·K)−1 ln(p/p0) = A − B/(T/K) 0 3 ∆HT*, ∆S T*, [24] Ir(acac) Subl.Ref. Complex 468–518 Process 15.65∆ T,K 10,397 86 ± 2 −1 130 ± 4 − AB kJ·mol J·(mol·K) 1 Subl. 423–453 26.30 14,013 116 ± 3 218 ± 17 trans-Ir(tfac)3 [24] Ir(acac) Subl. 468–518 15.65 10,397 86 ± 2 130 ± 4 Vap. 458–4883 16.29 9462 79 ± 3 136 ± 6 trans- Subl. 423–453 26.30 14,013 116 ± 3 218 ± 17 Subl. Ir(tfac) 414–4263 Vap. 20.62 458–488 16.29 11,389 9462 95 ± 8 79 ± 3 171 136 ± ±186 cis-Ir(tfac)3 Subl. 414–426 20.62 11,389 95 ± 8 171 ± 18 Vap. cis-Ir(tfac) 426–4413 15.15 9045 75 ± 3 ± 126 ±± 6 [21] [21] Vap. 426–441 15.15 9045 75 3 126 6 Subl. 358–396 23.29 10,688 88 ± 6 193 ± 9 Subl. Ir(hfac) 358–396 23.29 10,688 88 ± 6 193 ± 9 Ir(hfac)3 3 Vap. 401–443 13.79 6973 56 ± 1 115 ± 2 Vap. Ir(thd) 401–4433 Vap. 13.79 418–522 14.55 6973 8927 56 ± 1 74 ± 2 115 116 ±± 24 Ir(ptac) Vap. 413–518 13.88 8259 68 ± 1 116 ± 2 Ir(thd)3 Vap. 418–5223 14.55 8927 74 ± 2 116 ± 4 Ir(ptac)3 Vap. 413–518 13.88 8259 68 ± 1 116 ± 2 It was shown by the method of in situ mass spectrometry that the decomposition of Ir(acac)It 3wasvapors shown in vacuumby the method and in of the in presence situ mass of spectrometry hydrogen on that a heated the decomposition surface began of ◦ atIr(acac) 410 C3 vapors with the in vacuum release of and fragments in the presence of the acetylacetonateof hydrogen on liganda heated into surface the gaseous began at ◦ phase.410 °C In with the presencethe release of of oxygen, fragments the initialof the decompositionacetylacetonate temperature ligand into reducesthe gaseous to 220 phase.C, andIn the the presence ﬁnal decomposition of oxygen, the products initial aredeco watermposition vapor temperature and carbon dioxidereduces [to25 ].220 Recently, °C, and ◦ athe mechanism final decomposition of Ir(tfac)3 vapor products under are electron water impactvapor and at 160 carbonC has dioxide been proposed [25]. Recently, [20]. a mechanism of Ir(tfac)3 vapor under electron impact at 160 °C has been proposed [20]. 2.1.2. Iridium(I) Precursors 2.1.2.The Iridium(I) chemistry Precursors of iridium(I) precursors is more diverse, since these complexes are multi-ligand, which makes it possible to expand the series of compounds not only by The chemistry of iridium(I) precursors is more diverse, since these complexes are varying the structure of ligands of the same type, but also by combining various moieties multi-ligand, which makes it possible to expand the series of compounds not only by var- of the molecule. These complexes are much more readily available synthetically than ying the structure of ligands of the same type, but also by combining various moieties of their Ir(III) counterparts. Alkenes or carbonyls act as neutral ligands in volatile iridium(I) the molecule. These complexes are much more readily available synthetically than their

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complexes, while ligands containing such donor atoms as carbon, oxygen, nitrogen, and sulfur act as the anionic part. Below we will consider the main classes of compounds proposed for MOCVD applications and discuss the effective synthesis methods and present their structure, and summarize the information of their thermal properties. It is worth mentioning that the synthesis of Ir(I) compounds is carried out in an inert atmosphere, since their solutions are sensitive to air components. However, as solids, most of these complexes are stable for storage in the air for months. Although information about crystal structures is sketchy, the data obtained by other methods clearly show that all Ir(I) precursors are mononuclear molecular complexes, with the exception of the carboxylate and acetate derivatives. Complexes with (O)- and (OˆO)-coordinated ligands are represented by the following three classes:

• alcoholate [Ir(cod)(OMe)]2 (OMe = methylate ion) [26–28], • carboxylate [Ir(cod)(OAc)]2 (OAc = acetate ion) [29,30], • β-diketonates [Ir(cod)(L)], L = acac [31–33], tfac [31,33], hfac [31,33,34], thd [32–34], btfac, ptac [33], zis [35] and [Ir(CO)2(L)], L = acac [36,37], hfac [37,38], dbac, bac, ttfac [37], and [Ir(C2H4)2(acac)] [39]. In the synthesis of complexes with cyclooctadiene [Ir(cod)(L)], regardless of the type of L ligand, [Ir(cod)Cl]2 is usually used as the initial iridium compound. The alcoholate complex [Ir(cod)(OMe)]2 is obtained by the reaction with a stoichiometric amount of potassium hydroxide in methanol with a yield of 85% [27]. For the synthesis of the [Ir(cod)(OAc)]2 acetate complex, a twofold excess of alkali metal acetate was used, and the yield was more than 80% [22]. For the synthesis of β-diketonate complexes, a similar approach is used, but an excess of the β-diketone salt is not required. Yields exceed 85%. In the case of non-ﬂuorinated ligands (L = thd, acac), it is preferable to generate the β-diketone salt in situ. In the case of ﬂuorinated ligands L = hfac, tfac, ptac, btfac, it is convenient to use pre-synthesized β-diketonate MI(L), and the variation of the alkali metal cation MI does not affect the product yield [33]. It was shown using ligands L = acac, tfac, hfac as examples, that HL β-diketones themselves could substitute methylate ions in the [Ir(cod)(OMe)]2 complex with the formation of [Ir(cod)(L)]; however, the yield of the reaction was noticeably lower (about 70%) [31]. The complexes [Ir(cod)(L)], in their turn, are effective starting reagents for further preparation of the corresponding carbonyl derivatives [Ir(CO)2(L)] via a bubbling of carbon monoxide. Indeed, it was shown for L = acac [36] and hfac [38] that this approach led to fairly high yields (>75%). At the same time, the yield of the target products was only 14%–21% in the reverse order of stages of anionic and neutral ligand introduction, i.e., when the β-diketone HL interacted with Na2[Ir2(CO)4Cl4.6] in the presence of an excess of sodium carbonate [37]. The only described complex with an acyclic alkene, [Ir(C2H4)2(acac)], was obtained using the [Ir(coe)Cl]2 complex (coe = cyclooctaene) as an iridium source with the yield of 45% via ethylene bubbling followed by the reaction with in situ generated K(acac) [39]. Crystal structures have been determined only for a number of β-diketonate derivatives, some of which have been studied recently: [Ir(cod)(L)], L = acac [40], hfac [34], tfac, thd, btfac, ptac [33]; [Ir(CO)2(L)], L = acac [36], hfac [38]; [Ir(C2H4)2(acac)] [39]. In these molecules, the iridium atom is located in a typical distorted square environment: the coordination node IrO2C’2 (C’ is the center of the C=C bond) is realized for complexes with alkenes or IrO2C2 is for carbonyl derivatives (Figure4). In the case of the [Ir(cod)(OAc)] 2 acetate complex, the binuclear structure was proposed based on the measurement of molecular mass by the method of osmometry [30]. Coatings 2021, 11, x FOR PEER REVIEW 8 of 39 Coatings 2021, 11, 78 8 of 36

FigureFigure 4.4. MolecularMolecular structuresstructures ofof iridium(I)iridium(I) heteroligandheteroligandβ β-diketonate-diketonate complexescomplexes [Ir(cod)(L)][Ir(cod)(L)] (a(a),), [Ir(C [Ir(C22HH44))22(L)] ((b)) andand [Ir(CO)[Ir(CO)22(L)] (c) using L = acac as an example.

Complexes withwith (OˆN)-coordinated(O^N)-coordinated ligands ligandsare are represented represented by by the the following following four four classes: clas- •ses: β-heteroarylketonate [Ir(cod)(ThTFP)] [41]; • αβ-heteroarylketonate-aminoalcoholates [Ir(cod)(amakN(Me) [Ir(cod)(ThTFP)] [41];2)] [42] and [Ir(CO)2(amakN(Me)2)] [43]; • β-iminoalcoholates [Ir(cod)(L)], L = Mei-hfda and nPri-hfda [42], and [Ir(CO) (L)], L = • α-aminoalcoholates [Ir(cod)(amakN(Me)2)] [42] and [Ir(CO)2(amakN(Me)2)]2 [43]; nPri-hfda, Eti-hfda, [43]; • β-iminoalcoholates [Ir(cod)(L)], L = Mei-hfda and nPri-hfda [42], and [Ir(CO)2(L)], L = • β nPr-iminoketonatesi-hfda, Eti-hfda, [Ir(cod)(L)], [43]; L = Eti-hfac [42], Mei-tfac [44], Mei-acac, i-acac [45], n • andβ-iminoketonates [Ir(CO)2(L)], L [Ir(cod)(L)], = Eti-hfac, LPr =i-hfac Eti-hfac [43], [42], TFB-TFEA Mei-tfac [46 [44],]. Mei-acac, i-acac [45], Inand general, [Ir(CO) approaches2(L)], L = Eti-hfac, to the nPr synthesisi-hfac [43], of Ir(I)TFB-TFEA complexes [46]. with (OˆN)-coordinated β ligandsIn general, are similar approaches to those describedto the synthesis above of for Ir(I)-diketonate complexes derivatives. with (O^N)-coordinated In particular, ﬂuorinated compounds with cyclooctadiene [Ir(cod)(L)] (L = ThTFP [41], Eti-hfac, nPri-hfda, ligands are similar to those described above for β-diketonate derivatives. In particular, and amakN(Me) [42], Mei-tfac [44]) were obtained by the reaction of [Ir(cod)Cl] with a fluorinated compounds2 with cyclooctadiene [Ir(cod)(L)] (L = ThTFP [41], Eti-hfac,2 nPri- sodium salt of the corresponding ligand. The yields were 75%–95%. For the synthesis of hfda, and amakN(Me)2 [42], Mei-tfac [44]) were obtained by the reaction of [Ir(cod)Cl]2 ﬂuorine-free complexes (L = Mei-acac, i-acac), the corresponding salts were generated in with a sodium salt of the corresponding ligand. The yields were 75%–95%. For the syn- the reaction mixture in situ [45]. thesis of fluorine-free complexes (L = Mei-acac, i-acac), the corresponding salts were gen- Carbonyl complexes [Ir(CO) (L)] were synthesized by passing carbon monoxide gas erated in the reaction mixture in situ2 [45]. into a solution of the corresponding cyclooctadiene derivatives [Ir(cod)(L)] [43,46]. How- Carbonyl complexes [Ir(CO)2(L)] were synthesized by passing carbon monoxide gas ever, in contrast to the methods of preparation of β-diketonate complexes, the [Ir(cod)(L)] into a solution of the corresponding cyclooctadiene derivatives [Ir(cod)(L)] [43,46]. How- complexes themselves were not isolated, but generated in situ through the reaction of ever, in contrast to the methods of preparation of β-diketonate complexes, the [Ir(cod)(L)] stoichiometric quantities of [Ir(cod)Cl]2, NaH, and HL with the high (70–80%) yields of all targetcomplexes complexes, themselves with thewere exception not isolated, of L = butnPr igenerated-hfac (50%). in situ through the reaction of stoichiometricAlthough quantities most of the of presented [Ir(cod)Cl] compounds2, NaH, and decompose HL with the in high a polar (70–80%) solvent yields in air, of and all n carbonyltarget complexes, complexes with decompose the exception faster, of solutions L = Pri-hfac of two (50%). imine complexes with cyclooctadiene [Ir(cod)(L)]Although most with of L =thenPr presentedi-hfac and compoundsnPri-hfda are decompose stable [42 in]. a polar solvent in air, and carbonylCrystal complexes structures decompose were determined faster, solutions only for singleof two representatives imine complexes of complexeswith cycloocta- with n n differentdiene [Ir(cod)(L)] anionic ligands: with L = [Ir(cod)(L)], Pri-hfac and L =Pr Mei-hfdai-tfac are [44 ],stable L = Me[42].i-hfda [42] and ThTFP [41] Crystal structures were determined only for single representatives of complexes with and [Ir(CO)2(L)], L = TFB-TFEA [46] and amakN(Me)2 [43]. Similar to β-diketonate complexes,different anionic anionic ligands ligands: here [Ir(cod)(L)], also perform L = a Me chelatingi-tfac [44], function, L = Me implementingi-hfda [42] and the ThTFP coordina- [41] and [Ir(CO)2(L)], L = TFB-TFEA [46] and amakN(Me)2 [43]. Similar to β-diketonate com- tion nodes IrONC’2 (C’ is the center of C=C bond) or IrONC2 (Figure5). The absence of conjugationplexes, anionic in theligands chelate here cycle also of perform an anionic a chelating ligand (e.g., function, L = amakN(Me) implementing2 [43]) the leads coordi- to a noticeablenation nodes spatial IrONC’ distortion2 (C’ is the of the center molecule, of C=C i.e., bond) deviation or IrONC from2 (Figure planarity. 5). The absence of conjugationComplexes in the with chelate cyclopentadienyl cycle of an ligands anionicare ligand a fairly (e.g., large L = class amakN(Me) of compounds2 [43]) leads not only to a duenoticeable to the spatial possibility distortion of variation of the ofmolecule, substituents i.e., deviation in the aromatic from planarity. ring, but also due to a large number of neutral ligands that can complement the coordination environment of iridium, such as alkenes, alkynes, dienes, carbonyls, phosphines, amines, imines, and so on. Among them, only compounds with a limited number of neutral ligands have been proposed for MOCVD applications: • complexes with cyclooctadiene [Ir(cod)CpX], CpX = Cp [47], CpMe [47,48], CpMe3 [47], CpMe4 [49], Cp* [50], CpEt [50], Cp-allyl [51], Cp-propenyl [51], Cp-ipropenyl [52]); • complexes with cyclohexadiene [Ir(chd)CpX] (CpX = Cp [53], CpMe [54], CpEt [55]);

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X X Et • complexes with ethylene [Ir(C2H4)2Cp ] (Cp = Cp [56], Cp [56,57]); • complexes with carbon monooxide [Ir(CO)2Cp*] [58].

FigureFigure 5.5. Molecular structures of iridium(I)iridium(I) complexescomplexes withwith (OˆN)-coordinated(O^N)-coordinated ligands:ligands: [Ir(cod)(Me[Ir(cod)(Mei-i-hfda)]hfda)] ((aa),),

[Ir(cod)(ThTFP)][Ir(cod)(ThTFP)] ( b(b),), and and [Ir(CO) [Ir(CO)22(amakN(Me)(amakN(Me)22)])] ((cc).).

