J. Chem. Sci. Vol. 124, No. 6, November 2012, pp. 1191–1204. c Indian Academy of Sciences.

Simple tertiary phosphines to hexaphosphane ligands: Syntheses, transition metal chemistry and their catalytic applications

MARAVANJI S BALAKRISHNA∗, SOWMYA RAO and BIMBA CHOUBEY Department of Chemistry, Phosphorus Laboratory, Indian Institute of Technology Bombay, Mumbai 400 076, India e-mail: [email protected]

Abstract. Designing efficient phosphorus-based ligands to make catalysts for homogeneous catalysis has been a great challenge for chemists. Despite a plethora of phosphorus ligands ranging from simple tertiary phosphines to polyphosphines are known, the enthusiasm to generate new ones is mainly due to the demand from industry for economical and robust catalytic system operational under normal atmospheric conditions. In this context, we have developed new synthetic methodologies for making unusual inorganic ring systems containing trivalent phosphorus centres, novel phosphorus-based multidentate and hybrid ligands and explored their rich transition metal chemistry and catalytic applications. We have also fine tuned a few existing ligand systems with donor functionalities to employ them in homogeneous catalysis. The details are summarized in this account.

Keywords. Phosphines; transition metal complexes; catalysis; carbon–carbon coupling reactions; hydrogenation reactions; multidentate ligands.

1. Introduction analysis. The 10-membered heterocycle 1 reacts with SbF3 to afford the corresponding fluoro derivative 2 in The ceaseless curiosity in designing new types of good yield. The compounds 1 and 2 act as tridentate phosphorus-based ligands is essentially due to their ligands with molybdenum carbonyl derivatives form- 3 flexible coordination behaviour with both early and late ing complexes of the type, [Mo(CO)3{η -PhN(PX)2- t t transition metals and their applications in organic syn- {(-OC6H2( Bu)2)(μ-S)(( Bu)2C6H2O-)}] (3,X= Cl; thesis. 1 The undeniable fact is that they provide colossal 4,X= F). A crystal structure of the fluoro deriva- agility in the incorporation of a range of steric and elec- tive 4 showed the facial tricarbonyl complex compris- tronic attributes at phosphorus atoms that in turn facili- ing of a relatively strain free tetracyclic structure with tates the generation of appropriate metal complexes molybdenum in an octahedral environment coordinated which can promote homogeneous catalysis under mild to two phosphorus and sulphur atoms. 5 The hetero- conditions with remarkable turnover numbers. 2 cyclic diphosphane 2 readily reacts with various tran- We have designed several phosphorus-based ligands sition metal derivatives exhibiting η2 and η3 modes of ranging from simple monodentate to tri-, tetra- or hexa- coordination as shown in scheme 2. dentate systems and also modified a few existing li- gands with donor functionalities and explored their rich transition metal chemistry 3 and catalytic applications. 4 3. Transition metal derivatives of = ) The details of our contributions are briefly summarized. aminophosphines, Ph2PN(H)R (8, R Ph; 9, C6H11

The reactions of aminophosphines, Ph2PN(H)R (8, = 2. 10-Membered heterocyclic diphosphanes R Ph; 9,C6H11) with Pd(COD)Cl2 lead to the P–N bond cleavage to produce a chloro-bridged μ Bis(dichlorophosphino)aniline, PhN(PCl2)2 reacts with dimer, [Pd(PPh2O)(PPh2OH)( -Cl)]2 (10), whereas  one equivalent of 2,2 -thiobis(4,6-di-tert-butylphenol) with Pt(COD)Cl2, disubstituted complexes, cis- = ) to afford a 10-membered heterocycle, PhN(PCl)2- [PtCl2{PPh2N(H)R}2](11,R Ph; 12,C6H11 were t t {(-OC6H2( Bu)2)(μ-S)(( Bu)2C6H2O-)} (1) in high obtained. The reaction of 8 or 9 with K2PtCl4 afforded yield (see scheme 1). The structure of the heterocy- the platinum complex, [Pt{(PPh2O)2H}2](13), via P–N cle 1 has been determined by a single-crystal X-ray bond cleavage, as shown in scheme 3.The31PNMR spectrum of 13 shows a single resonance at 53.4 ppm ∗ 1 6 For correspondence with a JPtP coupling of 3958 Hz. 1191 1192 Maravanji S Balakrishna et al.

Et3N/Et2O SbF3/n-heptane S S S O O , 3 h O O OH OH r.t., 18 h P P P P + Cl N Cl F N F Cl Cl Ph Ph P P Cl N Cl 1 2 Ph

Scheme 1. Preparation and fluorination of heterocyclic diphosphane 1.

X O P S CO [Mo(CO) NBD]/hexane S 4 Mo N Ph O O 50 oC, 1-1.5h CO P P P CO O X X N X X = Cl, 1 Ph [RuCl (PPh ) )]/CH Cl X = Cl, 3 X = F, 2 2 3 3 2 2 r.t., 6 h X = F, 4 r.t., 6-7 h [M(COD)Cl2]/CH2Cl2

Cl Cl Cl Cl O P P O Cl P O N Ru N S S Ph M N S Ph Ph Cl O P P O P O Cl Cl Cl Cl M = Pd, 6 M = Pt, 7 5

Scheme 2. Reactions of cyclodiphosphane 1.

