Pergamon Poiyhedron Vol. 13, No. 617, pp. 919-925, 1994

Copyright Q 1994 Elsevier Science Ltd Printed in Great Britain. All rights reswved

0277-5387/94 $6.00+0.00

THE X-RAY STRUCTURE AND SUBSTITUTION REACTIONS OF THE AMID0 BRIDGED Ir” DIMER

~~~~~~,l~u-NH(p_~~~~~~l,~CO~,

MANOJ K. KOLEL-VEETIL, JESSICA F. CURLEY, PRAMILA R. YADAV and KAZI J. AHMED*

Department of Chemistry, University of Vermont, Burlington, VT 05405, U.S.A.

(Received 30 August 1993 ; accepted 3 November 1993)

Abstract-The addition of I2 to Ir2j&NH@-tolyl)]z(CO)4 (1) produces the Ir”--Ir” dimer

Ir,(I)2[~-NH(p-toly1)1,(C0)4 (21, h w ere the two iridium centres are connected by a metal- metal single bond of length 2.591 (1) A. The complex 2 undergoes substitution of only one CO with phosphines (PPh, and PPh,Me), even when an excess of the phosphines is used. NMR and IR data of all the complexes are presented, along with an X-ray structure of 2.

Recently, we have been studying’ the chemistry of Ir2~-NH@-tolyl)]2(C0)4 (l), a schematic drawing of which is shown below. In addition to exploring the chemistry of the bridging amido groups, the results of which will be communicated in the future, we have investigated the nature of oxidative addi- tion-type reactions at the dinuclear centre in 1.’

ptolyl’ H

1

The common reagents, such as halogens, hydrogen halides, alkyl halides, etc., all undergo addition at the dinuclear centre ; however, the mode of addition depends on the particular reagent. Methyl iodide, for example, adds to one of the two iridium centres to afford an Ir’-Ir”’ complex, shown below. We have not found any evidence for the formation of the adduct resulting from the addition of two equi- valents of Mel or the adduct where Mel adds across

ptolyl. ‘*. rH

oc*,. I”\ /*JO

oc,..: ‘r\NOJ.‘.,,co

-‘\ Me

pto1yl‘ -H

1’MBI

*Author to whom correspondence should be addressed.

the dinuclear centre. Similar Ir’-I?” complexes are formed when one equivalent of Cl,, Br, or HCl is added. However, with one equivalent of I,, addition across the dinuclear centre results in the formation of the following complex, 2, which bears two Ir” centres, each containing an iodide ligand, and joined by a formal metal-metal bond. Herein, we discuss the X-ray crystal structure of 2, along with a discussion of its substitution chemistry with aryl phosphines.

ptolyl;. ,H

EXPERIMENTAL

All preparations were carried out under N, using standard dry-box or Schlenk line techniques. Sol- vents were purified before use and stored under N, over 4 8, molecular sieves. Hexanes were distilled from CaH,, methylene chloride was distilled from P4010, and toluene and THF were distilled from Na/K alloy and benzophenone. Iodine was purified by sublimation before use. PPh3 and PPh,Me were purchased from Strem Chemicals and were used without any further purification. Deuterated sol- vents for NMR experiments were purchased from Cambridge Isotope Laboratories. The synthesis of

919

920 M. K. KOLEL-VEETIL et al.

Ir&-NH(p-tolyl)]2(CO)~ (1) has been described pre- viously.’

IR spectra were obtained on a Nicolet 7000 series FT-IR instrument. ‘H (250 MHz) and “P (101 MHz) NMR spectra were recorded using a Bruker instrument. Elemental analyses were performed by Quantitative Technologies Inc., Whitehouse, NJ.

Synthesis

Ir,(I),[p-NH@-tolyl)1,(CO), (2). Compound 1 (0.10 g, 0.14 mmol) was dissolved in toluene (25 cm’) in a Schlenk flask. One equivalent of I2 (0.04 g, 0.14 mmol), dissolved in 15 cm3 of toluene, was added dropwise to the above solution, causing a gradual colour change of the solution from yel- lowish-brown to green to finally reddish-brown. After the addition was complete, the solution was concentrated under reduced pressure and hexane was added to precipitate a reddish-brown micro- crystalline solid. The yield was 0.13 g, 95%. IR (carbonyl region), CH2C12, cm-’ : 2103,2082,2047. ‘H NMR, CDC13: 6 7.08 and 7.04 (4H, C&&Me), 5.35 (br s, lH, HN-), 2.29 (s, 3H, C,H,Me). 13C NMR, CDCl,: 6 167.0 (s, CO), 148.7, 136.6, 129.9, 122.5 (s, CsH,Me), 20.8 (s, C,H,Me). Found: C, 22.4; H, 1.7; N, 2.8. Calc.: C, 22.5; H, 1.7; N, 2.9%.

Ir,(I),~-NH(p-tolyl)],(CO)3(PPh3) (3). To a solu- tion of 2 (0.20 g, 0.21 mmol) in CH+.& (15 cm3) was added PPh3 (0.061 g, 0.22 mmol) as a solution in CHICIP (5 cm’) at 0°C. Once the addition was complete, the solution was brought to room tem-

perature and was stirred for 10 min. A darkening of the initial red-brown solution was observed. At this point, the volume of the solution was reduced to -5 cm3, transferred to a straight narrow-bore tube and layered with 10 cm3 hexane. Dark red, needle-shaped crystals formed after 2 days at room temperature, which were collected by filtration and washed with hexane. The yield was 0.115 g, 45%. IR (carbonyl region), CH& cm-’ : 2081, 2030 and 2018. ‘H NMR (CDC4) : 6 7.55-7.21 (m, 17H, P-C6H5 and part of C&Me), 6.97, 6.55 and 6.48 (d, 6H, rest of C&Me), 4.92 and 4.81 (br s, 2H, -NH--), 2.21 and 2.01 (s, 6H, &H&e). “P{‘H} NMR (CDCl,, relative to H3P03 : - 3.74. Found : C, 35.2; H, 2.7; N, 2.3. Calc.: C, 35.1; H, 2.6; N, 2.3%.