TheComplexes synthesis with of cyclopentadienyl cyclopentadienyl ligands complexes are a fairly of iridium large (I), class as inof othercompounds cases, is not carried only outdue into anthe inert possibility atmosphere, of variation which of is substitu caused notents onlyin the by aromatic the possibility ring, but of oxidationalso due to of a iridium,large number but also of neutral by the complexityligands that of can working complement with MCp the Xcoordinationsalts (M is an environment alkali metal), of sinceiridium, they such decompose as alkenes, in air alkynes, and are dienes, often pyrophoric. carbonyls, phosphines, amines, imines, and so on. AmongA technique them, described only compounds by Angelici with in 1991a limit [47ed], number in which of the neutral reagents ligands are [Ir(cod)Cl] have been2 andproposed the substituted for MOCVD cyclopentadiene applications: is salt, is mainly used to obtain the complexes [Ir(cod)CpX]. The yields in the reactions vary from 30% to 95% depending on the sub- • complexes with cyclooctadiene [Ir(cod)CpX], CpX = Cp [47], CpMe [47,48], CpMe3 [47], stituents in the Cp ligand. CpMe4 [49], Cp* [50], CpEt [50], Cp-allyl [51], Cp-propenyl [51], Cp-ipropenyl [52]); The highest yields (90%–95%) are obtained in the synthesis of complexes with a • complexes with cyclohexadiene [Ir(chd)CpX] (CpX = Cp [53], CpMe [54], CpEt [55]); monosubstituted CpX (X = Me, Et) ligand [54–56], although they decrease with an increasing • complexes with ethylene [Ir(C2H4)2CpX] (CpX = Cp [56], CpEt [56,57]); substituent length (80% at X = allyl) [51]. • complexes with carbon monooxide [Ir(CO)2Cp*] [58]. Complexes with polysubstituted cyclopentadienyl ligands are obtained with the lowerThe yields synthesis of 45%–85% of cyclopentadienyl [47]. In addition, complexes the yield of value iridium strongly (I), as depends in other on cases, the cationis car- inried the out MCp in anX saltinert and atmosphere, the selected which solvent. is caused For not example, only by the the yield possibility of [Ir(cod)Cp of oxidationMe4] complexof iridium, is 45%but also when by using the complexity LiCpMe4 in of tetrahydrofuran working with MCp (THF)X salts [47], (M whereas is an alkali the product metal), yieldssince they vary decompose from 15% toin air 54% and when are often using pyrophoric. NaCpMe4 or KCpMe4 as a ligand source and benzene,A technique THF or diethyldescribed ether by asAngelici a solvent in 1991 [49]. [47], Alternative in which methods the reagents of the are preparation [Ir(cod)Cl]2 ofand [Ir(cod)Cp the substitutedX] complexes cyclopentadiene are also described. is salt, is The mainly ﬁrst oneused is to the obtain direct the interaction complexes of X cyclopentadiene[Ir(cod)Cp ]. The with yields the in complex the reactions [Ir(cod)(OH)] vary from2 [ 5930%]. Thisto 95% method depending eliminates on the the substit- stage ofuents synthesis in the ofCp cyclopentadiene ligand. salts and further formation of insoluble chlorides in the reactionThe mixture,highest whichyields (90%–95%) results in a are cleaner obtained product. in the Despite synthesis the attractivenessof complexes with of the a method,monosubstituted as alkyl substituentsCpX (X = Me,are Et) addedligand to[54–56], the cyclopentadienyl although they decrease ring, the with acidity an increas- of the C–Hing substituent proton decreases, length (80% which at leadsX = allyl) to a [51]. signiﬁcant decrease in the yield in this reaction. The secondComplexes method with is basedpolysubstituted on the reduction cyclopen oftadienyl dimeric cyclopentadienylligands are obtained complexes with the of iridium(III)lower yields in of the 45%–85% presence [47]. of cyclooctadiene. In addition, the The yield interaction value strongly of [Ir(Cp*)( dependsµ-H)( onµ -Cl)(Cl)the cation2] within the an MCp excessX salt of and cyclooctadiene the selectedin solvent. the presence For example, of sodium the yield carbonate of [Ir(cod)Cp makes itMe4 possible] complex to isolateis 45% [Ir(cod)Cp*]when using LiCp withMe4 the in yield tetrahydrofuran of 73% [60].Complexes (THF) [47], withwhereas cyclohexadiene the product canyields also vary be obtainedfrom 15% using to 54% this when technique. using NaCpMe4 or KCpMe4 as a ligand source and benzene, THF or diethylIt should ether as be a noted solvent that [4 the9]. neutralAlternative cod-ligand methods in cyclopentadienyl of the preparation complexes of [Ir(cod)Cp [Ir(cod)CpX] com-X] isplexes more are strongly also described. bound to the The central first atomone is than the indirect [Ir(cod)(L)] interaction complexes of cyclopentadiene with β-diketonate with X ligandsthe complex and their [Ir(cod)(OH)] derivatives.2 [59]. This This does notmethod allow eliminates to obtain [Ir(CO)the stage2Cp of] complexessynthesis of directly cyclo- bypentadiene the substitution salts and of the further cod-ligand formation in the of reaction insoluble with chlorides CO, as described in the reaction above. Therefore, mixture, alternativewhich results synthetic in a cleaner strategies product. are developed. Despite th Fore attractiveness example, the of complex the method, [Ir(CO) as2 alkylCp*] wassub- obtainedstituents byarethe added reduction to the cyclopentadienyl from the cyclopentadienyl ring, the acidity complex of ofthe iridium(III), C–H proton [IrCp*Cl decreases,2]2, usingwhich the leads iron to carbonyl a significant complex decrease Fe3(CO) in12 the[58 yield]. in this reaction. The second method is basedDespite on the the reduction fact that of methods dimeric forcyclopentadi the synthesisenyl of complexes cyclopentadienyl of iridium(III) complexes in the of pres- irid- ium(I)ence of have cyclooctadiene. been known The since interaction the mid-20th of [Ir(Cp*)(µ-H)(µ-Cl)(Cl) century, only five complexes2] with with an excess cyclooctadi- of cy- Me eneclooctadiene and onewith in the cyclohexadiene presence of sodium have been carbonat structurallye makes characterized: it possible to isolate [Ir(cod)Cp [Ir(cod)Cp*]][48], with the yield of 73% [60]. Complexes with cyclohexadiene can also be obtained using this technique.

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It should be noted that the neutral cod-ligand in cyclopentadienyl complexes [Ir(cod)CpX] is more strongly bound to the central atom than in [Ir(cod)(L)] complexes with β-diketonate ligands and their derivatives. This does not allow to obtain [Ir(CO)2CpX] complexes directly by the substitution of the cod-ligand in the reaction with CO, as described above. Therefore, alternative synthetic strategies are developed. For example, the complex [Ir(CO)2Cp*] was obtained by the reduction from the cyclopentadienyl complex of iridium(III), [IrCp*Cl2]2, using the iron carbonyl complex Fe3(CO)12 [58]. Coatings 2021, 11, 78 Despite the fact that methods for the synthesis of cyclopentadienyl complexes of10 irid- of 36 ium(I) have been known since the mid-20th century, only five complexes with cyclooctadiene and one with cyclohexadiene have been structurally characterized: [Ir(cod)CpMe] [48], [Ir(cod)CpMe4] [49], [Ir(cod)CpMe5] [50], [Ir(cod)(Cp-propenyl)] [51], [Ir(cod)(Cp-ipro- [Ir(cod)CpMe4][49], [Ir(cod)CpMe5][50], [Ir(cod)(Cp-propenyl)] [51], [Ir(cod)(Cp-ipropenyl)] [52], penyl)] [52], [Ir(chd)CpMe] [54]. In all complexes, the cyclopentadienyl ligand is coordi- [Ir(chd)CpMe][54]. In all complexes, the cyclopentadienyl ligand is coordinated by the cen- nated by the central atom according to the η5-type, while cyclooctadiene and cyclohexa- tral atom according to the η5-type, while cyclooctadiene and cyclohexadiene are coordinated diene are coordinated according to the η4-type (Figure 6). according to the η4-type (Figure6).

Me Figure 6. 6. MolecularMolecular structures structures of ofiridium(I) iridium(I) cyclopentadienyl cyclopentadienyl complexes complexes [Ir(cod)Cp [Ir(cod)CpMe] (a) and](a) and Me [Ir(chd)(CpMe)])] ( (bb).).

Thermal properties of iridium(I) precursors Information about thermal properties of the considered iridium compounds is lim- Information about thermal properties of the considered iridium compounds is limited and is mainly represented by melting characteristics, thermogravimetry results, and ited and is mainly represented by melting characteristics, thermogravimetry results, and sublimation parameters [32–34,41–43,46,58]. Moreover, among a wide range of synthe- sublimation parameters [32–34,41–43,46,58]. Moreover, among a wide range of synthesized cyclopentadienyl complexes, only precursors containing CpX = CpMe, CpEt, CpMe4, sized cyclopentadienyl complexes, only precursors containing CpX = CpMe, CpEt, CpMe4, and Cp* [48–50,57] were studied in the thermochemical aspect. The thermochemical data and Cp* [48–50,57] were studied in the thermochemical aspect. The thermochemical data for dimeric acetate [Ir(cod)(OAc)]2 and alcoholic [Ir(cod)(OMe)]2 complexes, with the for dimeric acetate [Ir(cod)(OAc)]2 and alcoholic [Ir(cod)(OMe)]2 complexes, with the ex- exception of the melting points, are completely absent [27,29]. ception of the melting points, are completely absent [27,29]. A feature of [Ir(cod)CpX] complexes is an extremely low melting point manifesting A feature of [Ir(cod)CpX] complexes is an extremely low melting point manifesting when one alkyl substituent is introduced into the cyclopentadienyl ligand. For exam- when one alkyl substituent is introduced into the cyclopentadienyl ligand. For example, ple, complexes with CpMe and CpEt melt at 38–40 and 14 ◦C, respectively. Thus, these complexes with CpMe and CpEt melt at 38–40 and 14 °C, respectively. Thus, these com- compounds are easy to use in MOCVD processes as liquid precursors, which provide poundsconvenient are dosageeasy to use and in a constantMOCVD surfaceprocesses area as during liquid precursors, vaporization. which provide convenient dosageOther volatile and a constant iridium(I) surf complexesace area haveduring noticeably vaporization. higher melting points. An increase in theOther number volatile of methyliridium(I) groups complexes in the Cphave ligand noticeably leads higher to an increasemelting inpoints. the melting An in- creasetemperature, in the number namely, of up methyl to 124 groups◦C in thein the case Cp of ligand four methylleads to groupsan increase (CpX in= the CMe4 melting) and X Me4 temperature,up to 170 ◦C innamely, the case up of to five 124 methyl°C in the groups case of (Cp fourX = methyl Cp*). In groups the series (Cp of = βC-diketonate) and up to 170 °C in the case of five methyl groups (CpX = Cp*). In the series of β-diketonate com- complexes [Ir(cod)(L)], the introduction of a CF3 group in L leads to a decrease in the plexesmelting [Ir(cod)(L)], temperature, the while introduction the presence of a ofCF a3t Bugroup or Ph in substituentL leads to a results decrease in its in increase the melting [33]. temperature,Interestingly, thewhile replacement the presence of the of a methyl tBu or groupPh substituent in the tBu results substituent in its withincrease a methoxide [33]. In- terestingly,group (L = zisthe vs. replacement thd) leads hereof the to methyl such a decreasegroup in in the the tBu melting substituent point thatwith the a methoxide properties groupof this (L non-fluorinated = zis vs. thd) leads complex here to approach such a dec thoserease of in fluorinated the melting analogues, point that which the properties was not ofobserved this non-fluorinated for all previously complex studied approach volatile thoseβ-diketonate of fluorinated precursors analogues, [35 ].which Thus, was in thenot series of considered compounds, the melting point varies in the range of 105–170 ◦C, where the complexes [Ir(cod)(tfac)] and [Ir(cod)(zis)] have the lowest melting points, while [Ir(cod)(thd)] has the highest one. The tendency for a decrease of the melting point when fluorine atoms are introduced

into the ligand is also characteristic of β-diketonate complexes [Ir(CO)2(L)] [36–38] and β- iminoketonate derivatives [Ir(cod)(L)] [42,44,45]. For the other classes of (OˆN)-coordinated ligands, only ﬂuorinated complexes were synthesized. It was shown that in the case of β-iminoketonate complexes, the introduction of the Me group at the nitrogen atom also led to a decrease in the melting point, apparently due to the absence of intermolecular N–H...O interactions [45]. In general, in the series of β-iminoketonate complexes [Ir(cod)(L)] (L = Eti-hfac < Mei-tfac < Mei-acac < i-acac), the melting point varies in the range of 111–167 ◦C. Presumably, the change in the melting point Coatings 2021, 11, 78 11 of 36

during the transition from [Ir(cod)(L)] to [Ir(CO)2(L)] does not have a definite character even within one class of L ligands. Specifically, this parameter can decrease (L = hfac: 116 ◦C vs. 92 ◦C, L = Cp*: 170 ◦C vs. 110 ◦C), change slightly (L = acac: 152 ◦C vs. 160 ◦C), or increase (L = tfac: 109 ◦C vs. 148 ◦C). However, iridium(I) carbonyl complexes are always noticeably more volatile than their cyclooctadiene-containing analogues. This result was obtained by thermogravimetry (TG) with a wide variation of anionic ligands L (β-diketonates, β- iminoketonates, β-iminoalcoholates, α-aminoalcoholates, cyclopentadienyls). A similar study of a series of cyclopentadienyl complexes [Ir(Q)CpEt] demonstrates the effect of changing the neutral Q ligand within the class of alkenes [57]. In this case, the volatility regularly increased with a decrease in the molecular weight of the ligand: Q = cod < chd << C2H4. The effect of the anionic ligand L on volatility has also been mainly investigated by the TG method. The most complete picture is observed for the complexes [Ir(cod)(L)] with β-diketonate derivatives investigated in a series of our recent papers [33,35,44,45]. The presence of fluorinated substituents in the β-diketonate ligand L leads to an increase in the volatility of the complex, while the presence of the Ph group leads to a decrease in the volatility, and in the latter case, the vaporization process is accompanied by significant decomposition. The introduction of bulk tBu substituents also reduces the volatility of the compound, which is a feature of complexes of this class in comparison with monoligand β-diketonates. The replacement of the Me group in the tBu substituent with OMe results in the opposite effect, and this effect of increasing volatility (L = zis vs. thd) is also opposite to that observed for monoligand planar-square complexes. Replacement of β-diketonate ligand (L = acac) with the corresponding β-iminoketonate derivative (L = i-acac, Mei-acac) leads to a decrease in volatility. It should be noted that vaporization of all the studied complexes [Ir(cod)(L)] with (OˆN)-donor ligands is accompanied by decomposition [42,44,45] and for β-heteroarylketonate [Ir(cod)(ThTFP)] the mass loss curve corresponds only to decomposition with a clear cleavage of the anionic ligand at the first stage [41]. According to [42] and [43], it can be assumed that β-iminoketonates are the most volatile among [Ir(cod)(L)] and [Ir(CO)2(L)] complexes with the studied classes of (OˆN)-coordinated ligands, whereas β-iminoalcoholates are the least volatile. However, very different masses and substituents n in the ligands of this series (L = Eti-hfac, amakN(Me)2, Pri-hfda) do not allow us to make a final conclusion. In a series of cyclopentadienyl complexes [Ir(cod)CpX], an increase in the size of a single substituent or the number of substituents leads to a slight decrease in volatility: CpX = CpMe > CpEt ≥ CpMe4 > Ir(cod)Cp* [50]. However, the temperature difference is very small, so quantitative studies are required to better identify the considered effect. The volatilization processes were studied quantitatively for a limited number of compounds, namely, for the following five complexes: [Ir(cod)(L)], L = acac [45,58], Me Cp [58,61], zis [35], i-acac, Mei-acac [45], and two complexes [Ir(CO)2(L)], L = acac, Cp* [58]. The data obtained are summarized in Figure7 and Table3. All Ir(I) complexes are more volatile than the traditional non-fluorinated Ir(III) MOCVD precursor, Ir(acac)3. The order of their volatility is consistent with thermogravimetry data. In the series of [Ir(cod)(L)] complexes with β-diketonate derivatives, the change in the vapor pressure values when changing the terminal substituents (L = acac vs. zis) and the coordination environment of the metal (L = acac vs. i-acac, Mei-acac) is quite small. It is interesting to note the unusual order of the minor decrease (L = acac > i-acac > Mei-acac) in volatility being partially opposite to that for the planar-square complexes [M(L)2] with the same ligands [45]. Replacing the cod ligand with carbonyl ligands, i.e., switching from [Ir(cod)(L)] to [Ir(CO)2)(L)], leads to a significant increase in the vapor pressure of complexes with both β-diketonate and cyclopentadienyl ligands L. In this case, the complex [Ir(CO)2Cp*] is the most volatile among the Ir(I) compounds under consideration; however, synthetic difficulties (see above) hinder its practical application. Coatings 2021, 11, x FOR PEER REVIEW 12 of 39

volatile than the traditional non-fluorinated Ir(III) MOCVD precursor, Ir(acac)3. The order of their volatility is consistent with thermogravimetry data. In the series of [Ir(cod)(L)] complexes with β-diketonate derivatives, the change in the vapor pressure values when changing the terminal substituents (L = acac vs. zis) and the coordination environment of the metal (L = acac vs. i-acac, Mei-acac) is quite small. It is interesting to note the unusual order of the minor decrease (L = acac > i-acac > Mei-acac) in volatility being partially opposite to that for the planar-square complexes [M(L)2] with the same ligands [45]. Replac- ing the cod ligand with carbonyl ligands, i.e., switching from [Ir(cod)(L)] to [Ir(CO)2)(L)], leads to a significant increase in the vapor pressure of complexes with both β-diketonate Coatings 2021, 11, 78 and cyclopentadienyl ligands L. In this case, the complex [Ir(CO)2Cp*] is the most volatile12 of 36 among the Ir(I) compounds under consideration; however, synthetic difficulties (see above) hinder its practical application.