Ph Ph2 2 O P P O H Pt H OP P O Ph2 Ph2 13

K2PtCl4 H PPh OH Pt(COD)Cl Cl 2 2 Cl PPh NHR M Ph P N Pt 2 Cl PPh OH 2 2 Pd(COD)Cl2 R Cl PPh2NHR

or K2PdCl4 R = Ph, 11; C6H11, 12

-2 HCl Ph2 Ph O P Cl P 2O H Pd Pd H OP Cl P O Ph 2 Ph2 10

Scheme 3. Reactions of aminophosphine with PdII and PtII derivatives.

+ The reaction of 9 with either RuCl2(DMSO)4 or a molecular ion peak (MS FAB: 986 [M ]) correspond- RuCl2(PPh3)3 in 4:1 molar ratio yielded the ionic ing to the cationic species. The rare low-coordination of 31 complex, [RuCl{Ph2PN(H)C6H11}3]Cl (14). The P ruthenium in the molecule is attributed to the sterically NMR spectrum of 14 consists of a single resonance demanding aminophosphine ligands. However, a trigo- at 76.4 ppm indicating the tetrahedral nature of the nal bipyramidal geometry around the ruthenium(II) molecule and the mass spectrum of the complex showed centre with two chlorides in axial positions and the three Multidentate and multifunctional phosphines 1193

H CH2Cl2 r.t. Ru CpRuCl(PPh3)2 + Ph P N Cl 2 PPh3 R Ph2P N R R = Ph, 15

H C6H11, 17 Δ 8 or 9, H Toluene, Δ Toluene CpRuCl(PPh ) 3 2 +2 Ph2P N R

R Ru Ph2 Cl P N R = Ph, 16 Ph P 2 H C H ,18 N R 6 11 H

Scheme 4. Prepatation of ruthenium(II) complexes 15–18.

R = Ph; 18,CH ) in ∼75% yield containing trace PPh Cl 6 11 2 Toluene, Δ CpRuCl(PPh ) amount of respective monosubstituted complexes, 3 2+ R' N Ru Ph3P [CpRuCl(PPh3)(PPh2N(H)R)] (15,R= Ph, 15;C6H11, PPh2 PPh Ph P 2 17). The 31P NMR spectra of complexes 16 and 18 2 N R' showed single resonances at 72.6 and 81.8 ppm, respec- R' = Et, nPr, iPr, nBu 19 20 21 22 tively. The monosubstituted complexes 15 and 17 with an excess of the corresponding ligand also afforded Scheme 5. Preparation of ruthenium(II) cationic com- the disubstituted complexes 16 and 18 as shown in plexes 19–22. scheme 4. The reactions of aminobis(phosphines), n i n Ph2PN(R)PPh2 (R = Et, Pr, Pr, Bu) with equimolar phosphorus centres in the trigonal plane could not be quantity of [CpRuCl(PPh3)2] yielded cationic com- 31 ruled out as the P NMR spectrum will show a single plexes, [CpRu(PPh3)(Ph2PN(R)PPh2)]Cl (R = Et (19), resonance. nPr(20), i Pr (21), nBu (22)) in 50–60% yield (see The reactions of aminophosphines, Ph2PN(H)R scheme 5). with CpRuCl(PPh3)2 afford monosubstituted [CpRuCl- The half-sandwich ruthenium complexes were (PPh3)(PPh2N(H)R)] or disubstituted [CpRuCl- employed in the cyclopropanation reaction of styrene (PPh2N(H)R)2] complexes depending upon the stoi- derivatives in the presence of diphenyldiazomethane. chiometry and the reaction conditions. The reactions Interestingly, all complexes afforded 1,1,3,3-tetra- of Ph2PN(H)R (R = Ph, 8;C6H11, 9) with [CpRuCl- phenyl cyclobutane along with cyclopropane deriva- n (PPh3)2] in dichloromethane in equimolar ratio at room tives with [CpRu(PPh2N( Bu)PPh2)(PPh3)]Cl (22) temperature, gave [CpRuCl(PPh3)(PPh2N(H)R)] (15, showing better selectivity in the formation of 1,1,2- 31 R = Ph; 17,C6H11) in good yield. The PNMR triphenylcyclopropane (see scheme 6). In all reac- spectra of complexes 15 and 17 show two doublets tions, appreciable amounts of cyclopropanation and at 42.9, 72.4 ppm and 42.9, 77.9 ppm, respectively. metathesis products, 1,2-diphenylcyclopropane and 1, The chemical shifts at high fields are due to the PPh3 1-diphenylethene, were obtained along with 1,1,3-tri- group whereas the aminophosphines appear at lower phenylpropene derivatives. The variable temperature 2 1 field. The JPRuP couplings are 42.4 and 48.5 Hz for H NMR studies have suggested that the cyclo- complexes 15 and 17, respectively. In contrast, the propanation reactions in the presence of ionic com- reactions of [CpRuCl(PPh3)2] with 8 and 9 in 1:2 plex, [CpRu(PPh2N(R)PPh2)(PPh3)]Cl (22) proceeds ◦ molar ratio in toluene at 80–90 C gave disubstituted via carbene intermediate, [CpRu(=CPh2)(PPh2N(R)- 7 complexes of the type, [CpRuCl(PPh2N(H)R)2](16, PPh2)(PPh3)]Cl.

Ph Ph Ph Ph Ph H [Ru] + + + Ph+ Ph2CN2 Ph Ph H [Ru] = 15-22 Ph Ph Ph Ph Ph

Scheme 6. Cycloproponation rections catalysed by RuII-aminophosphine complexes. 1194 Maravanji S Balakrishna et al.