Figure 7. Temperature dependences of the saturated vapor pressure over the solid (sublimation Figure 7. Temperature dependences of the saturated vapor pressure over the solid (sublimation process) and liquid (vaporization process) Ir(I) complexes and Ir(acac)3 for comparison. process) and liquid (vaporization process) Ir(I) complexes and Ir(acac)3 for comparison. Table 3. Equations of the temperature dependences of saturated vapor pressure over the solid (sublimation, subl.) and Table 3. Equations of the temperature dependences of saturated vapor pressure over the solid (sublimation, subl.) and liquid (vaporization, vap.) Ir(III) β-diketonates. The sublimation and vaporization enthalpies (ΔHT*) and entropies (ΔS0T*) β 0 liquidare referred (vaporization, to the average vap.) Ir(III) temperature-diketonates. (T*) of The the sublimation interval measured and vaporization (ΔT), p0 = enthalpies 760 Torr (=∆ H1 T*atm) and = 10 entropies5 Pa. The (∆ ligandS T*) 5 areabbreviations referred to thecorrespond average to temperature Table 1. (T*) of the interval measured (∆T), p0 = 760 Torr = 1 atm = 10 Pa. The ligand abbreviations correspond to Table1. ln(p/p0) = A − B/(T,K) ΔHT*, ΔS0T*, Ref. Complex Process ΔT, K ln(p/p0) = A − B/(T,K) ∆H , −1 0 −1 Ref. Complex Process ∆T,K A B kJ·molT* J·(mol·K)∆S T*, kJ·mol−1 J·(mol·K)−1 [45] [Ir(cod)(acac)] Subl. 363–423 23.03AB 12,817 106.6 ± 0.7 191 ± 2 [45] [Ir(cod)(acac)] Subl. 363–423 23.03 12,817 106.6 ± 0.7 191 ± 2 Subl. 353–376353–376 28.04 28.04 14,886 14,886 124 124 ±± 4 233 ± ±10 10 [35][35] [Ir(cod)(zis)][Ir(cod)(zis)] Vap. 381–403 381–403 18.95 18.95 11,477 11,477 96 96 ±± 5 158 ± ±13 13 [Ir(cod)(i-acac)] Subl. 383–431 25.31 14,312 119 ± 2 210 ± 5 [45] [Ir(cod)([Ir(cod)(Mei-acac)]i-acac)] Subl. 383–431383–420 25.31 22.56 14,312 13,398 119 111 ±± 32 188 210± ±7 5 [45] Me Subl. 304–310 33.43 14,990 124.6 ± 5.0 279 ± 16 [Ir(cod)(Me[Ir(cod)Cpi-acac)]] Subl.Vap. 383–420 310–330 22.56 19.39 13,398 10,592 88.1 111± ±1.3 3 161.2 188± ±4.2 7 [58] [Ir(CO)2Cp*]Subl. 304–310297–332 33.43 27.91 14,990 12,641 124.6 105.0 ±± 3.45.0 232279± ±11 16 [Ir(cod)Cp[Ir(CO)2(acac)]Me] Subl. 306–333 22.40 11,293 94 ± 2 186 ± 8 Vap. 310–330 19.39 10,592 88.1 ± 1.3 161.2 ± 4.2 [58] [Ir(CO)2Cp*] Subl. 297–332 27.91 12,641 105.0 ± 3.4 232 ± 11 As for the stability during volatilization, sublimation is congruent for almost all [Ir(CO)2(acac)] Subl. 306–333 22.40 11,293 94 ± 2 186 ± 8 the studied complexes, except for [Ir(cod)(Mei-acac)], for which partial decomposition is observed. After melting, the thermal stability of the compounds usually decreases. This As for the stability during volatilization, sublimation is congruent for almost all the is most pronounced for [Ir(CO) (L)] complexes, and least pronounced for [Ir(cod)CpX] studied complexes, except for [Ir(cod)(Me2 i-acac)], for which partial decomposition is ob- complexes. For example, the composition of the gas phase of [Ir(cod)CpMe] was shown served. After melting, the thermal stability of the compounds usually decreases. This is by mass spectrometry to be stable over time during vaporization at 60 ◦C, and only its X most pronounced◦ for [Ir(CO)2(L)] complexes, and least pronounced for [Ir(cod)CpMe ] com- holding at 120 C results in the appearance of signal corresponding to [Ir(cod)Cp ]2 and plexes. For example, the composition of the gas phase of [Ir(cod)CpMe] was shown by mass a decrease in the intensity of signal corresponding to the molecular ion [61]. spectrometryRecently, to we be studied stable over the thermaltime during stability vaporization of vapors at of60 a°C, representative and only its holding series of at β-diketonate complexes [Ir(cod)(L)] (L = hfac, tfac, ptac, acac, thd, btfac) on a heated surface [33]. It turned out that the vapor of complex [Ir(cod)(thd)] was the most stable, which appeared to be due to the presence of two bulk tert-butyl substituents. Indeed, a relatively low degree of vapor decomposition even at maximum temperatures (475–500 ◦C) was also observed for complexes with one bulk substituent in the ligand, i.e., tert-butyl (L = ptac) or phenyl (L = btfac). The [Ir(cod)(hfac)] complex is characterized by the lowest vapor stability, which indicates the possibility of deposition of coatings at deposition temperatures lower than those for the other compounds from this series. This property together with its highest volatility makes this compound a particularly interesting precursor. More detailed information about the thermal behavior of vapors on a heated surface is provided for two Ir(I) volatile Me complexes: [Ir(cod)Cp ][62] and [Ir(CO)2(acac)] [25,63]. Coatings 2021, 11, x FOR PEER REVIEW 13 of 39

120 °C results in the appearance of signal corresponding to [Ir(cod)CpMe]2 and a decrease in the intensity of signal corresponding to the molecular ion [61]. Recently, we studied the thermal stability of vapors of a representative series of β- diketonate complexes [Ir(cod)(L)] (L = hfac, tfac, ptac, acac, thd, btfac) on a heated surface [33]. It turned out that the vapor of complex [Ir(cod)(thd)] was the most stable, which appeared to be due to the presence of two bulk tert-butyl substituents. Indeed, a relatively low degree of vapor decomposition even at maximum temperatures (475–500 °C) was also observed for complexes with one bulk substituent in the ligand, i.e., tert-butyl (L = ptac) or phenyl (L = btfac). The [Ir(cod)(hfac)] complex is characterized by the lowest vapor stability, which in- Coatings 2021, 11, 78 dicates the possibility of deposition of coatings at deposition temperatures lower than 13 of 36 those for the other compounds from this series. This property together with its highest volatility makes this compound a particularly interesting precursor. More detailed information about the thermal behavior of vapors on a heated surface is provided for two Ir(I) volatile complexes:Using [Ir(cod)Cp in situ mass-spectrometryMe] [62] and [Ir(CO)2 [Ir(cod)Cp(acac)] [25,63].Me] vapors have been shown to be stable up Using into 280situ ◦mass-spectrometryC in the absence of [Ir(cod)Cp a gas-reagent,Me] vapors but the have rate been of thermolysis shown to be was stable greatly increased at up to 280 °C500 in◦ theC (the absence intensity of a ofgas-reagent, ions corresponding but the rate to HCpof thermolysisMe and cod was increases greatly sharply) in- [62]. When creased at 500oxygen °C (the is intensity introduced of ions into corresponding the reaction zone, to HCp vaporMe and decomposition cod increases beginssharply) at a temperature [62]. When oxygen is◦ introduced into the reaction zone, vapor decomposition begins at a of ~200 C; the main gaseous products of thermolysis are CO, CO2, and H2O. In this case, temperaturethe of ~200 molecular °C; theion main peak gaseous [Ir(cod)Cp productsMe ]of+ thermolysisand the peaks are CO, of its CO defragmentation2, and H2O. products Me + In this case,under the molecular electron impaction peak practically [Ir(cod)Cp disappear] and the already peaks atof temperaturesits defragmentation above 230 ◦C, which products underindicates electron complete impact decomposition practically disappear of the already precursor at temperatures [62]. above 230 °C, which indicates complete decomposition of the precursor [62]. For the [Ir(CO)2(acac)] complex, experimental results obtained by in situ mass spec- For thetrometry [Ir(CO)2(acac)] [25] were complex, recently experimental supplemented results by obtained quantum by chemical in situ mass modeling, spec- which allowed trometry [25]us towere present recently a more supplemented complete pictureby quantum of the chemical process [modeling,63]. Calculations which al- have shown that lowed us to present a more complete picture of the process [63]. Calculations have shown the decomposition of the molecule in the gas phase begins with the cleavage of one of the that the decomposition of the molecule in the gas phase begins with the cleavage of one Ir–O bonds. When the molecule is adsorbed on the substrate, a structural rearrangement of the Ir–O bonds. When the molecule is adsorbed on the substrate, a structural rearrange- occurs, which consists of a noticeable increase in the lengths of the Ir–O and Ir–C bonds, ment occurs, which consists of a noticeable increase in the lengths of the Ir–O and Ir–C and the carbonyl and methyl groups are located out of the complex plane. Thus, the bonds, and the carbonyl and methyl groups are located out of the complex plane. Thus, decomposition of the precursor on a heated surface proceeds by a different mechanism the decomposition of the precursor on a heated surface proceeds by a different mechanism than in the thangaseous in thephase, gaseous i.e., with phase, the i.e.,cleavage with of the Ir–C cleavage bonds ofat Ir–Cthe first bonds stage at during the ﬁrst a stage during a removal of removalCO molecule. of CO The molecule. process is The schematically process is schematicallypresented in Figure presented 8. in Figure8.

Figure 8. Scheme of Ir(acac)(CO)2 thermal destruction based on the results of quantum chemical Figure 8. Scheme of Ir(acac)(CO)2 thermal destruction based on the results of quantum chemical calculations [63]. calculations [63].

Threshold temperature of the beginning of [Ir(CO)2(acac)] vapor decomposition in Thresholdvacuum temperature is 200 ± of10 the◦C. beginning It is shown of [Ir(CO) that the2(acac)] introduction vapor decomposition of gases-reagents in (oxygen or vacuum is 200hydrogen) ± 10 °C. It does is shown not signiﬁcantly that the introduction affect the of processgases-reagents temperatures. (oxygen or Moreover, hy- the main drogen) doesdecomposition not significantly products affect (CO,the pr Hacac)ocess temperatures. in vacuum and Moreover, in the presence the main of de- hydrogen are the composition products (CO, Hacac) in vacuum and in the presence of hydrogen are the same. Peaks corresponding to the appearance of water and CO2 were registered in the same. Peaks corresponding to the appearance of water and CO2 were registered in the oxidizing atmosphere. oxidizing atmosphere. 2.1.3. Application of Iridium Precursors in MOCVD Processes

Nowadays, the most commonly used iridium precursor remains Ir(acac)3. The main

regularities of MOCVD processes with the use of Ir(acac)3 and the characteristics of the resulted coatings were described in detail in our previous review [18] and supplemented in the recent paper [64] focused on the coating deposition in the presence of hydrogen. In general, this precursor is successfully used to produce both metallic iridium (in the presence of hydrogen or oxygen) and its oxide (in an oxidizing atmosphere). At the same time, this compound has some notable disadvantages, such as high thermal stability of vapors in hydrogen atmosphere, which requires high deposition temperatures (>460 ◦C) and relatively low volatility, as a result of which the source temperature is usually set above 200 ◦C to create the necessary concentration of precursor vapors, and, ﬁnally, a rather intricate synthesis. Comparing Ir(I) volatile precursors with Ir(acac)3 based on these parameters (see above), we can conclude that this type of compound looks more attractive for MOCVD processes. In addition to the advantages associated with these aspects, the variety of combinations of neutral and anionic ligands provides the ability of Coatings 2021, 11, 78 14 of 36

“fine-tuning” of the thermochemical properties of the precursor, based on the characteristics of the target material and the object to be coated. For example, if we want to deposit a coating on a substrate with low thermal stability, we can choose a compound with a low thermal stability of vapors or use ligands that are sensitive to additional activation of the decomposition process. Despite this potential, the use of Ir(I) precursors in MOCVD processes is still limited. The representative examples are summarized in Table4 in order to show the range of possibilities for varying deposition conditions, composition, growth rate, and some other characteristics of the resulting coatings depending on the specific precursor. This information will be briefly discussed below. The first MOCVD experiments using Ir(I) complexes were conducted in 1967, when high-purity iridium coatings (90–99 mass.% Ir) were deposited from [Ir(cod)(OMe)]2 and [Ir(cod)(acac)] in the presence of hydrogen at 550 and 600 ◦C, respectively [26]. Then the row of precursors used for the deposition of pure metal films (about 1 mass.% C and O) in a reducing atmosphere was expanded. It was shown that when using [Ir(cod)(hfac)] the coating formed already at 350 ◦C, and the growth rate was quite high (15 nm/min) [34]. At the same time, the formation of coatings from [Ir(cod)(thd)] at this temperature was possible only by additional passing of hydrogen through isopropanol [32]. These results are consistent with our data on the thermal stability of the vapors of these complexes: high stability in the case of [Ir(cod)(thd)] and a relatively low one for [Ir(cod)(hfac)] [33]. Using [Ir(cod)CpMe] it is possible to obtain coatings with a high growth rate (up to 25 nm/min) at 300–400 ◦C[65], and the precursor is removed from the reactor without decomposition at the lower temperatures, which also correlates perfectly with in-situ mass spectrometry data [62]. Finally, [Ir(CO)2(acac)] vapors were shown by mass-spectrometry to be the least stable in the presence of hydrogen, and the possibility of deposition of coatings from this precursor at the lowest temperature to date (240–300 ◦C) was confirmed using pulsed MOCVD [25]. As for the use of (OˆN)-coordinated precursors in a reducing atmosphere, it has been shown using a fluorinated β-iminoalcoholate complex [Ir(cod)(nPri-hfda)] as an example that the growth rate was relatively low (1.3 nm/min) even at a deposition temperature of 500 ◦C[42]. The latter temperature was higher than that used for deposition of coatings from a fluorinated β-diketonate precursor [Ir(cod)(hfac)]. At the same time, the obtained samples had a high degree of contamination (41 mass.% C). Replacing the gas-reagent with oxygen results in a decrease in both the amount of impurities (2 mass.% O) and the deposition temperature (375 ◦C), as well as a noticeable increase in the growth rate. The formation of metal coatings under similar conditions was also demonstrated for other (OˆN)-coordinated cyclooctadiene complexes [42]. The use of oxygen as a gas-reagent for the production of metallic iridium has proven itself well in the case of liquid cyclopentadienyl complexes. When using the [Ir(cod)CpMe] precursor, pure coatings (<1 at.% C and O) are formed already at 270 ◦C[29,66]; however, the growth rate is low. The application of the special system of “liquid delivery” of the precursor using a tetrahydrofuran (THF) solution allows attainment of exceptionally high growth rates (up to 70 nm/min at 350 ◦C) [67]. However, the purity of the coatings noticeably reduces (12.5 at.% C and 4 at.% O). Orientation of crystallites in films deposited from [Ir(cod)CpMe] strongly depends on the substrate material and deposition temperature [62,68]. An interesting example is the preparation of highly oriented <111> films on amorphous native silicon oxide at 700 ◦C: the ratio of intensities of the (111) and (200) reflections is two orders of magnitude higher than in the non-textured sample [68]. It is important that despite the high deposition temperature, these coatings are also pure metallic iridium, whereas when using [Ir(cod)CpEt] with an increase in the deposition temperature (300–500 ◦C), the probability of formation of iridium oxide increases [69–73]. Another factor affecting the phase composition of films formed from [Ir(cod)CpEt] is the Et oxygen concentration. With a significant increase in the O2/[Ir(cod)Cp ] ratio, the content of the oxide phase increases [70]; however a balance that allows obtaining metal coatings with a high growth rate (up to 5 nm/min) can be found [72,73]. Coatings 2021, 11, 78 15 of 36

Table 4. Deposition conditions and some characteristics of the coatings obtained by the MOCVD method from volatile iridium(I) complexes. The ligand abbreviations correspond to Table1 a.

Precursor Reagent Gas Source/Deposition Thickness, Growth Rate, Composition Ref. Temperature, ◦C Substrate nm nm/min (XRD) Purity and Other Details [Ir(cod)(OMe)] H –/550 ––– 5–10 mass.% impurities [26] 2 2 Ir [Ir(cod)(acac)] H2 –/600 ––– 1 mass.% C and O

[29] [Ir(cod)(OAc)]2 no 130/250 Si (100) 40 13.3 Ir 1 mass.% C and O µ H SiO2/Si, – 15 Ir 0.1–0.3 m agglomerates, 1 [34] [Ir(cod)(hfac)] 2 60–65/250–350 Pt/Si, Cu/Si mass.% C i 130/350 25 4.1 Ir 1 mass.% C and O, islands [32] [Ir(cod)(thd)] H2 + PrOH Quartz glass H2 130/350–550 – <0.7 Ir –

[25] [Ir(CO)2(acac)] H2 or O2 –/240–300 Si (100) – – Ir Crystallite size 4–20 nm

[Ir(cod)(Eti-hfac)] O2 –/400 Si (100) 278 7.0 2 mass.% O [42] H –/500 Si (100) 322 1.3 30–90 nm agglomerates, 41 [Ir(cod)(nPri-hfda)] 2 Ir mass.% C O2 –/375 Si (100) 320 3.8 1 mass.% O [Ir(cod)(amakN(Me)2)] O2 –/350 Si (100) 264 4.4 2 mass.% O

50/400 290 4.8 Ir 60–90 nm agglomerates, 2 mass.% Si (100) impurities n 75/425 400 1.7 Polycrystalline [43] [Ir(CO)2( Pri-hfda)] O2 75/425 LiTaO (012) 2000 8.3 IrO tilted needles: 15–25 nm in 3 2 diameter, 1.5 µm in length 75/425 LiNbO3 (100) 3000 12.5 vertical needles 95/425 LiNbO3 (100) 430 7.1 vertical pillars

[41] [Ir(cod)(ThTFP)] no 110–130/700–800 Si (100), SiO2, Al2O3 30–140 0.5–0.6 Ir Uniform, crystallites < 30 nm Untextured constituted of small [65] H2 100/300–400 W 700–1500 11.7–25 Me grains [29] [Ir(cod)Cp ] 80/270 Si (100) 95 0.42 Ir <1 mass.% C and O O2 Si(100), SiO2/Si, C TiN/SiO /Si, = 0.05M(THF) 2 420 70 0.9 mass.% C, 0.4 mass.% O [67] 200/300–450 Pt/Ti/Si, IrOx/poly- Si/SiO2/Si

Et Si, SiO2/Si, [73] [Ir(cod)Cp ] O2 150–180/300–500 1.2–300 0.02–5 Ir 0.4 mass.% O TiO2/SiO2/Si

Et Ir Low O2 ﬂow (0–2 sccm) [57] [Ir(C2H4)2Cp ] O2 40/400 SiO2/Si –– IrO2 Higher O2 ﬂow (10–160 sccm) Uniform crystallites, [55] Et O 70/250, 350 SiO 65, 120 1.6, 30 Ir [Ir(chd)Cp ] 2 2 roughness—1.2 nm a XRD = powder X-Ray diffraction, iPrOH = propanol-2, C = concentration, THF = tetrahydrofurane. Coatings 2021, 11, 78 16 of 36

The possibility of deposition of metallic iridium from carbonyl precursors in an oxidizing atmosphere was also shown. For example, films containing iridium in a single form (Ir0, according to X-ray photoelectron spectroscopy, XPS) were obtained from n ◦ [Ir(CO)2(acac)] [25] and [Ir(CO)2( Pri-hfda)] [43] at 280 and 400 C, respectively. n It was shown for (OˆN)-coordinated precursor [Ir(CO)2( Pri-hfda)] that for the simul- taneous increase in the total pressure in the reactor, the source and deposition temperatures led to the formation of IrO2 films [43]. Both the temperature parameters and the substrate nature had a significant effect on the shape and orientation of particles, as well as on the growth rate of these layers. In particular, replacing the Si(100) substrate with LiTaO3 (012) or LiNbO3 (100) led to the formation of obliquely or vertically oriented needles, respectively, and to a significant increase in the growth rate. With the same LiNbO3 (100) substrate, an increase in the source temperature led to the formation of vertical columns instead of needles, accompanied by a decrease in the growth rate. The possibility of preparation of iridium oxide films has also been tested for a series of Et Et Et such cyclopentadienyl complexes as [Ir(cod)Cp ], [Ir(chd)Cp ] and [Ir(C2H4)2Cp ][55,57]. Et It was shown that only the use of [Ir(C2H4)2Cp ] precursor made it possible to obtain pure oxide films with an increase in the amount of oxygen. In the case of [Ir(cod)CpEt], the iridium oxide phase did not form even at the maximal oxygen concentrations at temperatures of ◦ Et 250–400 C, and in the case of [Ir(chd)Cp ] only mixed Ir–IrO2 layers could be obtained. Experiments on the deposition of coatings in the absence of a gas-reagent were carried out using precursors [Ir(cod)(ThTFP)] and [Ir(cod)(OAc)]2. In the case of the first precursor, metal layers formed at high temperatures (700–800 ◦C) [41]. An increase in the deposition time led to an increase in the film crystallinity and thickness, and the substrate material did not affect the decomposition of the precursor. The purity of the obtained samples was not discussed. An unexpected result was obtained when using [Ir(cod)(OAc)]2: the possibility of the formation of pure metal films at a very low temperature (250 ◦C) was shown [29]. The growth rate was two orders of magnitude higher than that observed in the case of [Ir(cod)(ThTFP)]. However, this compound was not further used as a precursor. In general, the use of Ir(I) precursors of different classes, as well as the variation of ligands within the same class, allowed us to obtain metal and oxide coatings in a wide range of deposition temperatures, and in some cases, to control the orientation of crystallites and the morphology of the formed layers.

2.2. Volatile Platinum Precursors Platinum is able to coordinate various types of C donor ligands, including oleﬁns, carbonyl, dienes, vinyls, etc., and form strong bonds with methyl groups [15,74,75], which causes a variety of organoplatinum compounds that can be promising precursors for MOCVD. In contrast to organoplatinum compounds, the number of platinum complexes with organic O, N, S, and Se-donor ligands (Table1) is signiﬁcantly limited due to the high chemical inertness of platinum in exchange reactions.

2.2.1. Platinum(0) Precursors

There are only a few examples of platinum(0) precursors, namely, Pt(PF3)4 and Pt(C2H4)3 [15]. Both compounds are obtained by heating anhydrous PtCl2 in the presence of reducing agents and gases (C2H4, PF3) at the pressure above 100 MPa [76]. Pt(PF3)4 and ◦ Pt(C2H4)3 are liquids (Pt(PF3)4 (boiling temperature is 87 C) and decompose to metal with the elimination of ligands in air. Due to low storage stability and synthetic unavailability, these compounds are currently almost not used in MOCVD.

2.2.2. Platinum(II) Precursors As a rule, MOCVD platinum(II) precursors are represented by the following two classes: organoplatinum compounds and platinum complexes with organic O,N,S,Se-donor ligands. Coatings 2021, 11, 78 17 of 36

Organoplatinum compounds, in turn, are represented by three classes:

• alkylplatinum derivatives with monodentate ligands: Me(R)PtQ2, where R = Me, CO, t 3 HC = CH2, BuC≡C, ï -C3H5; Q = HCN, R’CN [74,75]; • dimethylplatinum derivatives with bidentate ligands: Me2Pt(Q), where Q = tmeda (N,N,N0,N0-tetramethylethylenediamine) [15], hd and its derivatives [77], nbd and its derivatives [78], cod and its derivatives [79,80] (Table1); • platinum homoleptic ω-alkenyl derivatives: Pt(X)2, where X = C6H11,C7H13 and C8H15 [81,82] (Table1). Alkylplatinum derivatives with monodentate ligands are among the first compounds synthesized and tested in MOCVD processes. Kumar et al. [74,75] were among the first who synthesized and studied the series of Me(R)PtQ2 compounds (R = Me, CO, HC = CH2, t 3 BuC≡C, ï -C3H5, Q = HCN, R’CN) by spectral methods in 1989. Compounds were obtained with yields up to 60% under inert conditions by exchange reactions using cis- [PtMe(Cl)(SMe2)] as an initial reagent. Most of Me(R)PtQ2 is a white or light-yellow powder unstable in air. Due to the complexity of synthesis and the high content of carbon impurities (up to 40 at.%) in the resulting films [74,75], Me(R)PtQ2 precursors have not found a real application in MOCVD processes. The first example of a dimethylplatinum derivative with a bidentate ligand, namely Me2Pt(cod), was presented as a MOCVD precursor in the same paper by Kumar et al. [38] This direction turned out to be more promising, and the chemistry of these derivatives has been intensively developing over the past decade. To date, a wide range of Me2Pt(Q) compounds, where Q is some olefins, namely, hd, nbd, cod, and their derivatives [77–79], has been synthesized. In addition, an example of the Me2Pt(Q) compound with Q = tmeda (N,N,N0,N0-tetramethylethylenediamine) is also known [15]. The developed approaches to the synthesis of Me2Pt(Q) compounds are summarized in Figure9. Note that the yields of Me2Pt(Q) complexes do not exceed 60% using approaches based on interactions of Pt(Q)Cl2 or Pt(Q)I2 intermediates with alkyl sources (MeLi, Cu2LiMe, MeMgX) (Figure9). On the Coatings 2021, 11, x FOR PEER REVIEWcontrary, the approach based on a single step interaction of Pt(acac)2 with AlMe3 in18 theof 39

presence of Q allows us to achieve yields of the Me2Pt(Q) compounds of up to 85% [77,78].

FigureFigure 9. 9.General General scheme scheme of of the the synthesis synthesis of of Me Me2Pt(Q)2Pt(Q) using using Me Me22Pt(cod)Pt(cod) asas anan example.example.

AccordingAccording to to single-crystal single-crystal XRD XRD data, data, the platinum the platinum atom in atom the molecules in the molecules of Me2Pt(Q) of compoundsMe2Pt(Q) compounds is located is in located a tetrahedral in a tetrahedral environment environment with PtC with2C’ PtC2 (C’2C’ is2 (C’ the is center the center of C=Cof C=C bond) bond) coordination coordination core core (Figure (Figure 10 10),), andand Pt–CPt–CMe bondbond lengths lengths is isapproximately approximately 0.1 0.1Å shorter Å shorter than than Pt–C Pt–CC=C ones.C=C ones. This trend This trend also persists also persists for Me for2Pt(cod) Me2Pt(cod) [80], while [80], the while replace- the replacementment of Q from of Q nbd from to nbd cod tois codacco ismpanied accompanied by a shortening by a shortening of the ofPt–C theMe Pt–C andMe Pt–CandC=C Pt–Cdistances.C=C distances.

Figure 10. Molecular structure of Me2Pt(nbd).

The platinum homoleptic ω-alkenyl derivatives Pt(X)2, X = C6H11, C7H13 and C8H15 (Table 1) are synthesized by interaction of (cod)Pt(Hal)2 with the corresponding LiX salts or Grignard reagents (XMgBr) in an inert atmosphere [81,82]. When using Grignard reagents, the target products Pt(C6H11)2, Pt(C7H13)2, and Pt(C8H15)2 are obtained in a mixture with by-products such as Pt(cod)(C7H13)2, Pt(cod)(Br)(C6H11). The use of LiX reagents leads to an increase in the yields of Pt(X)2 compounds to 60%. Compounds Pt(C6H11)2, Pt(C7H13)2 and Pt(C8H15)2 adopt solid-state structures with (idealized) C2 symmetries, in which the two α-carbon atoms are mutually cis (Figure 11). Significant influence of steric and packing effects on the conformations of Pt(C6H11)2, Pt(C7H13)2 and Pt(C8H15)2 suggest that the oleﬁn−Pt interactions are relatively weak. In- deed, Pt(C6H11)2 and Pt(C8H15)2 compounds are dynamic in solution, i.e., C=C bonds reversibly decomplex at the rates that are fast on the NMR time scale, while Pt(C7H13)2 shows the slowest decomplexation rates and the greatest thermal stability.

Coatings 2021, 11, x FOR PEER REVIEW 18 of 39

Figure 9. General scheme of the synthesis of Me2Pt(Q) using Me2Pt(cod) as an example.

According to single-crystal XRD data, the platinum atom in the molecules of Me2Pt(Q) compounds is located in a tetrahedral environment with PtC2C’2 (C’ is the center of C=C bond) coordination core (Figure 10), and Pt–CMe bond lengths is approximately 0.1 Coatings 2021, 11, 78 Å shorter than Pt–CC=C ones. This trend also persists for Me2Pt(cod) [80], while the replace-18 of 36 ment of Q from nbd to cod is accompanied by a shortening of the Pt–CMe and Pt–CC=C distances.

Figure 10. Molecular structure of Me2Pt(nbd). Figure 10. Molecular structure of Me2Pt(nbd).

The platinumThe platinum homoleptic homoleptic ω-alkenylω-alkenyl derivatives derivatives Pt(X) Pt(X)2, X =2, C X6 =H11 C,6 HC711H,C13 7andH13 Cand8H15 C 8H15 (Table(Table 1) are1 )synthesized are synthesized by interaction by interaction of (cod)Pt(Hal) of (cod)Pt(Hal)2 with2 with the corresponding the corresponding LiX LiX salts salts or or GrignardGrignard reagents reagents (XMgBr) (XMgBr) in inan an inert inert atmosphere atmosphere [81,82]. [81,82 ].When When using using Grignard Grignard rea- reagents, gents,the the target target products products Pt(C Pt(C6H6H1111))22,, Pt(CPt(C77H1313))22,, and and Pt(C Pt(C88HH1515)2) 2areare obtained obtained in in a a mixture mixture with with by-productsby-products such such as as Pt(cod)(C Pt(cod)(C7H7H1313)2,) 2Pt(cod)(Br)(C, Pt(cod)(Br)(C6H611H).11 The). The use use of LiX of LiX reagents reagents leads leads to to an increasean increase in the in theyields yields of Pt(X) of Pt(X)2 compounds2 compounds to 60%. to 60%. CompoundsCompounds Pt(C6H Pt(C11)26, HPt(C11)27,H Pt(C13)2 7andH13 )Pt(C2 and8H Pt(C15)2 adopt8H15)2 solid-stateadopt solid-state structures structures with with (idealized)(idealized) C2 symmetries, C2 symmetries, in which inthe whichtwo α-carbon the two atomsα-carbon are mutually atoms cis are (Figure mutually 11). cis Significant(Figure influence 11). Significant of steric influenceand packing of steric effects and on packing the conformations effects on the of conformations Pt(C6H11)2, of Pt(C7HPt(C13)2 6andH11 )Pt(C2, Pt(C8H157)H2 13suggest)2 and that Pt(C 8theH15 ole)2 fisuggestn−Pt interactions that the olefin are −relativelyPt interactions weak. In- are rela- deed, tivelyPt(C6H weak.11)2 and Indeed, Pt(C8H Pt(C15)2 6compoundsH11)2 and Pt(C are8 Hdynamic15)2 compounds in solution, are i.e., dynamic C=C bonds in solution, re- i.e., Coatings 2021, 11, x FOR PEERversibly REVIEWC=C decomplex bonds reversibly at the rates decomplex that are fast at theon the rates NMR that time are fastscale, on while the NMR Pt(C7H time13)2 shows scale,19 while of 39

the slowestPt(C7H decomplexation13)2 shows the slowestrates and decomplexation the greatest thermal rates and stability. the greatest thermal stability.

6 11 2 7 13 2 Figure 11. Molecular structures of Pt(C 6HH11)) 2(a(a) )and and Pt(C Pt(CH7H13) )(2b).(b ).