Ph2 P Ph O P 2 Ru

O O Ph2 P P Ph2 Ru O Ph3P 23 24

Ph P 2 Rh PPh O O Cl + 2 O Rh Cl 26 27 O

[Rh(COD)Cl]2

Ph2 Ph2 P PPh P [Rh(COD)Cl]2 2 [Rh(CO) Cl] OC Rh BF 2 2 Rh 4 Cl O AgBF O 4 O O O O 28 25

Scheme 7. Rhodium complexes of functionalized phosphine {Ph2PC6H4(OCH2OMe-o)}.

4. Phosphines with ether and alcohol The reaction of Ph2PC6H4OCH2OCH3-o with functionalities [PdCl2(COD)] led to the isolation of two mononuclear complexes, [PdCl(Ph2PC6H4O-o)(Ph2PC6H4OH-o)] The chemistry of phosphine ethers is interesting due to (29) cocrystallized with phosphonium salt, [Ph2P- the hemilabile nature of the ether oxygen which can (CH2OCH3)C6H4OH-o]Cl (31) and [Pd(Ph2PC6H4O- coordinate to soft metals along with soft-phosphorus o)2](30) (see scheme 8) as confirmed by X-ray donors. Such complexes are very useful in homo- diffraction study. The former shows extensive hydro- geneous catalysis. Hemilabile phosphines can coor- gen bonding interactions between the complex dinate to the metal centre and stabilize it in lower and the phosphonium salt. The reaction between oxidation state and enhance the chelating possibilities Ph2PC6H4OCH2OCH3-o and [PdCl2(COD)] in the through ether O-centre. Further, the labile M–O coor- presence of AgBF4 afforded cationic complex [PdCl- dinate bond can be readily cleaved as and when it is (Ph2PC6H4OCH2OCH3-o)2][BF4]2 (32) in quantitative required during the catalytic and biological processes. yield. 8 i In view of this, we have extensively studied the tran- A novel tetranuclear titanium complex, [{( PrO)2- i sition metal chemistry of phosphine ethers of the type Ti(μ3-O)TiCl( PrO)((OC6H4)2PPh)}2](33) containing Ph2PC6H4OCH2OCH3-o and PhP(C6H4OCH2OCH3- penta- and hexacoordinated titanium centres was o)2 and phosphinophenol, Ph2PC6H4OH-o. The reac- tion of Ph PC H OCH OCH -o with RuCl .3H O 2 6 4 2 3 3 2 PPh [Pd(COD)Cl ] 2 2 Cl Ph2 gave trischelated octahedral ruthenium(III) complex, Ph2 HO P P + ) O O Pd + [Ru(Ph2PC6H4O-o 3](23) through metathetical elimi- Ph2P OH Cl O O nation of three equivalents of CH3OCH2Cl, whereas the [Pd(COD)Cl2] 29 AgBF 31 reaction with CpRu(PPh3)2Cl resulted in the formation 4 + ) Ph Ph of [CpRu(Ph2PC6H4O-o PPh3](24). Ph2 Ph2 2 2 P P P P 2BF4 Treatment of Ph2PC6H4OCH2OCH3-o with Pd Pd rhodium(I) derivatives resulted in the formation of O O O O 30 complexes 25–28 with phosphine ligand exhibiting O O 32 both mono- and bidentate modes of coordination in- volving the phosphorus centre and the phenolic oxygen Scheme 8. Palladium complexes of functionalized phos- as shown in scheme 7. phine {Ph2PC6H4(OCH2OMe-o)}. Multidentate and multifunctional phosphines 1195 obtained in the reaction of bis(o-phenol)phenyl- centres for possible coordination to low-valent platinum phosphine with titanium tetrachloride. The X-ray struc- metals, the preferential binding of soft-phosphorus ture depicted the presence of both the bridging and atoms to oxophilic titanium centres to form 33 is due the terminal isopropoxy groups. 9 Although we antici- to the diphenolate substituents on phosphorus centres, pated a dimeric or tetrameric aryloxy complexes which bring the Ti and P atoms in close proximity to of the type 34 with uncoordinated phosphorus(III) establish Ti–P bonds.

5. Large bite bis(phosphine) ligands with aryl boronic acids in MeOH at room temperature or at 60◦C, giving generally high yields even under The ligating properties of bisphosphine ligands depend low catalytic loads. The cationic rhodium(I) com- to a large extent on the nature of the spacer besides plex, [Rh(COD){Ph2P(-OC10H6)(μ-CH2)(C10H6O-) the phosphorus substituents. In stereogenic ligands such PPh2}]BF4 (43) catalyses the hydrogenation of as binap, restricted rotations makes them ideal li- styrenes to afford the corresponding alkyl benzenes at gands for asymmetric synthesis. If the bulky groups can room temperature or at 70◦C with excellent turnover rotate freely about a pivoting group, the induced ring frequencies. 13 strain can facilitate the dissociation of one of the metal- phosphorus bonds so that a catalyst precursor may be generated. 10 In such cases, the large bite angle will 6. Diphenylether based bisphosphine enhance the steric congestion around the metal cen- and phosphino-phosphinimine ligands tre, which favours the less sterically demanding transi- tion state leading to selectivity in catalysis. 11 In view The large bite bis(2-(diphenylphosphino)phenyl)ether of this, several large-bite bis(phosphine) ligands were (DPEphos) (46) with a relatively rigid diphenyl ether synthesized 12 and their transition metal chemistry and backbone and containing both oxygen and phospho- catalytic reactions were investigated. rus donor sites 14 offers different coordination modes Bis(2-diphenylphosphinoxynaphthalen-1-yl)methane exhibiting rich coordination and organometallic che- (35) reacts with Group 6 metal carbonyls, mistry with various metal centres. van Leeuwen and 3 15 [Rh(CO)2Cl]2, anhydrous NiCl2, [Pd(η -C3H5)Cl]2/ coworkers have extensively studied the coordina- AgBF4 and M(COD)X2 to give the corresponding tion chemistry and catalytic utility of DPEphos. As 10-membered chelate complexes 36–42 as shown part of our research interest, we have investigated the 16 17 in scheme 9. Reaction of 35 with [Rh(COD)Cl]2 ruthenium and copper chemistry DPEphos and also in the presence of AgBF4 afforded a cationic com- catalytic hydrogenation of styrene. 5 plex, [Rh(COD){Ph2P(-OC10H6)(μ-CH2)(C10H6O-)- The half-sandwich complexes [(η -C5H5)RuCl- 6 PPh2}]BF4 (43). Treatment of 35 with AuCl(SMe2) (DPEphos)] (47)and[{(η -p-cymene)RuCl2}2(μ- gives mononuclear chelate complex, [(AuCl){Ph2- DPEphos)] (48) were synthesized by the reaction of P(-OC10H6)(μ-CH2)(C10H6O-)PPh2}] (44)aswell bis(2-(diphenylphosphino)phenyl)ether (DPEphos) (46) as a binuclear complex, [Au(Cl){μ-Ph2P(-OC10H6)- with a mixture of ruthenium trichloride trihydrate and 6 (μ-CH2)(C10H6O-)PPh2}AuCl] (45) with ligand 35 cyclopentadiene and with [(η -p-cymene)RuCl2]2, exhibiting both chelating and bridged bidentate modes respectively. Treatment of 46 with cis-[RuCl2(dmso)4] 3 of coordination respectively (see scheme 9). The mix- afforded fac-[RuCl2(η -DPEphos)(dmso)] (49). The ture of Pd(OAc)2 and 35 effectively catalyses Suzuki dmso ligand in 49 can be substituted by pyridine,   cross-coupling reactions of a range of aryl halides 2,2 -bipyridine, 4,4 -bipyridine and PPh3 to yield 1196 Maravanji S Balakrishna et al.

Ph O 2 P Cl Rh Ph CO O 2 CO P CO O P Ph O 2 M Ph2 P CO Rh O P CO Ph 42 BF 2 O P 4 [Rh(CO)2Cl]2 Ph2 40 M = Mo; 41, W [M(CO) ] 6 43 [Rh(COD)Cl]2 AgBF4 Ph Ph O 2 2 P X O P NiCl M 2 AuCl(SMe2) X M(COD)Cl O P 2 O P Ph2 Ph Ph2 2 O P [Pd(allyl)Cl] 2 35 37 M = Ni, X = Cl AgBF Au Cl 38 M = Pd, X = Cl, I 4 2AuCl(SMe2) P 39 M = Pt, X = Cl, I O Ph2 44 Ph Ph O 2 2 P O P Au Cl

Pd BF4 P O O P Au Cl Ph2 Ph2 36 45

Scheme 9. Reactions of 35 transtion metal derivatives.

O Ru P 47 O Ph Cl 2 Cl Ph P P 2 Ru Ph 2 P Cl (i) Ph (iii) 2 S O PPh 2 49 O P Ru Cl (ii) (iv) Ph 2 Cl PPh O 2 Cl O PPh P Ru Cl 46 2 Ph2 Cl OH2 Ru

48 Ph P NCCH3 54 2 Cl

Scheme 10. Reactions of DPEphos with (i) RuCl3.3H2O and Cp in ethanol; (ii) [RuCl2(p-cymene)]2 in CH2Cl2; (iii) cis-[RuCl2(dmso)4] in CH2Cl2; (iv) [RuCl2(p-cymene)]2 in CH3CN.

trans,cis-[RuCl2(DPEphos)(C5H5N)2](50), cis,cis- 7. Iminophosphoranephosphine ether  [RuCl2(DPEphos)(2,2 -bipyridine)] (51), trans,cis- as a heterodifunctional ligand  [RuCl2(DPEphos)(μ-4,4 -bipyridine)]n (52)andmer, 3 trans-[RuCl2(η -DPEphos)(PPh3)](53), respectively. Partially oxidized hemilabile iminophosphorane- 6 Refluxing [(η -p-cymene)RuCl2]2, with DPEphos in phosphane ligand 55 was synthesized by treating moist acetonitrile leads to the elimination of the p- bis[2-(diphenylphosphanyl)phenyl]ether (46) with cymene group and the formation of the octahedral phosphoryl azide by Staudinger reaction. 18 The complex cis,cis-[RuCl2(DPEphos)(H2O)(CH3CN)] (54) iminophosphorane shows both monodentate and biden- (see scheme 10). The catalytic activity of these tate chelating coordination modes. The platinum(II), complexes for the hydrogenation of styrene is palladium(II), and rhodium(I) complexes 56a, 56b,and studied. 17 57, respectively, are obtained as trans isomers as shown Multidentate and multifunctional phosphines 1197