Platinum complexes complexes with with organic organic O,N,S,Se- O,N,S,Se-donordonor ligands ligands are are represented represented by the by fol- the lowingfollowing five five classes: classes: β-diketonatesβ-diketonates [83–86], [83–86 β],-iminoketonatesβ-iminoketonates [87,88], [87,88 β],-alkenolsβ-alkenols [89], [89 α],- aminoalcoholatesα-aminoalcoholates [90], [90 and], and dithio/diselen dithio/diselenoimidodiphosphinatesoimidodiphosphinates [91] [91 (Table] (Table 1).1 ). Although the first first mention of the successful isolation of a volatile coordination coordination compound ofof platinum,platinum, namely,namely, Pt(acac) Pt(acac)2,2 was, was published published in in the the first first half half of theof the 20th 20th century, century, the thesearch search for synthesisfor synthesis methods methods aimed aimed at increasing at increasing the yields the yields of platinum of platinumβ-diketonate β-diketonate com- complexesplexes is still is relevant. still relevant. In fact, In this fact, first this synthetic first synthetic approach approach that consisted that consisted in to the reaction in to the of reactionK2PtCl4 withof K2 H(acac)PtCl4 with in an H(acac) alkaline in medium an alkaline gives medium a yield of gives 33%. a The yield most of successful33%. The most strat- 2+ successfulegy, apparently, strategy, is the apparently, synthesis is through the synthesis the formation through ofthe a formation labile aqua-ion of a labile Pt[(H aqua-ion2O)4] , whichPt[(H2O) is4 then]2+, which introduced is then into introduced the reaction into withthe reaction potassium withβ potassium-diketonate. β-diketonate. This way allows This wayobtaining allows the obtaining target complexes the target with complexes yields up with to 95% yields [83 up,84 ,to92 ].95% Since [83,84,92]. the formation Since the of such formation of such an ion is possible only in an acidic medium, this approach is not applicable for obtaining platinum(II) complexes with other organic ligands (Table 1). The platinum complexes with β-iminoketonate, β-alkenol, α-aminoalcoholate, and dithio/diselenoimidodiphosphinate ligands (Table 1) are synthesized by substitution reactions with appropriate ligands in neutral or alkaline media using K2PtCl4 or cis-(Py)2PtI2 (Py = pyridine) as initial reagents, and the yields do not exceed 50%. Regardless of the class of platinum(II) complexes, all the above complexes have monomeric molecular structures, in which platinum ions are in a plane-square geometry (Figure 12). In the case of asymmetric ligands, the formation of cis,trans-isomeric platinum(II) complexes is observed [83,87,88,93].

Coatings 2021, 11, x FOR PEER REVIEW 19 of 39

Figure 11. Molecular structures of Pt(C6H11)2 (a) and Pt(C7H13)2 (b).

Platinum complexes with organic O,N,S,Se-donor ligands are represented by the following five classes: β-diketonates [83–86], β-iminoketonates [87,88], β-alkenols [89], α- aminoalcoholates [90], and dithio/diselenoimidodiphosphinates [91] (Table 1). Although the first mention of the successful isolation of a volatile coordination compound of platinum, namely, Pt(acac)2, was published in the first half of the 20th century, the search for synthesis methods aimed at increasing the yields of platinum β-diketonate complexes is still relevant. In fact, this first synthetic approach that consisted in to the reaction of K2PtCl4 with H(acac) in an alkaline medium gives a yield of 33%. The most Coatings 2021, 11, 78 successful strategy, apparently, is the synthesis through the formation of a labile aqua-ion19 of 36 Pt[(H2O)4]2+, which is then introduced into the reaction with potassium β-diketonate. This way allows obtaining the target complexes with yields up to 95% [83,84,92]. Since the formation of such an ion is possible only in an acidic medium, this approach is not applicable an ion is possible only in an acidic medium, this approach is not applicable for obtaining for obtaining platinum(II) complexes with other organic ligands (Table 1). The platinum platinum(II) complexes with other organic ligands (Table1). The platinum complexes with complexes with β-iminoketonate, β-alkenol, α-aminoalcoholate, and dithio/dise- β-iminoketonate, β-alkenol, α-aminoalcoholate, and dithio/diselenoimidodiphosphinate lenoimidodiphosphinate ligands (Table 1) are synthesized by substitution reactions with ligands (Table1) are synthesized by substitution reactions with appropriate ligands in appropriate ligands in neutral or alkaline media using K2PtCl4 or cis-(Py)2PtI2 (Py = pyri- neutral or alkaline media using K PtCl or cis-(Py) PtI (Py = pyridine) as initial reagents, dine) as initial reagents, and the yields2 4 do not exceed2 250%. and the yields do not exceed 50%. Regardless of the class of platinum(II) complexes, all the above complexes have mon- Regardless of the class of platinum(II) complexes, all the above complexes have omeric molecular structures, in which platinum ions are in a plane-square geometry monomeric molecular structures, in which platinum ions are in a plane-square geom- (Figure 12). In the case of asymmetric ligands, the formation of cis,trans-isomeric plati- etry (Figure 12). In the case of asymmetric ligands, the formation of cis,trans-isomeric num(II) complexes is observed [83,87,88,93]. platinum(II) complexes is observed [83,87,88,93].

Figure 12. Molecular structures of Pt(II) complexes: cis-Pt(tfac)2 (a), trans-Pt(i-tfac)2 (b) Pt(amN(Me)2)2 (c), and Pt(alk(CF3))2 (d). The hydrogen atoms are omitted for clarity.

Thermal properties of platinum(II) precursors Despite the variety of synthesized organoplatinum compounds, the data on their thermal properties are almost absent and presented only by melting characteristics and thermogravimetry results [75,78,79]. The melting points of Me2Pt(Q) compounds can vary over a wide range depending on the Q ligand: from liquids (Q = cod derivatives) [79] and low-melting compounds (31–49 ◦C, Q = hd derivatives) [77] to solids (80–110 ◦C, Q = nbd derivatives) [78]. The melting points of platinum homoleptic ω alkenyl derivatives Pt(X)2 increase with the growth of the carbon chain in the X ligand: from liquid (Pt(C6H11)2, ◦ freezing point −15 C) to solid Pt(C7H13)2 and Pt(C8H15)2 compounds with melting points 20 ◦C and 46–47 ◦C, respectively [81,82]. The majority of Me(R)PtQ2 and Me2Pt(Q) compounds decompose when heated. The Me(R)PtQ2 and Me(R)PtQ2 compounds with Q = RCN, nbd and their derivatives pos- sess the lowest thermal stability, while Me2Pt(cod) and related compounds are partially decomposes when heated. According to [75], the decomposition of Me(R)Pt(R’CN)2 com- 3 pounds (R = Me, C = CH, ï -C3H5, R’ = H, Me) in the condensed phase is accompanied by the elimination of the R’CN ligand and its subsequent oligomerization. The analysis of gaseous products (methane, ethane, hexadiene, and butadiene) indicates elimination 3 and further recombination of the alkyl radical R (Me, HC = CH2, ï -C3H5). Decomposition Pt(C5H9)2 in aromatic solvents revealed the formation of a transient three-coordinate plat- Coatings 2021, 11, 78 20 of 36

inum hydride [81] that makes PtNPs deposited from Pt(C5H9)2 a promising catalyst for the hydrogenation of chloronitrobenzene. Thermal properties of volatile platinum complexes with organic O,N,S,Se-donor ligands, viz., Pt(β-diketonate)2, Pt(β-iminoketonato)2, Pt(β-alkenols)2, Pt(amN(Me)2)2, are studied mainly by the thermogravimetry method [83–90]. Most of all platinum (II) complexes of the classes considered above have a melting point above 100 ◦C. In contrast to organoplatinum compounds, platinum(II) complexes usually pass into the gaseous phase almost quantitatively under the conditions of TG experiments (inert atmosphere), with the exception of Pt(acac)2 and Pt(alk(Me))2 complexes. A tendency to increase the stability during vaporization with the introduction of a ﬂuorinated substituent into the ligand was shown in the series of Pt(β-diketonate)2 and Pt(β-alkenols)2. A comparison of a series of platinum complexes with ligands of different classes but containing the same terminal substituent (Me) shows that the volatility of platinum complexes decreases in the order Pt(amN(Me)2)2 > Pt(acac)2 > Pt(alk(Me))2. The lower volatility of the Pt(alk(Me))2 complex can be clearly explained by the presence of an aromatic fragment in the β-alkenol ligands. To study the effect of the ligand on the volatility of platinum(II) complexes in detail, quantitative data on the volatility of the complexes are necessary. Until now, the volatilization processes have been quantitatively studied using ten- simetic data (p-T dependences of saturated vapor pressure) only for two organoplatinum compounds, one platinum(II) β-iminoketonate and several platinum(II) β-diketonate complexes. This information is summarized in Table5 and Figure 13. The partial vapor pressure −6 of Me2Pt(MeCN)2 measured only at 298 K was 3.3 × 10 Torr [75].

Table 5. Equations of the temperature dependences of saturated vapor pressure over the solid (sublimation, subl.) and 0 liquid (vaporization, vap.) Pt(II) compounds. The sublimation and vaporization enthalpies (∆HT*) and entropies (∆S T*) 5 are referred to as the average temperature (T*) of the interval measured (∆T), p0 = 760 Torr = 1 atm = 10 Pa. The ligand abbreviations correspond to Table1.

ln(p/p ) = A − B/(T,K) ∆H , ∆S0 , Compounds Process ∆T,K 0 T* T* Ref. kJ·mol−1 · · −1 AB J (mol K)

[94] Me2Pt(cod) Subl. 343–353 17.2 5441 45.2 142.9 [82] Pt(C7H13)2 Subl. – 14.2 5520 44.1 118.2 [94] Subl. 323–358 29.30 9320 77 ± 2 243.5 Pt(hfac) Subl. 377–419 21.9 10,520 83.6 ± 0.4 183 [95] 2 Vap. 419–439 12.9 6670 52.8 ± 1.5 107.2 cis-Pt(tfac) Subl. 412–461 23.8 12,900 106.6 ± 0.4 196.46 ± 4.59 [85] 2 trans-Pt(tfac)2 Subl. 437–506 23.14 13,210 109.93 ± 2.92 192.28 ± 5.43 [95] Pt(thd)2 Vap. 447–467 16.2 9280 77.1 ± 0.7 134.3 ± 1.6 Subl. 393–453 23.40 14,136 117.52 ± 1.38 117.52 ± 1.38 Pt(acac) [85] 2 Subl. 332–399 22.15 13,391 111.33 ± 0.81 184.18 ± 2.20 Pt(i-tfac)2 Subl. 393–453 23.62 13,429 111.54 ± 4.17 196.20 ± 9.82

In general, the vapor pressure of organoplatinum compounds is several orders of magnitude higher than that of platinum complexes. The volatility of platinum complexes with organic ligands depends on the combination of terminal groups in the ligand and to a lesser extent on the combination of donor atoms in the coordination environment of platinum. Like iridium(III) β-diketonates, the vapor pressure of platinum(II) β-diketonates t increases by more than 2–3 orders of magnitude when CF3 or Bu groups are introduced into ligands [85]. For example, at 353 K, the vapor pressures of Pt(hfac)2 and Pt(acac)2 are −1 −4 10 and 10 Torr, respectively. In contrast, Pt(tfac)2 and Pt(i-tfac)2 complexes, which differ only in the combination of donor atoms in ligands ((OˆO) vs. (OˆNH)), have a very close volatility (at 393 K): 2 × 10−2 vs. 1.5 × 10−2 Torr, respectively [85]. Coatings 2021, 11, x FOR PEER REVIEW 21 of 39

[94] Subl. 323–358 29.30 9320 77 ± 2 243.5 Pt(hfac)2 Subl. 377–419 21.9 10,520 83.6 ± 0.4 183 [95] Vap. 419–439 12.9 6670 52.8 ± 1.5 107.2 cis-Pt(tfac)2 Subl. 412–461 23.8 12,900 106.6 ± 0.4 196.46 ± 4.59 [85] trans-Pt(tfac)2 Subl. 437–506 23.14 13,210 109.93 ± 2.92 192.28 ± 5.43 [95] Pt(thd)2 Vap. 447–467 16.2 9280 77.1 ± 0.7 134.3 ± 1.6 Coatings 2021, 11, 78 Subl. 393–453 23.40 14,136 117.52 ± 1.38 117.52 ± 1.38 21 of 36 Pt(acac)2 [85] Subl. 332–399 22.15 13,391 111.33 ± 0.81 184.18 ± 2.20 Pt(i-tfac)2 Subl. 393–453 23.62 13,429 111.54 ± 4.17 196.20 ± 9.82

Figure 13. 13. TemperatureTemperature dependences dependences of ofthe the saturated saturated vapor vapor pressure pressure over over Pt(II) Pt(II) compounds. compounds.

InThe general, information the vapor on pressure the decomposition of organoplatinum of vapors compounds of platinum(II) is several compounds orders of is magnitude higher than that of platinum complexes. The volatility of platinum complexes highly sporadic. According to [96], the pyrolysis of Me2Pt(cod) vapors in a high vac- with organic −ligands8 depends on the combination of terminal groups in the ligand and to uum (5 × 10 Torr) is accompanied by the loss of the CH3· radical, which reacts actively witha lesser an extent organic on substrate the combination and recombines of donor withatoms PtMe. in the The coordination introduction environment of a radical of trap platinum.minimizes Like vapor iridium(III) recombination β-diketonates, and promotes the vapor an increase pressure in of the platinum(II) growth rate β-diketo- of the nano- nates increases by more than 2–3 orders of magnitude when CF3 or tBu groups are intro- material. In the presence of hydrogen, Me2Pt(cod) vapor decomposition is accompanied by duced into ligands [85]. For example, at 353 K, the vapor pressures of Pt(hfac)2 and the release of methane and cod ligands [15]. In an oxidative atmosphere, oxygen dissociates Pt(acac)2 are 10−1 and 10−4 Torr, respectively. In contrast, Pt(tfac)2 and Pt(i-tfac)2 complexes, into adsorbed Me Pt(cod-R) precursors and binds to platinum, releasing methane. On the which differ only in2 the combination of donor atoms in ligands ((OÔ) vs. (O^NH)), have second step, the organic ligands are decomposed and platinum species are created [79]. De- a very close volatility (at 393 K): 2 × 10−2 vs. 1.5 × 10−2 Torr, respectively [85]. composition of adsorbed platinum compounds with ω-alkenyl ligands is accompanied by The information on the decomposition of vapors of platinum(II) compounds is highly the α-elimination and decomplexation of the olefin groups from Pt, resulting in the forma- sporadic. According to [96], the pyrolysis of Me2Pt(cod) vapors in a high vacuum (5 × 10−8 tion of highly reactive platinum species that serve as nucleation initiators [82]. The ligand Torr) is accompanied by the loss of the CH3∙ radical, which reacts actively with an organic substratestructure and affects recombines the nature with of gaseousPtMe. The byproducts. introduction In of particular, a radical thetrap main minimizes gaseous vapor product recombinationof vapor decomposition and promotes of Pt(C an 7increaseH13)2 is in the th hydrogenatede growth rate of ligand the nanomaterial. [82], while both In the the hy- drogenated ligand and its dehydrogenated fragments (pentandiene isomers) are observed presence of hydrogen, Me2Pt(cod) vapor decomposition is accompanied by the release of methaneat decomposition and cod ligands of Pt(C [15].5H9) 2Invapors an oxidativ [97].e The atmosphere, rapid desorption oxygen dissociates of most of into byproducts ad- sorbedin the presenceMe2Pt(cod-R) of a precursors reactive gas and leads binds to to the platinum, growth releasing of active methane. Pt surface-demonstrating On the second step,autocatalytic the organic behavior. ligands are decomposed and platinum species are created [79]. Decom- positionUnlike of adsorbed organoplatinum platinum compounds, compounds with platinum ω-alkenyl forms ligands strong is bonds accompanied with ligands by the in the −1 αcoordination-elimination and compounds. decomplexation For example, of the olefin thebinding groups from energy Pt, resulting of Pt–Oacac in the(180 formation kJ·mol ) is −1 ofcomparable highly reactive with thatplatinum of C–C species bonds (140that kJserve·mol as )[nucleation15]. This meansinitiators that [82]. the The decomposition ligand structureof adsorbed affects vapors the nature of Pt(acac) of gaseous2 occur byprod withoutucts. ligand In particular, decoordination, the main which gaseous causes prod- a large uctamount of vapor of carbon decomposition in the films of Pt(C [15].7H A13 similar)2 is the conclusionhydrogenated has ligand been formulated [82], while both for Pt(hfac) the 2 hydrogenatedusing the mass ligand spectrometry and its dehydrogenated data since the highly fragments intense (pentandiene [(CO)Pt(hfac)] isomers)+ fragment are ob- was registered [94]. Moreover, the fluorosubstitution in the ligand of platinum β-diketonates leads to an increase of the vapor thermal stability. Specifically, the full decomposition of ◦ Pt(hfac)2 vapors to metallic platinum and gaseous carbon oxides takes place only at 527 C as it has been shown using temperature-programmed reaction spectroscopy. Note that the temperature of decomposition of Pt(hfac)2 vapors can be reduced by using copper substrates, which is due to the formation of Cu-hfac fragments during the adsorption of Pt(hfac)2 [98].