8. Mesocyclic thioetherphosphonites RN X Ph2 O Ph P P M PPh PPh R = P(O)(OPh)2 2 2 2 The coordination chemistry and catalytic utility of ether O NR M = Pd, 56a; Pt, 56b; X = Cl Cl M = Rh, X = CO, 57 and diphenyl ether-based ligands have been studied. However, the corresponding thioether-based bisphos- (iii) Ph P 2AuCl phines or phosphonites are less extensive. Ligands com- (i) (ii) O bining phosphorus centres as well as sulphur centres O PPh O PPh2 2 PPh are especially interesting. Both phosphorus and sulphur PPh2 2 P NP(O)(OPh)2 46 NP(O)(OPh) Ph2 55 2 60 are excellent donor atoms for a wide range of tran- (iv) (v) sition metals, while the low ionization energy of sul- N phur and the existence of several lone pair of electrons P P(OPh)2 N Ph P P(OPh) 2 2 (three in the case of a thiolate anion) offer the possibil- O Ph2 O O P BF O ity of a rich sulphur-based chemistry of the complexes. Rh 4 Ph2 Pd O P P To the best of our knowledge, there are no reports Ph2 Ph2 O P on cyclic thioether-aminophosphonite type of ligands, 58 (OPh)2P 59 N Ph 2 either in coordination chemistry or in catalysis. Holmes and co-workers have reported several eight-membered Scheme 11. Reactions of DPEphos with (i) N P(O)(OPh) 3 2 cyclic P, S compounds where sulphur shows coor- CH3CN; (ii) AuCl(SMe2) in CH2Cl2; (iii) M(COD)Cl2 19 or [Rh(CO)2Cl]2 in CH2Cl2;(iv)Pd2dba3 in C6H6;(v) dinative interaction towards phosphorus. They have [Rh(COD)Cl]2,2AgBF4 in CH2Cl2. shown that the donor action provided by sulphur leads to an increase in coordination at phosphorus from trico- ordinate to pseudopentacoordinate. However, there are no reports on coordination behaviour or catalytic acti- in scheme 11. The reaction of 55 with [{Rh(COD)Cl}2] vity of such ligands. It will be interesting to see the and AgBF4 produced the 11-membered macrocyclic coordination chemistry of such ligands as sulphur square-planar complex 58 with iminophosphorane li- shows coordinative tendency towards phosphorus. In gand showing chelating-bidentate mode of coordina- this context, following mixed P, S mesocyclic ligands tion. The cationic rhodium(I) complex 58 is catalyt- were prepared and their transition metal chemistry and ically active for the hydrogenation of olefins with a catalytic reactions were investigated (scheme 12). TON of 2 × 105 and a TOF of 6 × 105 h−1.ThePd0 Mesocyclic thioether–aminophosphonite ligands, complex 59, in which ligand 55 binds in a chelating {-OC10H6(μ-S)C10H6O-}PNC4H8O(62a) and {-OC10- fashion, was synthesized by the reaction of 55 with H6(μ-S)C10H6O-}PNC4H8NCH3 (62b) are obtained by 0 [Pd2(dba)3]. The Pd complex 59 is catalytically active reacting {-OC10H6(μ-S)C10H6O-}PCl (61) with corres- for Suzuki cross-coupling reactions of various aryl ponding nucleophiles. 20 Similar reaction with aniline bromides and phenylboronic acid. A lower catalytic led to the isolation of nitrogen bridged bis(phosphonite) loading of 0.05 mol% of 59 allows complete conversion 63 in good yield. 21 The ligands 62a and 62b react with II II of several aryl bromides into biaryls. (PhCN)2PdCl2 or M(COD)Cl2 (M = Pd or Pt ) to

OH S X OH

N (i) P Ph O O O N O S P P O S S (ii) S P Cl (iii) O O O 63

61 62a X = O 62b X = NMe

◦ Scheme 12. (i) PCl3,Et3N, DMAP, −20 C, THF; (ii) morpholine or ◦ N-methyl piperazine, 0 C, THF; (iii) PhNH2,Et3N, DMAP, Et2O. 1198 Maravanji S Balakrishna et al.

X

N Cl O P M Cl S O O P S N 64a M = Pd, X = O O 64b M = Pd, X = NMe 65b M = Pt, X = NMe X 65a M = Pt, X = O (i) - X O X O O P (iii) N S O (ii) + S O P OTf N S Pd Pd H H O O Cl Cl P X = O, 67a; X = NMe, 67b N X = O, 66a; X = NMe, 66b

X

◦ 3 Scheme 13. (i) M(COD)CI2,CH2CI2,25C; (ii) [Pd(η -C3H5)CI]2/ ◦ ◦ AgOTf, CH2CI2,25 C; (iii) (PhCN)2PdCI2,H2O(trace), toluene, 90 C.