2.2.3. Platinum(IV) Precursors The volatile platinum(IV) precursors are represented by derivatives of trimethylplat- X X Me Et inum with cyclopentadienyl Me3Pt(Cp ) (Cp = Cp, Cp , Cp ) or β-diketonate ligands Me3Pt(L)(H2O) (L = hfac, tfac, ptac, btfac), Me3Pt(L)(Py) (Py = pyridine, L corresponds to Table1). Coatings 2021, 11, x FOR PEER REVIEW 22 of 39

served at decomposition of Pt(C5H9)2 vapors [97]. The rapid desorption of most of byproducts in the presence of a reactive gas leads to the growth of active Pt surface-demonstrating autocatalytic behavior. Unlike organoplatinum compounds, platinum forms strong bonds with ligands in the coordination compounds. For example, the binding energy of Pt–Oacac (180 kJ∙mol−1) is comparable with that of C–C bonds (140 kJ∙mol−1) [15]. This means that the decomposition of adsorbed vapors of Pt(acac)2 occur without ligand decoordination, which causes a large amount of carbon in the films [15]. A similar conclusion has been formulated for Pt(hfac)2 using the mass spectrometry data since the highly intense [(CO)Pt(hfac)]+ fragment was registered [94]. Moreover, the fluorosubstitution in the ligand of platinum β-diketonates leads to an increase of the vapor thermal stability. Specifically, the full decomposition of Pt(hfac)2 vapors to metallic platinum and gaseous carbon oxides takes place only at 527 °C as it has been shown using temperature-programmed reaction spectroscopy. Note that the temperature of decomposition of Pt(hfac)2 vapors can be reduced by using copper substrates, which is due to the formation of Cu-hfac fragments during the adsorption of Pt(hfac)2 [98].

Coatings 2021, 11, 78 2.2.3. Platinum(IV) Precursors 22 of 36 The volatile platinum(IV) precursors are represented by derivatives of trimethylplatinum with cyclopentadienyl Me3Pt(CpX) (CpX = Cp, CpMe, CpEt) or β-diketonate ligands The MeMePt(Cp3Pt(L)(HX)2O) complexes (L = hfac, withtfac, ptac, 50% btfac), yields Me are3Pt(L)(Py) synthesized (Py = bypyridine, exchange L corresponds reactions to Table3 1). using [Me3PtI]4 as an initial reagent [99]. An alternative “one-pot” technique based on The Me3Pt(CpX) complexes with 50% yields are synthesized by exchange reactions the interaction of K PtCl with CH Br and MeLi with the subsequent addition of NaCpX using [Me23PtI]4 6as an initial2 reagent2 [99]. An alternative “one-pot” technique based on the X allows to obtain Me Pt(Cp ) complexes with yields of 60% [15]. X interaction3 of K2PtCl6 with CH2Br2 and MeLi with the subsequent addition of NaCp al- Two syntheticlows to obtain approaches Me3Pt(Cp toX) complexes obtain Me with3Pt(L)A yields (A of =60% H 2[15].O, Py) complexes with β- diketonate ligandsTwo aresynthetic known. approaches The earlier to obtain one isMe similar3Pt(L)A to (A those = H2O, described Py) complexes in [99 ]with and β- based on thediketonate reaction ligands of [Me are3PtI] known.4 with The potassium earlier oneβ-diketonates is similar to [those100,101 described]. A shift in in[99] the and equilibriumbased towards on the the reaction target Meof [Me3Pt(L)A3PtI]4 with product potassium is achieved β-diketonates by AgF [100,101]. introducing, A shift which in the leads to theequilibrium formation towards of an AgIthe target precipitate. Me3Pt(L)A Thus, product the is use achieved of AgF by makes AgF introducing, it possible which to leads to the formation of an AgI precipitate. Thus, the use of AgF makes it possible to obtain Me3Pt(L)A complexes with yields up to 85%, whereas without this reagent, the obtain Me3Pt(L)A complexes with yields up to 85%, whereas without this reagent, the yields do not exceed 50%. This approach was used mainly for the synthesis of Me3Pt(L)A complexesyields with ﬂuorinateddo not exceedβ 50%.-diketonate This approach ligands. was An used alternative mainly for technique the synthesis based of Me on3Pt(L)A the complexes with fluorinated β-diketonate ligands. An alternative technique based on the interaction of [Me3PtI]4 with lead β-diketonates with the formation of a PbI2 precipitate is interaction of [Me3PtI]4 with lead β-diketonates with the formation of a PbI2 precipitate is universal and allows obtaining Me3Pt(L)Py complexes with yields up to 90% [102]. universal and allows obtaining Me3Pt(L)Py complexes with yields up to 90% [102]. All Pt(IV) compoundsAll Pt(IV) compounds under consideration under consideration are monomeric are monomeric molecular molecular complexes. complexes. Ac- Ac- X cording tocording single-crystal to single-crystal XRD data, XRD platinum data, platinum atoms atoms in Me in3 Pt(CpMe3Pt(Cp) moleculesX) molecules lie lie above above a a point approximatelypoint approximately located located in the in the centers centers of of the the ﬁve-membered five-membered cyclopentadienyl cyclopentadienyl rings rings [103][103] (Figure (Figure 14a). 14a). Since Since a β a-diketonate β-diketonate ligand ligand isis onlyonly bidentate, bidentate, a afree free coordination coordination posi- position remainstion remains in the in “Me the3 Pt(“Meβ3-diketonato)”Pt(β-diketonato)” fragment. fragment. A A donor donor molecule, molecule, namelynamely water or or pyridine,pyridine, complements complements this position. this position. In such In adducts,such adducts, the platinumthe platinum cation cation is located is located in in a a distorted octahedraldistorted octahedral environment environment (Figure (Figure14b,c). 14b,c).

Me Figure 14. Molecular structures of Pt(IV) complexes: Me3Pt(Cp )(a), Me3Pt(acac)Py (b), Me3Pt(hfac)H2O(c).

Thermal properties of platinum(IV) precursors X Thermal properties of Me3Pt(Cp ) and Me3Pt(L)A (A = H2O, Py) complexes are X studied in detail in Refs. [15,100–102] Cyclopentadienyl complexes Me3Pt(Cp ) are low- melting, and the introduction of an alkyl substituent X and an increase in its carbon chain is accompanied by a decrease in melting temperatures. For example, complexes with CpMe Et ◦ ◦ and Cp melt at 30 C and −78 C, respectively [15]. The pyridine adducts of Me3Pt(L)Py ◦ are characterized by the lower melting points (30–97 C) compared to their Me3Pt(L)H2O analogues (>100 ◦C). X In contrast to Me2Pt(Q) compounds, the most of Me3Pt(Cp ) and Me3Pt(L)Py complexes pass into the gas phase almost quantitatively under the conditions of TG experiments (inert atmosphere). Only ﬂuorine-free Me3Pt(L)Py compounds partially decompose with a mass loss of 20%–30% when heated. The appearance of the steps corresponding to pyridine loss on TG curves of Me3Pt(zif)Py and Me3Pt(ttfa)Py complexes indicates that the processes of vaporization and decomposition of these complexes proceed in parallel. The thermal stability of Me3Pt(L)A complexes with the same β-diketonate ligands deteriorates when the neutral Py ligand is replaced with H2O[100,101]. The quantitative data on volatilization processes of several cyclopentadienyl complexes X of the series Me3Pt(Cp ) and Me3Pt(L)Py complexes and one Me3Pt(hfac)(H2O) complex that has the highest thermal stability are presented in Table6. In general, the vapor pressure X of complexes differs by three orders of magnitude in the transition from Me3Pt(Cp ) to Me3Pt(L)A, with cyclopentadienyl complexes being the most volatile (Figure 15). For exam- Coatings 2021, 11, 78 23 of 36

ple, the vapor pressures for Me3Pt(Cp), Me3Pt(hfac)H2O, Me3Pt(hfac)Py, and Me3Pt(acac)Py, measured at the same temperature of 323 K, are 5 × 10−1, 1.5 × 10−2, 6.5 × 10−2, and 10−4 Torr, respectively.

Table 6. Equations of the temperature dependences of saturated vapor pressure over the solid (sublimation, subl.) and 0 liquid (vaporization, vap.) Pt(II) compounds. The sublimation and vaporization enthalpies (∆HT*) and entropies (∆S T*) 5 are referred to the average temperature (T*) of the interval measured (∆T), p0 = 760 Torr = 1 atm = 10 Pa. The ligand abbreviations correspond to Table1.

ln(p/p ) = A − B/(T,K) ∆H , 0 Compounds ∆T,K 0 T* ∆S T*, Ref. Process · −1 · · −1 AB kJ mol J (mol K)

Me3Pt(Cp) Subl. – 25.9 8590 71.4 215 [15] Me Me3Pt(Cp ) Subl. – 26.1 8690 72.21 217 [95] Me3Pt(hfac)H2O Subl. 338–365 32.78 14,103 117.2 ± 0.5 272.4 ± 1.5 Me3Pt(hfac)Py Vap. 343–393 11.23 6787 56.4 ± 0.5 93.3 ± 1.5 [100] Me3Pt(tfac)Py Vap. 348–393 18.51 10,060 83.6 ± 0.3 153.8 ± 0.8 Me3Pt(thd)Py Vap. 366–403 12.72 8147 67.7 ± 0.7 105.7 ± 1.8 Me Pt(ptac)Py Subl. 309–343 34.17 15,631 129.9 ± 2.5 284.2 ± 7.7 [102] 3 Me3Pt(zis)Py Subl. 326–353 32.79 15,896 132.1 ± 1.8 272.5 ± 5.3 ± ± Coatings 2021, 11, x FORMe PEERPt(acac)Py REVIEW Subl. * – 12407 23.46 103.1 0.7 195.0 241.3 of 39 [104] 3 Vap. 390–414 9699 16.47 80.6 ± 0.5 136.9 ± 1.2 * Calculated from differential scanning calorimetry data.

FigureFigure 15. TemperatureTemperature dependences of of the saturated vapor pressure over Pt(IV) complexes and Pt(acac)2 for comparison Pt(acac)2 for comparison

TheThe introduction introduction of of Me Me substituent substituent into into the the cyclopentadienyl cyclopentadienyl ligand ligand leads leads to an to ap- an proximately threefold increase in the vapor pressure of Me3Pt(CpXX) complexes: 0.1 and approximately threefold increase in the vapor pressure of Me3Pt(Cp ) complexes: 0.1 and 0.044 Torr at 300 K for Me3Pt(CpMe)Me and Me3Pt(Cp), respectively. The saturated vapor pres- 0.044 Torr at 300 K for Me3Pt(Cp ) and Me3Pt(Cp), respectively. The saturated vapor 3 surepressure (in Torr) (in Torr) of Me ofPt(L)Py Me3Pt(L)Py complexes complexes measured measured at 323 at 323K decreases K decreases in the in thefollowing following order:order: 6.5 6.5× 10×−2 10(L− =2 hfac)(L = > hfac) 2.5 × > 10 2.5−3 (L× =10 tfac)−3 (L > =2 × tfac) 10−3 >(L 2 =× ptac)10− 3> (L8 × = 10 ptac)−4 (L >= thd) 8 × >10 5− ×4 10(L−4 = (L thd) = acac) > 5 >× 2 10× 10−4−4(L (L= = acac)zis). The > 2 obtained× 10−4 (Lorder = zis). is in The a good obtained correlation order with is in TG agood data forcorrelation Me3Pt(L)Py with complexes. TG data for Thus, Me3 Pt(L)Pythe introduction complexes. of each Thus, CF the3 group introduction into the ofligand each (L CF =3 hfacgroup vs. into tfac the vs. ligand acac) (Lleads = hfac to vs.the tfacincrease vs. acac) of the leads vapor to the pressure increase by of about the vapor 0.5–1 pressure order, t t whileby about replacing 0.5–1 order,one Bu while group replacing with (CH one3)2OCHBu group3 (L = thd with vs. (CH zis)3 )reduces2OCH3 (Lthe = volatility thd vs.zis) by fourreduces times. the Comparison volatility by of four the times. p(T) data Comparison for Me3Pt(L)Py of the pwith(T) data the forcorresponding Me3Pt(L)Py plati- with num(II)the corresponding β-diketonates, platinum(II) Pt(L)2, indicatesβ-diketonates, an increase Pt(L) in2, indicatesvolatility anby increaseapproximately in volatility two or- by dersapproximately of magnitude, two orderswith an of increase magnitude, in the with coordination an increase innumber the coordination of Pt. For numberexample, of the Pt. −2 −2 vaporFor example, pressure the of vaporMe3Pt(acac)Py pressure at of 373 Me 3KPt(acac)Py is 4 × 10 Torr, at 373 while K is 4in× the10 caseTorr, of Pt(acac) while in2 this the −4 −4 valuecase of is Pt(acac)equal to2 5this × 10 value Torr. is equal to 5 × 10 Torr. TheThe study ofof thermalthermal decomposition decomposition of of platinum(IV) platinum(IV) precursor precursor vapors vapors was was carried carried out outby severalby several scientiﬁc scientific groups groups [15 ,[15,100,102,1100,102,105,10605,106]] with with the the main main focus focus on cyclopentadienylon cyclopentadi- enylcomplexes. complexes. In the presence of hydrogen, the decomposition of Me3Pt(CpX) vapors via an oxidative addition on the platinum followed by a reductive elimination of cyclopentane and methane [15]. According to IR data, decomposition of absorbed {Me3Pt(CpMe)} complex molecules on Si substrate in the oxygen presence occurs through a formation of {Me2Pt(CpMe)} adsorbed species that further oxidize forming {Pt} species and releasing of CO2 and CH4 gases [105]. According to [106], the most kinetically favorable elimination pathway of Me3Pt(CpMe) vapors on the hydroxylated graphene surfaces also includes the formation of {Me2Pt(CpMe)} adsorbed species when methyl interacts with hydrogen from the surface OH-groups with the release of methane. Decomposition of vapors of Me3Pt(L)Py (L = acac, ptac, zis, zif) in the presence of hydrogen on SiO2/Si substrates is also accompanied by the release of methane (Figure 16). However, the most stable fragment in the gas phase recorded in all mass spectra of Me3Pt(L)Py complexes is [Me3Pt]+ [100,102]. This may indicate an increase in the strength of Pt–CMe bonds in Me3Pt(L)Py complexes in comparison with their cyclopentadienyl Me3Pt(CpX) analogues. Indeed, according to single-crystal XRD data, the Pt–CMe distances shorten by about 0.1 Å when moving from Me3Pt(CPX) to Me3Pt(L)A.

Coatings 2021, 11, 78 24 of 36

X In the presence of hydrogen, the decomposition of Me3Pt(Cp ) vapors via an oxidative addition on the platinum followed by a reductive elimination of cyclopentane Me and methane [15]. According to IR data, decomposition of absorbed {Me3Pt(Cp )} complex molecules on Si substrate in the oxygen presence occurs through a formation of Me {Me2Pt(Cp )} adsorbed species that further oxidize forming {Pt} species and releasing of CO2 and CH4 gases [105]. According to [106], the most kinetically favorable elimination Me pathway of Me3Pt(Cp ) vapors on the hydroxylated graphene surfaces also includes the Me formation of {Me2Pt(Cp )} adsorbed species when methyl interacts with hydrogen from the surface OH-groups with the release of methane. Decomposition of vapors of Me3Pt(L)Py (L = acac, ptac, zis, zif) in the presence of hydrogen on SiO2/Si substrates is also accompanied by the release of methane (Figure 16). However, the most stable fragment in the gas phase recorded in all mass + spectra of Me3Pt(L)Py complexes is [Me3Pt] [100,102]. This may indicate an increase in the strength of Pt–CMe bonds in Me3Pt(L)Py complexes in comparison with their cyclopen- Coatings 2021, 11, x FOR PEER REVIEW X 25 of 39 tadienyl Me3Pt(Cp ) analogues. Indeed, according to single-crystal XRD data, the Pt–CMe X distances shorten by about 0.1 Å when moving from Me3Pt(CP ) to Me3Pt(L)A.

Figure 16. The temperature dependencies of the most intense ion peaks in the mass-spectra of Figure 16. The temperature dependencies of the most intense ion peaks in the mass-spectra of Me3Pt(acac)Py vapors showing their decomposition on the heated surface in hydrogen presence. Me3Pt(acac)Py vapors showing their decomposition on the heated surface in hydrogen presence.

The main gaseous by-products of decomposition of Me3Pt(L)Py vapors in an oxida- The main gaseous by-products of decomposition of Me3Pt(L)Py vapors in an oxidative tive atmosphere are CO, CO2, H2O; the peak corresponding to [Me3Pt]+ + fragments are also atmosphere are CO, CO2,H2O; the peak corresponding to [Me3Pt] fragments are also detected in the mass spectra. In In a a recent wo workrk [102], [102], the effect of terminal groups in β- diketonate ligandsligands onon thethe binding binding strength strength of of platinum platinum with with neutral neutral ligands ligands in in the the series series of of Me3Pt(L)Py complexes (L = hfac, tfac, ttfac, ptac, acac, thd, zis) has been examined. It Me3Pt(L)Py complexes (L = hfac, tfac, ttfac, ptac, acac, thd, zis) has been examined. It has has been shown that the introduction of each CF3 group into L increased the Pt–N bond been shown that the introduction of each CF3 group into L increased the Pt–N bond energy byenergy ~2.5 by kJ/mol. ~2.5 kJ/mol.