10 10 afford P-coordinated cis-complexes, [{(-OC10H6(μ- dynamic coordination chemistry and weak d –d 25 S)C10H6O-)PNC4H8X}2MCl2](64,M= Pd; 65,M= metallophilic interactions involving rigid N-donor Pt) as shown in scheme 13. Compounds 62a and ligands, such as 4,4-bipyridine, 4,4-dibenzonitrile, 3 62b on treatment with [Pd(η -C3H5)Cl]2 in the pre- pyrazine which can offer multi-dimensional, metal– sence of AgOTf produce the P, S-chelated cationic organic materials with diverse properties. 4,4- complexes, [{(-OC10H6(μ-S)C10H6O-)PNC4H8X}Pd- Bipyridine has served as an effective bridging group 3 (η -C3H5)](CF3SO3) (66a,X= O; 66b,X= NMe). and hundreds of interesting supramolecular architec- 26 Treatment of 62a and 62b with (PhCN)2PdCl2 in the tures have been reported. Instances of the use of rigid  presence of trace amount of H2O afforded P, S-chelated linear phosphines, analogous to 4,4 -bipyridine, in the 27 anionic complexes, [{(-OC10H6(μ-S)C10H6O-)- synthesis of polynuclear complexes are less extensive. P(O)}PdCl2](H2NC4H8X) (67a,X= O, 67b,X= In this context, we have designed novel tetraphos- NMe), via P–N bond cleavage. The crystal structures phane ligands of the type {(X2P)2NC6H4N(PX2)2} of most of these compounds have been determined and explored their rich transition metal chemistry and by X-ray diffraction studies. The compound 67a is a catalytic applications. These tetraphosphanes can be rare and first example of crystallographically charac- compared to two 4,4-bipyridine units fused sideways terized anionic transition-metal complex containing containing both electronically and sterically tunable a thioether-phosphonate ligand. 20a The reactions of phosphorus donor centres. A few important reactions thioether-aminophosphonites with Pt(COD)Cl2 gave of these ligands with transition metals are described. exclusively phosphorus coordinated cis-complexes The reaction of p-phenylenediamine with with high σ -donor strength. Most of these palladium excess of PCl3 in the presence of pyridine affords complexes proved to be very active catalysts for (Cl2P)2NC6H4N(PCl2)2 (68) in good yield. Fluorination the Suzuki–Miyaura, Heck–Mizaroki carbon–carbon of 68 with SbF3 produces (F2P)2NC6H4N(PF2)2 (69) cross coupling and amination reactions with excellent in moderate yield. 28 The aminotetra(phosphonites), 4 turnover numbers (TON up to 9.2 × 10 using complex p-C6H4[N{P(OC6H4OMe-o)2}2]2 (70)andp- 21 67a as catalyst). C6H4[N{P(OMe)2}2]2 (71) have been prepared by reacting 68 with appropriate amount of 2-(methoxy)- phenol or methanol, respectively, in the presence of 9. Tetra- and hexaphosphane ligands triethylamine (see scheme 14). Interestingly, the compounds of the type 68–71 can The chemistry of Group 11 metal complexes in their adopt several conformations depending upon the ori- +1 oxidation state have attracted much attention due entation of the P–N–P skeleton with respect to the to their catalytic applications, 22 role in biochemistry 23 phenylene ring. Three major idealized possibilities are: and photochemical areas. 24 Also, Group 11 metals (i) both phenylene and P–N–P skeletons can be copla- serve as versatile connecting nodes for the synthesis nar; (ii) the phenylene ring can be perpendicular to the of supramolecular architectures through the use of P–N–P skeletons; (iii) the phenylene and one of the Multidentate and multifunctional phosphines 1199

Scheme 14. Preparation of octachlorotetraphosphane and its derivatives.

molecular structures of many of these compounds are PX PX2 X P 2 X2P PX X2P 2 2 29 N N N N N N confirmed by single crystal X-ray diffraction studies. ··· X P PX2 The weak intermolecular P P interactions observed X2P PX2 X2P PX2 2 I II III in 68 leads to the formation of a 2-D sheet like structure R R R

X X N X N : : (figure 1) which is also examined by DFT calculations. N : X X X :

: P P P : P P P The palladium(II) complex 72 is an efficient catalyst for X X X X X X C2v C2V ' Cs the coupling of several activated and deactivated aryl bromides and chlorides with phenylboronic acid and Chart 1. Possible conformations of tetraphosphane also for the one-pot multiple carbon–carbon couplings derivatives. at room temperature. The reactions of 70 with [Rh(COD)Cl]2 in 1:2 and 1:1 molar ratio gave chelate complexes, [Rh4(COD)2- P–N–P skeletons can be in one plane and orthogonal (μ-Cl)4{R2PN(C6H4)NPR2}] (76)and[Rh2(μ-Cl)2- to the other P–N–P skeleton. Further, the P–N–P moie- {R2PN(C6H4)NPR2}]n (77), whereas similar reaction  ties in each conformation can adopt C2V , C2V or CS of 71 with [Rh(COD)Cl]2 in dichloromethane– conformations depending on the mutual orientation of acetonitrile mixture gave a dinuclear complex, phosphorus lone pairs with respect to the phosphorus- [Rh2Cl2(CH3CN)2{R2PN(C6H4)NPR2}] (78). The substituents as shown in chart 1, so there is a total of 18 reaction of 77 or 78 with CO afforded a dinuclear car- possible conformations. bonyl derivative, [Rh2Cl2(CO)2{R2PN(C6H4)NPR2}] Reactions of 70 with [M(COD)Cl2](M= Pd or Pt) (79) (see scheme 16). Treatment of 78 with two resulted in the formation of chelate complexes, equivalents of pyrazine or 4,4-bipyridine produced [M2Cl4-p-C6H4{N{P(OC6H4OMe-o)2}2}2](72,M= one-dimensional coordination polymers, [Rh2Cl2- Pd; 73,M= Pt). The reactions of 70 with four (C4H4N2){R2PN(C6H4)NPR2}]n (80) (figure 2)and equivalents of CuX (X = Br and I) produce the [Rh2Cl2(C10H8N2){R2PN(C6H4)NPR2}]n (81), in quan- 30 tetranuclear complexes, [Cu4(μ2-X)4(NCCH3)4-p- titative yield. These polymers have the metals in con- C6H4{N(P(OC6H4OMe-o)2)2}2](74,X= Br; 75, jugation with aromatic π-systems through P–N–P X = I) in quantitative yield as shown in scheme 15.The skeletons with P–N bonds showing multiple bond

Scheme 15. Palladium, platinum and copper complexes of 70. 1200 Maravanji S Balakrishna et al.

Scheme 17. Transfer hydrogenation reaction of ketones using rhodium(I)-tetraphosphane complexes 76–81.

character evinced by X-ray structure determination. By choosing appropriate redox-active metals and sub- stituents at the P-centres it is possible to design efficient conducting polymers. The catalytic activity of rhodium(I) complexes 76– 81 and some ruthenium(II) complexes 31 have been investigated in transfer hydrogenation reactions (see scheme 17). Among them, the tetra metallic complex 76 appeared to be the most active precursor for the reduc- tion of acetophenone (8 h, TON = 199 h−1) and further it was used for the reduction of ketones other than ace- Figure 1. Octachlorotetraphosphane (68) showing inter- tophenone. The reduction performed with benzophe- molecular P···P interactions. none yielded 80% of diphenylmethanol after 24 h with

Scheme 16. Reactions of tetraphosphane 70 with rhodium derivatives.

Figure 2. Molecular structure of one-dimensional RhI zigzag coordination polymer [Rh2Cl2(C4H4N2){R2PN(C6H4)NPR2}]n (80). Multidentate and multifunctional phosphines 1201

Scheme 18. Preparation and derivatization of dodecachlorohexaphosphane 82. complex 76. The complex 76 also showed good activity study, the ligand 84 has been used for the prepara- in the transfer hydrogenation of six-membered cyclic tion of platinum group metal complexes. Treatment of ketones such as α-tetralone and cyclohexanone but at 84 with three equivalents of [M(COD)Cl2](M= Pd different rates. Further, the reduction of 4-bromo ace- or Pt) in dichloromethane afforded the chelate com- tophenone tends to proceed at significantly lower rate plexes, 1,3,5-C6H3[p-C6H4N{P(OR)2}2(MCl2)]3 [R = with low yield because of the higher mesomeric effect −C6H3OMe(C3H5)](85,M= Pd; 86,M= Pt) in caused by bromide substitution. good yield. The 31P NMR spectra of complexes 85 and 86 show single resonances at 63.2 and 57.9 ppm, respectively, which are considerably shielded compared 10. Hexaphosphanes and metallocene-based to the free ligand. The platinum complex exhibits a 1 bisphosphanes large JPtP coupling of 5913 Hz, which is consistent with the proposed cis geometry around the platinum  The reaction of 1,3,5-tris(4 -aminophenyl)benzene with centre. 32 phosphorus trichloride in the presence of three equi- Ferrocenyl–phosphine ligands are versatile and are valents of pyridine afforded the novel dodecachloro- able to form complexes with transition metals in a varie- ) hexaphosphane, 1,3,5-C6H3[p-C6H4N(PCl2 2]3 (82)as ty of coordination geometries and oxidation states, a pale yellow crystalline solid in 20% yield. The yield which have proven to be efficient catalysts in homoge- has been further improved by carrying out the reac- neous catalysis. Several ferrocenyl–phosphines have tion in the presence of a strong base like triethy- been extensively studied and have shown good catalytic lamine with a catalytic amount of N, N-dimethyl-4- activity in organic synthesis. In contrast, ferrocenyl– aminopyridine (see scheme 18). The compound 82 phosphonites or –phosphite derivatives are less exten- readily decomposes on exposure to air and moisture. sive although, due to their easy preparation methods, The fluorination of 82 with antimony trifluoride yielded can be an attractive alternative to ferrocenyl- ) the fluoro analogue, 1,3,5-C6H3[p-C6H4N(PF2 2]3 (83) phosphines. In view of this, we have made several in good yield. The 31P NMR spectrum of 82 con- bisphosphonites, phosphites and aminophosphines sists of a single peak at 153.7 ppm, whereas 83 based on ferrocenyl framework and explored their tran-   shows the A portion of an AA X2X2 multiplet cen- sition metal chemistry and catalytic applications. Few |1 | |3 | |2 | tred at 130.4 ppm with JPF , JPF and JPP cou- of these ligands and their transition metal chemistry plings of 1246, 124 and 372 Hz, respectively. The sin- gle crystal X-ray diffraction analysis of 82 showed the distorted pyramidal geometry about the phosphorus P(NEt ) 2 2 PCl2 O centres and a planar environment around the nitro- (i) S (ii) (iii) P gen centres with the sum of the angles around nitro- Fe Fe Fe O ◦ P(NEt ) PCl Fe gen almost 360 in all cases. Further the bridging 2 2 2 88 O phenylene rings are almost perpendicular to the plane 87 P (iv) S of the P–N–P skeletons. The reaction of 82 with 12 OR 89 O P equivalents of 4-allyl-2-methoxyphenol in the pres- OR Fe ence of triethylamine afforded hexaphosphonite, 1,3,5- OR = {OC H (OMe-o)(C H -p), 90 = ) OR 6 3 3 5 C6H3[p-C6H4N{P(OR)2}2]3 [R -C6H3OMe(C3H5 ] P OR = {-OC6H4(OMe-o)}, 91 OR= {OC H (C H -o), 92 (84) in quantitative yield. Compounds 82–84 are OR 6 4 3 4 potential hexadentate ligands and are expected to n Scheme 19. (i) 1. BuLi, TMEDA; 2. CIP(Net2)2; (ii) ◦ behave as simple aminobisphosphine units mimick- HCI(g), Et2O, −78 C; (iii) Diol, Et3N, Et2O; (iv) ROH, ◦ ing their coordination behaviour. In a preliminary Et3N, Et2O, −20 C. 1202 Maravanji S Balakrishna et al.