2.2.4. Application of Platinum Precursors in MOCVD Processes Since 2000, MOCVD processes of fabrication of Pt nanomaterialsnanomaterials have been actively developing mainly in the direction of deposition of smooth layers for microelectronics and Pt nanoparticles forfor catalysis.catalysis. DueDue to to the the peculiarities peculiarities of of the the synthesis synthesis and and thermal thermal behavior behav- ofior volatile of volatile platinum platinum compounds, compounds, there there are are only only a a few few numbers numbers of of platinum platinum compoundscompounds used in MOCVD processes.processes. The mostmost widelywidely usedused PtPt precursorsprecursors areare MeMe22Pt(Q),Pt(Q), Pt(acac)Pt(acac)22, Me and Me33Pt(CpMe),), while while the the examples examples of applicationapplication ofof otherother compounds,compounds, e.g.,e.g.,Pt(C Pt(C55HH99))22, Pt(C7H1313))22, ,Pt(amN(Me) Pt(amN(Me)2)22),2 are, are sporadic sporadic (Table (Table 7).7). A modern trend trend in in the the field field of of platinum platinum MOCVD MOCVD is isthe the use use of ofnew new modifications modifications of ofthe the method method aimed aimed at atincreasing increasing the the efficiency efficiency of ofboth both the the mass mass transfer transfer of ofprecursors precursors to tothe the reaction reaction zone zone (fluidizes (fluidizes bed-MOCVD, bed-MOCVD, FB-MOCVD, FB-MOCVD, liquid liquid delivery-MOCVD, delivery-MOCVD, LD- MOCVD) and and the the deposition deposition process process itself itself by by means means of ofadditional additional activation activation (plasma-as- (plasma- assisted-MOCVD,sisted-MOCVD, PA-MOCVD, PA-MOCVD, ultraviolet ultraviolet stimulation-MOCVD, stimulation-MOCVD, UV-MOCVD). UV-MOCVD). The The FB- MOCVD has been successfully used for one-step preparation of Pt-supported catalysis with an average size of Pt 1–5 nm and a narrow size distribution. In addition, utilization of FB-MOCVD installations allows the use of Me2Pt(Q) precursors with a low thermal stability when heated. The LD-MOCVD was adapted to produce smooth Pt films on Si structures with trenches. PA-MOCVD is suitable for the preparation of smooth Pt films, since the use of plasma helps to reduce carbon impurities and achieve the lowest resistivity of the films [82]. Nowadays, platinum(II) acetylacetonate, Pt(acac)2, is the most demanded MOCVD precursor used for the deposition of platinum nanomaterials for various purposes, including catalysis [107], microelectrodes [108,109], and microelectronic [110] applications. Its relevance is related to its ability to change the morphology of Pt films deposited from Pt(acac)2 in a wide range depending on MOCVD parameters. In particular, according to the zone growth model, Pt films can be deposited with porous cauliflower (zone I, Th < 0.3), columnar (zone II, 0.3 < Th < 0.5), or equiaxed (zone III, Th > 0.5) structures (Th = Tdepo- sition/m.p.(Pt), m.p.(Pt) = 2045 K) can be deposited (Tdeposition—temperature of deposition process, m.p.—melting point) [111]. The features of Pt films growth depending on the deposition temperature, the type of gas reagent, and the substrate were studied in detail in

Coatings 2021, 11, 78 25 of 36

MOCVD has been successfully used for one-step preparation of Pt-supported catalysis with an average size of Pt 1–5 nm and a narrow size distribution. In addition, utilization of FB-MOCVD installations allows the use of Me2Pt(Q) precursors with a low thermal stability when heated. The LD-MOCVD was adapted to produce smooth Pt films on Si structures with trenches. PA-MOCVD is suitable for the preparation of smooth Pt films, since the use of plasma helps to reduce carbon impurities and achieve the lowest resistivity of the films [82]. Nowadays, platinum(II) acetylacetonate, Pt(acac)2, is the most demanded MOCVD precursor used for the deposition of platinum nanomaterials for various purposes, including catalysis [107], microelectrodes [108,109], and microelectronic [110] applications. Its relevance is related to its ability to change the morphology of Pt films deposited from Pt(acac)2 in a wide range depending on MOCVD parameters. In particular, according to the zone growth model, Pt films can be deposited with porous cauliflower (zone I, Th < 0.3), columnar (zone II, 0.3 < Th < 0.5), or equiaxed (zone III, Th > 0.5) structures (Th = Tdeposition/m.p.(Pt), m.p.(Pt) = 2045 K) can be deposited (Tdeposition—temperature of deposition process, m.p.—melting point) [111]. The features of Pt films growth depending on the deposition temperature, the type of gas reagent, and the substrate were studied in detail in several papers [111–113]. According to [112], the morphology of Pt films was mainly influenced by the nature of the growth surface rather than by the reaction atmosphere. The addition of H2O promotes homogeneous growth of Pt films, while the addition of oxygen increases the growth rates of Pt films, which is accompanied by an increase in the level of impurities. In the low-pressure MOCVD process, depending on the deposition conditions, the impurity level of Pt samples deposited from Pt(acac)2 can reach 50 at.% due to the high strength of Pt-O bonds in Pt(acac)2. The possibility of deposition of Pt films with a 95% step of coverage coating in Si trench structures from Pt(acac)2 using a LD-MOCVD method was reported in [110]. The Pt(acac)2 precursor is also of interest for the deposition of PtOx layers by ALD (atomic layer deposition) [114]. Me Et Note that in contrast to Pt(acac)2 the use of Me3Pt(Cp ) and its analogue Me3Pt(Cp ) in LD-MOCVD processes leads to the growth of layers with a moderate coverage step of 35%–51% in Si trench structures [110,115]. This may be due to the fact that during Me Et LD-MOCVD, the growth of Pt layers from Me3Pt(Cp ) and Me3Pt(Cp ) precursors is Me limited by mass transfer. According to [82], Me3Pt(Cp ) exhibits long nucleation delays in CVD, which leads to the formation of smoothed layers. This feature determines its demand for ALD applications [114]. Therefore, the precursors of this type are typically applied to produce smooth films with low resistance values, as well as Pt nanoparticles. In the last decade, interest in the deposition of Pt layers from Me3Pt(L)Py in oxidative and reducing atmospheres has increased markedly [100,116]. Comparison of data on the deposition of Pt coatings in hydrogen from Me3Pt(L)Py (L = acac, hfac) indicates an increase in the stability of precursor vapors when fluorinated groups are introduced into the ligand, which makes it possible to achieve relatively high growth rates of Pt layers up to 3.2 nm/min when using Me3Pt(L)Py. At the same time, activation of the gas phase (UV-MOCVD) allows to minimize the level of fluorine impurities, and to obtain Pt layers ◦ already at 200 C. It was shown using Me3Pt(acac)Py as an example that the replacement of the reagent gas from hydrogen to oxygen while maintaining the deposition temperature led to a significant decrease in the amount of carbon impurities and an approximately threefold increase in the growth rate. It should be noted that for both Me3Pt(L)Py precursors, the deposition temperatures sufficient for the formation of high-quality films are comparable, and in many cases even lower than those used to obtain platinum nanomaterials from more commonly used precursors. This fact, as well as the possibility of varying the thermal properties of Me3Pt(L)A complexes over a wide range by directed modification of several types of ligands, encourages further development of MOCVD processes using precursors of this class. Coatings 2021, 11, 78 26 of 36

Table 7. Deposition conditions and some characteristics of Pt nanoparticles (PtNPs) and ﬁlms obtained by MOCVD from different classes of platinum precursors. The ligand abbreviations correspond to Table1 a.

Composition Precursor Reagent Gas Source/Deposition Growth Rate, Ref. Temperature, ◦C Substrate Thickness, nm nm/min (XRD) Purity and Other Details FB-MOCVD, PtNPs with average d = 1–3 nm [94] Me Pt(cod) H –/90–120 SiO + TEOS – – 2 2 2 Pt0, 2.3–3.8 mass.%; C, 4 mass.% Me Pt(cod-Et) 100/380 CVS/MOCVD, PtNPs with average d = 2–2.2 nm 2 Aerosil®200 [79] Me2Pt(cod-Bu)O2 100/380 –– Pt0, 4–5.7 mass.%; C, 4 mass.% i SiO2 Me2Pt(cod- Bu) 110/380 PtO, up to 8.6 mass.%; PtO2 up to 0.7 mass.%

Me2Pt(hd) [77] Me2Pt(hd-Me) H2 40/200–300 SiO2 + TEOS 100 – Smoothed ﬁlms (Ra~1 nm), 25 µΩ·cm or less Me2Pt(hd-Me2) Pt

[78] Me2Pt(nbd) N2 + O2 C = 0.1 M(tolulene)/300 Si/SiO2 60 2 LD-MOCVD, 1 mass.% C and O PtNPs with average d = 4.6 ± 1.1nm, [81] Pt(C H ) no 25/500 mesoporous silica –– 5 9 2 Pt0, 2.27–4.95 mass.%,

Al2O3 12 (PACVD) PA-MOCVD (Ra~1.7–2.2 nm) or MOCVD [82] Pt(C7H13)2 O2 75–80/250–330 – SiO2, Si, Au 300 (MOCVD) 47–56% Pt, 38–40% C, and 5–13% O µ · [90] Pt(amN(Me) ) H 80/400–500 TaN/Ta/Si – 0.35 Smoothed ﬁlms (Ra~1 nm), 13.6 Ω cm 2 2 2 9 mass.% C and 1 mass.% O Pt(S/S-idpp) PtS Large particles of PtS with d = 2–3 µm [91] 2 no 150/200–600 glass 250–300 – 2 2 Pt(Se/Se-idpp)2 PtSe2 PtSe2 ﬁlms with a worm-like morphology

[108] O2 180–220/260–380 Ti 200–1700 – Pt Electrode life up to 698 hrs

[109] H2O 140/280–380 glass rods, TiO2 10–200 0.1–1.1 Pt or Pt-TiO2 100 nm Pt ﬁlm behaves like a bulk Pt electrode

O C 200–600 1.8 LD-MOCVD, Ra = 5 nm [110] 2 = 0.04 M(THF) 200/350 Si—trench structures 27.6 µΩ·cm, 2 mass.% C Pt(acac)2 140/260–300(O2) vs. 0.7–2.4(O2) vs. [111] O2,H2O Soda glass – 110 vs. 91 kJ/mol activation energy 280–400(H2O) 0.1–1.1(H2O) Pt N –O ,N –O -H O, quartz 50–80 [112] 2 2 2 2 2 160–170/420 15 53–16 mass.% C N2–H2 CaF2 280–310

[117] O2 130/300–440 sappﬁre – 1.3–21.2 Pt-SiO2 11–18 mass.% C (111)-oriented ﬁlms, R = 15 nm [110] O C = 0.06 M(THF) 70/350 Si—trench structures 12–140 2 a 2 23.6 µΩ·cm, 6 mass.% C

[113] O2 35/200–400 Si 20–600 – (111)-oriented ﬁlms, >15 µΩ·cm Me FB-MOCVD, PtNPs with average d = 2.1 nm [118] Me3Pt(Cp ) O 35/150 TiO , ZrO –– 2 2 2 Pt Pt0, 3.4–5.7 mass.%; C, 4 mass.%

TiO2/SiO2/Si, LD-MOCVD, Ra = 3.1–4.5 nm(TiO2/SiO2/Si), [119] O2 48/350–400 60–100 0.5 SiO2/Si Ra = 4.9–7.6 nm(SiO2/Si), 8–10 mass.% C

Et C = 0.05 M(THF) Si—trench structures, LD-MOCVD, (111)-oriented ﬁlms, [115] Me3Pt(Cp ) O2 –– 130/300–450 LiCoO2 Ra = 5.4 nm, 11.8–25 µΩ·cm

110–270 1.1(H2) 15 mass.% C and 1 mass.% O (H2) [100] Me3Pt(acac)Py H2/O2 85/280–310 Si(100), Ti electrodes 800–850 3.6 (O2) 5 mass.% C and 1 mass.% O (O2) UV-MOCVD, fractal Pt ﬁlms were deposited at a high [116] Me Pt(hfac)Py H 50/200–270 Si(100), Ti electrodes – 0.7–3.2 3 2 hydrogen concentration a XRD = powder X-Ray diffraction, C = concentration, THF = tetrahydrofurane, FB-MOCVD = ﬂuidizes bed-MOCVD, LD-MOCVD = liquid delivery-MOCVD, PA-MOCVD = plasma-assisted-MOCVD, UV-MOCVD = ultraviolet stimulation-MOCVD, d = diameter, Ra = average roughness, TEOS = tetraethoxysilane. Coatings 2021, 11, 78 27 of 36

3. Precursors for the Preparation of Bimetallic Platinum- and Iridium-Containing Materials by MOCVD Bimetallic materials are of particular interest due to the mutual influence of components, which provides improved characteristics compared to monometallic analogues (synergetic effect). In MOCVD processes, bimetallic systems can be obtained using a single heterometallic precursor or a combination of individual precursors of each component. The advantage of the first approach is that the metals are already mixed at the molecular level. However, a given ratio of metals limits the possibility of varying the composition of the resulting material. The second approach does not have this problem; however, the success of its implementation depends entirely on the chosen combination of precursors. Such precursors must be compatible in terms of thermochemical properties, which primarily means that there is a common temperature range for the decomposition of compound vapors and the absence of ligand exchange reactions that complicate the process control. In general, studies dealing with the preparation of bimetallic materials containing platinum and iridium by the MOCVD method are sporadic, which is mainly due to the precursor factor. At the same time, this direction is more developed for platinum than for iridium. In particular, no iridium compounds, which can be used as a heterometallic precursor in gas-phase processes, have yet been described. For platinum, such a complex was obtained using ruthenium cyclopentadienyl, modified so as to be able to coordinate 5 the second metal atom: [CpRu(η -C5H3CH2NMe2)Pt(hfac)] [120]. This precursor was successfully tested for the deposition of bimetallic PtRu films in the temperature range up to 400 ◦C in an oxygen atmosphere; however, synthetic difficulties limit the use of the precursors of such types. For this reason, co-deposition from combinations of individual precursors is a priority approach to the formation of Pt–M and Ir–M systems by the MOCVD method. The literature analysis shows that homoligand metal acetylacetonates are mainly used for this purpose. In particular, PtRu and PtCo nanoparticles [107], as well as thin films containing PtNi, Pt3Ni, PtCo, Pt3Co and PtCoNi phases [121], were obtained from such combinations. However, a simple combination of Pt(acac)2 and Ir(acac)3 does not look optimal for the preparation of Pt–Ir metal coatings that are most interesting for medical applications. In fact, in the only example of the usage of this precursor combination, the deposition of bimetallic nanoparticles was carried out at 400 ◦C without an additional gas-reagent and without control of the purity and composition of the resulting product [122]. The introduction of a gas-reagent changes the picture dramatically. In the presence of hydrogen, ◦ [Pt(acac)2] vapors are less thermally stable than Ir(acac)3, and this difference exceeds 200 C (see Section2). In the presence of oxygen, a region of possible co-deposition exists; however, the process is complicated by the tendency to form IrO2 from Ir(acac)3. For this reason, for the preparation of metallic PtxIr1–x (x = 0.5–0.9) coatings in an oxidizing environment, we proposed an alternative combination of the precursors, viz., [Pt(acac)2] and [Ir(cod)(acac)], that seemed to be more convincing [123]. Typically, co-deposition is performed using independent heated sources in a MOCVD setup for each precursor. However, a single-source delivery system for the vapor trans- portation regardless of the number of precursors used is much more preferable from a technological point of view, since it allows for reducing the number of MOCVD parameters to be controlled and simplifying the reactor design. The single-source delivery system implies the usage of precursor mixtures. In this case, the processes of volatilization of precursors become crucial, since there is always the possibility of mutual influence of thermal properties on each other in the mixture, especially if the volatility of the precursors differs significantly [124]. The methods of thermogravimetry (TG) and XRD were used to study mixtures of [Ir(cod)(acac)] and [Pt(acac)2] precursors. The precursor mixtures were prepared in two ways: (i) by mechanical mixing of the necessary substance samples (mech) and (ii) by dissolving the substance samples in chloroform and evaporating the solution in an inert medium (solv). The second method was used to detect the effect of co-crystallization. To identify the dynamics of changes in physical and chemical properties, Coatings 2021, 11, x FOR PEER REVIEW 30 of 39 Coatings Coatings2021 2021, ,11 11,, 78 x FOR PEER REVIEW 2830 ofof 3639

crystallization. To identify the dynamics of changes in physical and chemical properties, thecrystallization. ratio of [Pt(acac) To identify] and [Ir(cod)(acac)]the dynamics of components changes in inphysical the mixtures and chemical varied properties, as 9:1 and the ratio of [Pt(acac)22] and [Ir(cod)(acac)] components in the mixtures varied as 9:1 and 1:1 the ratio of [Pt(acac)2] and [Ir(cod)(acac)] components in the mixtures varied as 9:1 and 1:1 1:1(ratios (ratios similar similar to those to those in the in theobtained obtained films) ﬁlms) and and1:9. 1:9.Comparison Comparison of the of diffraction the diffraction pat- (ratios similar to those in the obtained films) and 1:9. Comparison of the diffraction pat- patternsterns of ofall allthe the investigated investigated mixtures mixtures with with different different ratios ratios of of components components (Figure 1717)) terns of all the investigated mixtures with different ratios of components (Figure 17) shows that, regardless regardless of of the the meth methodod of of preparation, preparation, they they are are a asuperposition superposition of ofphases phases of shows that, regardless of the method of preparation, they are a superposition of phases of oftwo two individual individual compounds. compounds. This This fact fact is isclearly clearly expressed expressed for for the the mixtures mixtures prepared by mechanicaltwo individual mixing compounds. (mech). In This the casefact ofis usingclearly a solventexpressed while for maintainingthe mixtures the prepared additivity by mechanical mixing (mech). In the case of using a solvent while maintaining the additivity ofmechanical the peaks mixing on the diffraction(mech). In the patters, case of a reproducibleusing a solvent peak while at 2 maintainingθ = 12.2◦ is observed the additivity (see of the peaks on the diffraction patters, a reproducible peak at 2θ = 12.2° is observed (see Figureof the peaks17, dotted), on the the diffraction appearance patters, of which a reproducible is apparently peak due at to 2θ partial = 12.2° decomposition is observed (see of Figure 17, dotted), the appearance of which is apparently due to partial decomposition of theFigure iridium 17, dotted), complex. the appearance of which is apparently due to partial decomposition of the iridium complex. the iridium complex.

Figure 17. 17. PowderPowder XRD XRD patterns patterns (Shimadzu (Shimadzu XRD-7000, XRD-7000, CuKα radiation, CuKα radiation, angle range angle of 2θ range = 5°– of Figure 17. Powder XRD patterns (Shimadzu XRD-7000, CuKα radiation, angle range of 2θ = 5°– 30°, room◦ temperature)◦ of the [Pt(acac)2]-[Ir(cod)(acac)] mixtures with the ratios of 9:1, 1:1, 1:9, 2θ = 5 –30 , room temperature) of the [Pt(acac)2]-[Ir(cod)(acac)] mixtures with the ratios of 9:1, 30°, room temperature) of the [Pt(acac)2]-[Ir(cod)(acac)] mixtures with the ratios of 9:1, 1:1, 1:9, prepared by mechanical mixing of the precursor samples (mech) and dissolving them in an or- 1:1,prepared 1:9, prepared by mechanical by mechanical mixing mixing of the ofprecursor the precursor samples samples (mech) (mech) and dissolving and dissolving them them in an in or- an organic solvent with its further evaporation (solv), and theoretical XRD patterns for [Ir(cod)(acac)] ganicganic solventsolvent withwith itsits further further evaporation evaporation (solv), (solv), and and theoretical theoretical XRD XRD patterns patterns for for [Ir(cod)(acac)] [Ir(cod)(acac)] [40 ] [40] and [Pt(acac)2] [125]. and[40] [Pt(acac)and [Pt(acac)2][1252] ].[125]. TG curves (all experiments were performed under identical conditions) describing TGTG curvescurves (all(all experimentsexperiments werewere performedperformed underunder identicalidentical conditions)conditions) describingdescribing the processes of volatilization of individual compounds [Ir(cod)(acac)] and [Pt(acac)2] thethe processesprocesses ofof volatilizationvolatilization ofof individualindividual compoundscompounds [Ir(cod)(acac)][Ir(cod)(acac)] andand [Pt(acac)[Pt(acac)22]] have a single-stage appearance; the temperature difference corresponding to 50% mass havehave aa single-stagesingle-stage appearance;appearance; thethe temperaturetemperature differencedifference correspondingcorresponding toto 50%50% massmass loss of precursors is 30 ◦°C (Figure 18). The platinum precursor sublimes completely (<0.3 lossloss ofof precursorsprecursors isis 3030 °CC (Figure(Figure 18 18).). TheThe platinumplatinum precursorprecursor sublimessublimes completely completely (<0.3 (<0.3 mass.% residue), while the iridium complex passes into the gaseous phase with partial mass.%mass.% residue),residue), whilewhile thethe iridiumiridium complexcomplex passespasses intointo thethe gaseousgaseous phasephase withwith partialpartial decomposition (3.4 mass.% residue). decompositiondecomposition (3.4(3.4 mass.%mass.% residue).residue).

Figure 18. TG curves (Netzsch TG 209 F1, He, 10 K/min, 30 mL/min, Al crucible, sample masses = Figure 18. TG curves (Netzsch TG 209 F1, He, 10 K/min, 30 mL/min, Al crucible, sample masses = Figure5.900–6.370 18. TG mg) curves of the (Netzsch individu TGal 209precursors F1, He, 10[Pt(acac) K/min,2] 30and mL/min, [Ir(cod)(acac)] Al crucible, and sampletheir mixtures masses 5.900–6.370 mg) of the individual precursors [Pt(acac)2] and [Ir(cod)(acac)] and their mixtures =[Pt(acac) 5.900–6.3702]: [Ir(cod)(acac)] mg) of the individual= 9:1, 1:1, 1:9, precursors prepared [Pt(acac)by simple2] mixing and [Ir(cod)(acac)] of the precursor and samples their mixtures (mech) [Pt(acac)2]: [Ir(cod)(acac)] = 9:1, 1:1, 1:9, prepared by simple mixing of the precursor samples (mech) [Pt(acac)and by dissolution]: [Ir(cod)(acac)] of the precursor = 9:1, 1:1, samples 1:9, prepared in an organic by simple solvent mixing with of theits fu precursorrther evaporation samples (mech) (solv). and by dissolution2 of the precursor samples in an organic solvent with its further evaporation (solv). and by dissolution of the precursor samples in an organic solvent with its further evaporation (solv).

Coatings 2021, 11, 78 29 of 36

TG curves of the mixtures with the ratio of components 9:1 and 1:1 (both mech and solv) have a two-stage form, in which the first stage completely reproduces the volatilization of the iridium compound, while the second stage corresponds to that of the platinum precursor. However, at the second stage, the transition to the gas phase is accelerated, and the mixture completely passes into the gas phase at temperatures lower by 10 ◦C (9:1) and 20 ◦C (1:1). Herewith, the mixture enriched with the iridium component (1:9) completely reproduces the TG curve of the individual compound [Ir(cod)(acac)]. These data may indicate an increase in the volatility of the platinum component with an increase in the content of the iridium complex in the mixture. The use of the solution method (solv) for the mixture preparation does not affect the speed of the mixture volatilization process; however, it significantly reduces the thermal stability of the system. Summing up the study of mixtures, one could conclude that the preparation method does not result in forming any solid solutions and, consequently, affect volatilization processes of the binary system. However, it significantly changes the mixture stability: a mutual effect of reducing the thermal stability of substances in the mixture is observed. When the mixtures are heated, a synergistic effect is observed, which is manifested in an increase in the rate of volatilization of the less volatile component [Pt(acac)2] with an increase in the content of the more volatile [Ir(cod)(acac)]. The effect is insignificant for the mixture [Pt(acac)2]-[Ir(cod)(acac)] 9:1 and pronounced for the mixture of the inverse proportion of 1:9. The obtained results are very important and should be taken into account when choosing the temperature conditions of a single-channel source of MOCVD setup when depositing multicomponent layers, as well as when doping the main composition of the coating. The use of [Pt(acac)2]-[Ir(cod)(acac)] mixture in the ratio of 9:1 and 1:1 most likely leads to coatings with a gradient distribution of metal elements in depth. The use of the [Pt(acac)2]-[Ir(cod)(acac)] mixture in the ratio of 1:9 may be quite justified, because the volatilization process appears to take place with a preserved ratio of components.

4. MOCVD of Platinum and Iridium for Medical Application When discussing the use of a MOCVD method for deposition of specific functional film structures, it is necessary to take into account both the characteristics of the material to be coated and the features that the target coating should have. A specific requirement for the coating material that is used for medical purposes is a developed topography, i.e., a large effective surface area. In fact, high-roughness surfaces have been proven to be the most effective for promoting osseointegration of medical implants [126,127]. As for the electrodes for cardio and neurological devices, an increase in the area of the electrochemically active surface gives both a higher capacitance and a lower polarization, which are required to optimize pacing and sensing parameters, respectively [7,8]. The prospects of the application of the MOCVD technique to fabricate high-surface- area noble metal coatings of various compositions on the pole tips of medical electrodes have been demonstrated in a series of our recent works [100,123,128–130]. In particular, metallic iridium coatings were produced using a conventional precursor Ir(acac)3 in a reducing atmosphere [128]. These samples were further employed as templates for the electrochemical preparation of activated iridium oxide films (AIROFs). The resulting AIROFs exhibited charge storage capacity values higher than those of AIROFs obtained from electron-beam evaporated iridium films and were comparable to commercial electrodes. Metallic platinum coatings with an enhanced surface area and, consequently, high capacitance characteristics and low impedance were obtained on endocardial and diagnostic electrodes made of titanium and stainless steel, respectively, using Pt(II) β-diketonate, Pt(acac)2 [129,130], and the related Pt(IV) complex Me3Pt(acac)Py [100]. It was shown that the roughness of coatings formed in an oxidizing atmosphere was higher than that of the coatings deposited in reducing conditions [100]. Bimetallic platinum-enriched coatings PtxIr(1−x) in a wide range of metal ratios (x = 0.5–0.9) were successfully obtained onto the endocardial electrodes using a combination of Pt(acac)2 and Ir(cod)(acac) precursors and oxygen as a gas-reagent [123]. The surface roughness was found to raise with an increase in Coatings 2021, 11, 78 30 of 36

the iridium content. The activated PtxIr(1−x) coatings formed by MOCVD were comparable or better than those obtained by electrodeposition and magnetron sputtering. One of the current trends in the development of medical devices and implants is the transition from titanium and metal alloys to new materials based on carbon fibers, carbon composites, and polymers. These materials are characterized by a relatively low thermal stability and/or sensitivity to oxygen. Recent work suggests strategies for the successful deposition of noble metals also on such carriers [116,130]. In particular, deposition, a special sub-layer in a reducing atmosphere, can be effective for protection against oxygen. We have shown that the use of an adhesive Pd layer protects the stainless steel substrate during further deposition of platinum from Pt(acac)2 in an oxidizing atmosphere without reducing the capacitance characteristics of the resulting platinum layers [130]. Another approach is to develop low-temperature deposition processes under moderate conditions by selecting special precursors and/or using additional activation. In particular, it has recently been shown that Pt layers with a high active surface area could be obtained in a ◦ reducing atmosphere already at 200–250 C using Me3Pt(hfac)Py under vacuum ultraviolet irradiation [117]. With regard to permanent medical products, i.e., those that imply a long-time stay in the human body, such as implants reconstructing body parts and pacemakers, the biocompatibility of the introduced material becomes a particularly serious aspect. The application of film materials from the considered noble metals can be useful to improve this characteristic. In fact, Pt, Ir, IrOx, Pt–Ir nanomaterials obtained by different methods have been proven to be perfectly biocompatible with various cell colonies [131–133]. This phenomenon is traditionally associated with the inertia and non-cytotoxicity of these materials. Recent reports [131–133] also emphasize the positive impact of noble metal surfaces and nanoparticles on the processes of growth, integration, and differentiation of some human cells, including the effects induced by specific binding. For example, the great affinity of IrOx to proteoglycans stimulates a direct growth of neural cells for network formation [131]. Although the noble metals have not yet been deposited on the surface of medical implants by MOCVD, the capability of this method to obtain pure nanomaterials (see Sections 2.1.3 and 2.2.4) guarantees the absence of any problems with the biocompatibility of such coatings. On the other hand, a number of recent results in the field of implantation materials, obtained by various research groups, also indicate that a MOCVD method is promising for the preparation of biologically compatible coatings with improved characteristics [134–138]. Finally, it should be noted that for medical permanent implants, it is extremely important to impart antibacterial properties to the surface in order to prevent infectious complications [139]. In this aspect, in our opinion, there are two promising areas of application for the MOCVD method. On the one hand, the introduction of iridium and platinum improves bactericidal properties of silver and gold traditionally used for this purpose. MOCVD provides a convenient single process chain for the preparation of such mixed film materials with a developed surface, because both gold and silver are also metals, which can be deposited by this method [140,141], and a common deposition temperature can be achieved by selecting combinations of volatile precursors. On the other hand, mono- and bimetallic platinum nanoparticles, PtNPs [142], AgPtNPs [133], and AuPtNPs [143], are now considered as alternative antibacterial agents with improved biocompatibility. Here, MOCVD allows us to obtain the nanoparticles without impurities that are typical for solution methods. This can also help to better understand the principle of the antibacterial action of PtNPs, which remains unproven [144]. The active development of the MOCVD method for the formation of PtNPs demonstrated in Section 2.2.4 creates the basis for successful research in this direction.

5. Conclusions The interest in platinum group metals, in particular iridium and platinum, has not disappeared for many years due to such unique properties as exceptional chemical inertia, Coatings 2021, 11, 78 31 of 36

high biological compatibility, mechanical strength, and corrosion resistance, which are the basis for their application in medical practice. MOCVD is a promising method to fabricate Ir and Pt nanoparticles and film materials, multilayers and heterostructures. Its advantages include precise control of the composition and microstructures of the formed materials in deposition processes at relatively low temperatures on a wide range of the substrates including non-planar ones. The development of MOCVD processes is inextricably linked with the development of the chemistry of volatile precursors, viz., specially designed coordination and organometallic compounds. This review describes the main classes of volatile precursors of iridium and platinum iridium in all the oxidation state, namely, Ir(I), Ir(III), Pt(0), Pt(II), and Pt(IV). The synthesis methods are briefly discussed and the most efficient approaches are presented. The detailed data on thermal properties of the compounds are collected here and are useful for choosing the temperature regimes of MOCVD processes. The effects of a ligand structure and a metal coordination environment on the precursor thermal behavior are demonstrated. A number of examples of the usage of various volatile precursors in MOCVD including the formation of Pt, Ir and IrO2 films, Pt nanoparticles, and bimetallic nanomaterials are presented. A comparative analysis of the precursor chemistry allows us to draw the following conclusion. Although metal acetylacetonates, Ir(acac)3 and Pt(acac)2 are currently the most widely used precursors, Ir(I) and Pt(IV) complexes seem to be more promising due to a relative simplicity of preparation and their higher volatility, while the films from these precursors can be deposited at comparable or lower temperatures. The main advantage of these precursors is their ability to control thermochemical properties by modifying several types of ligands. This makes them tunable both to deposit films onto different substrate materials having specific requirements to deposition conditions and to select a combination of compatible compounds for obtaining the bimetallic materials. Finally, the prospects for using the MOCVD approach to obtain medical Ir- and Pt- based nanomaterials are discussed through the prism of recent works and current trends in this area.

Author Contributions: Conceptualization, N.B.M.; Validation, T.V.B. and N.B.M.; Writing—Original Draft Preparation, K.I.K. (Section 2.1), S.I.D. (Section 2.2), E.S.V. (Sections 2.1 and4), I.Y.I. (Section 2.1), K.V.Z. (Section3), and N.B.M. (Sections1,4 and5); Writing—Review and Editing, T.V.B. and N.B.M.; Visualization, I.Y.I. and K.I.K.; Supervision, N.B.M.; Project Administration, N.B.M. All authors have read and agreed to the published version of the manuscript. Funding: This work was financially supported by RSF (grant N 20-15-00222). Conflicts of Interest: The authors declare no conflict of interest.

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