O O O O O NCMe P P P Au Cl O Cu CuX AuCl(SMe ) Fe 2 Fe Fe X X Cl CH Cl / Au 2 2 Cu CH2Cl2 O CH CN P P 3 P NCMe O O 97 O O 89 O 95 M(COD)Cl2 [Ag(PPh3)OTf]

O CH2Cl2 CH2Cl2 O O P O Cl P OTf Fe M Cl Fe Ag

P PPh3 O P M = Pd, 93; Pt, 94 O O 96 O

Scheme 20. Metal complexes of Ferrocenyl–phosphonite 89. is described. The synthetic method adopted for the isoproponol yielded a novel heptacyclic tetranuclear preparation of various ferrocenyl-phosphanes is given titanium complex containing four different types of in scheme 19. oxygen binding along with formal titanium–phosphorus The chloro-precursor 88 readily undergoes nucleo- bonds. Large-bite bis(phosphonite) 35 with Pd(OAc)2 philic substitution at phosphorus centres to form bis- catalyses Suzuki–Miyaura C–C coupling reactions, phosphonites of the type 89–92 in good to moderate whereas the rhodium complex 43 catalyses the hydro- yields. Ferrocenyl-phosphonite ligand 89 readily react genation of various styrenes with excellent turnover with platinum and Group 11 metals to form inter- numbers. Bis(diphenylphosphino)phenylether 46 and esting metal complexes (see scheme 20). The palla- its partially oxidized iminophosphorane–phosphane dium complexes show good catalytic activity towards derivative 55 form interesting complexes with various Suzuki–Miyaura cross coupling reactions. 33–35 transition metals and the palladium(0) and rhodium(I) complexes of later show excellent catalytic activity towards Suzuki–Miyaura C–C coupling reactions and 11. Summary hydrogenation of styrenes and acetylenes, respectively. The mesocyclic thioether–aminophosphonites due to Several phosphorus based ligands ranging from sim- their flexible framework display rich coordination ple tertiary phosphines to hexaphosphane ligands chemistry, especially, the anionic palladium complex 67 have been synthesized and their transition metal che- promotes Suzuki–Miyaura, Mizoroki–Heck coupling mistry and catalytic applications have been investi- reactions as well as amination reactions with very high gated. The synthetic flexibility, easy synthetic methodo- catalytic activity (TON up to 1.5 x 106). Octachlorote- logy with readily accessible nucleophilic sites and the traphosphane 68 shows intermolecular P···P inter- presence of three soft donor centres make the hete- actions whose estimated strength is around −5to rocyclicdiphosphanes (1-and2) a valuable ligand −10 kJ mol−1. Tetraphosphane derivatives form simple system. Aminophosphines and aminobis(phosphines) binuclear to tetranuclear and polynuclear metal com- form interesting ruthenium(II) compelxes and they plexes and 1D-, 2D- and 3D-coordination polymers catalyse cycloproponation reactions. Formation of with Group 11 metals. Tetraphosphanes in combina- 1,1,3,3-tetraphenyl cyclobutane was observed for the tion with pyridyl ligands could serve as efficient tec- first time in the cycloproponation reactions. tons for the generation of interesting conglomerates Phosphinoethers show both mondentate and biden- which might find useful applications. For the first time tate coordination modes involving P(III) and ether an efficient one-pot synthesis of a novel dodecachloro- oxygen centres. These ligands also generate metal- hexaphosphane is achieved in our laboratory and its phenoxide bonds through metathetical elimination preliminary reactions have been carried out. These of methoxymethylchloride on treatment with plati- polydentate phosphorus ligands on aromatic frame- num metal halide-derivatives. The reaction of bis(o- work are best suited to generate polynuclear complexes phenol)phenylphosphine with TiCl4 in toluene and with metals in close proximity to understand their Multidentate and multifunctional phosphines 1203 mutual cooperativitiy, in case, such metal complexes (i) Kaboudin B and Balakrishna M S 2007 Synth. Com- are found suitable for homogeneous catalysis. mun. 31 2773; (j) Mohanty S and Balakrishna M S 2010 J. Chem. Sci. 122 137 5. Balakrishna M S, Panda R and Mague J T 2001 Inorg. Acknowledgements Chem. 40 5620 The majority of the work described here has been per- 6. (a) Priya S, Balakrishna M S and Mague J T 2003 J. Organomet. Chem. 679 116; (b) Priya S, Balakrishna formed by my past graduate students; Drs. S Priya, M S, Mague J T and Mobin S M 2003 Inorg. Chem. 42 R Panda, P P George, B Punji and C Ganesamoorthy. 1272 I am thankful to them for their synthetic skills and dedi- 7. Priya S, Balakrishna M S, Mobin S M and McDonald R cation. I am indebted to Prof. Joel T Mague, Tulane 2003 J. Organomet. Chem. 688 227 University, New Orleans, for X-ray structure determi- 8. Priya S, Balakrishna M S and Mague J T 2004 nation. Our work was supported by the Department of J. Organomet. Chem. 689 3335 9. Priya S, Balakrishna M S and Mague J T 2004 Chem. Science and Technology (DST), the Council of Scien- Lett. 33 308 tific and Industrial Research (CSIR), New Delhi, and 10. (a) Guo R, Lough A J, Morris R H and Song D 2004 we are grateful for their continued support. Organometallics 23 5524; (b) Kesanli B and Lin W 2004 Chem. Commun. 2284; (c) Hölscher M, Franci`ö G and Leitner W 2004 Organometallics 23 5606; (d) Wiles References J A, Daley C J A, Hamilton R J, Leong C G and Bergens S H 2004 Organometallics 23 4564 1. (a) Applied homogeneous catalysis with organometal- 11. Carb´o J J, Maseras F, Bo C and van Leeuwen P W N M lic compounds 2002 2nd Edition, B Cornils, W A 2001 J. Am. Chem. 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