Ir,(I),[~-NH(p-tolyl)]r(C0)3(PPhzMe) (4). Com- pound 2 (0.15 g, 0.16 mmol) was dissolved in CH$& (30 cm3) in a Schlenk flask, to which one equivalent of PPh,Me (0.032 g, 0.16 mmol), dis- solved in CH2C1, (6 cm’), was added dropwise. The initial red-brown colour of the solution darkened as the addition of PPh,Me was completed, at which point the mixture was reduced in volume and hex- ane was added to precipitate a red solid. The yield was 0.15 g, 85%. IR (carbonyl region), CH#& cm-’ : 2081,2029,2019. ‘H NMR (CDC13): 6 8.10- 7.30 (m, lOH, P-C,H,), 7.15, 6.96, 6.72 and 6.50 (d, 8H, C,H,Me), 4.83 and 4.8 1 (br, 2H, -NH-), 2.22 and 2.20 (s, 6H, C,H,Me), 1.53 (d, 3H, PPh&fe). 3’P{‘H} NMR (CDC13, H3P04 as external reference) : 6 - 13.86. Found : C, 32.1 ; H, 2.7 ; N, 2.2. Calc.: C, 31.8; H, 2.6; N, 2.5%.

Table 1. Summary of crystallographic data for 2 - OSCHCl,

Empirical formula Space group Temperature (“C) Cell dimensions

a (A) b (A) c (A) a (“) P (“) Y (“) Z (molecules cell - ‘) Volume (A3) Calculated density (g cm-‘) Wavelength (8) Molecular weight (amu) Linear absorption coefficient (cm-‘)

R(Q R,(F)

C,gH,41ZIr2N204* 0.5CHC13 P-l 23

13.347(3) 14.916(3) 16.484(4) 101.00(2) 109.00(2) 112.97(2) 4 2659.4 2.55 0.71073 1020.2

124.9 0.051 0.053

Structure and reactions of an amido bridged 18’ dimer 921

X-ray structural analysis of Ir,(I)&u-NH@tolyl)1, (CO), - O.SCHC&

Crystallographic data are collected in Table 1. The complex was crystallized by cooling a solution in chloroform to 0°C for ca 12 h. A dark red paral- lelepiped-shaped crystal, measuring 0.30 x 0.20 x 0.40 mm, was mounted on the tip of a glass fibre with epoxy cement. The observed cell constants, which were based on 25 centred reflections in the range 20” < 28 < 30”, revealed a triclinic system, and the centrosymmetric space group P- 1 was initially chosen due to its frequency of occurrence. The results of refinement support this choice. w scans of some strong low angle reflections revealed sharp diffraction peaks. Wyckoff scans over the 28 range 4.@-50.0” produced 9352 unique reflections, of which 7355 (I > 30(Z)) were used for refinement. The iridium atoms were located from a Patterson map, and the remaining non-hydrogen atoms were detected from difference Fourier maps. Each asymmetric unit was found to contain two structurally almost identical but crystallo- graphically independent dimer molecules, along with a molecule of CHC& as the solvent of crys- tallization. Full-matrix least-squares refinement was carried out with anisotropic thermal param- eters for all non-hydrogen atoms. The hydrogen atoms were placed in calculated positions. Refine- ment converged at R = 0.051 and R, = 0.053. The maximum shift/e.s.d. was less than 0.1 and the larg- est residual electron density in the final difference Fourier map was 2.65 e A-’ near one of the iridium atoms. An empirical absorption correction based on cp scans was applied. All computations used SHELXTL-PC software (G. Sheldrick, Siemens, Madison, WI).

Atomic coordinates have been deposited with the Cambridge Crystallographic Data Centre.

RESULTS

Molecular structure of Ir,(I),b-NH@-tolyl)],(CO),

(2)

Figure 1 shows a ball-and-stick diagram and the atom numbering scheme of one of the molecules found in the asymmetric unit. Although the two molecules in each asymmetric unit are crys- tallographically independent, they have virtually identical bond lengths and angles. Relevant bond distances and angles of both the molecules are given in Table 2.

Each of the two iridium centres in 2 adopts a distorted square-based pyramidal geometry ; the two pyramids share a common basal edge defined by the nitrogen atoms of the two p-tolylamido groups. The axial position in each square-based pyramid is occupied by an I- in such a way that the two I- ligands are mutually cis to each other. The two halves of the molecule are related to each other by a virtual mirror plane that passes through the bridging nitrogen atoms and bisects the Ir-Ir bond axis. The imaginary mirror plane also contains the two phenyl rings of the p-tolylamido groups. The short Ir-Ir distance of 2.591(l) 8, clearly indicates a formal Ir”-1r” single bond, as also shown by the diamagnetism of the complex. Table 3 lists some representative metal-metal distances observed in Irn2 complexes. A comparison of the values listed in the table suggests that the metal-metal distances observed in 2 are among the shortest observed thus far and are close to those observed for [Ir(COD)(p- pyrazolyl)],(X-Y) complexes, where X-Y is a strongly electron withdrawing alkyne, such as CZ(CF3)2 or C2(C02Me)z.3,4

The two Ir-I distances are very similar, with an average value of 2.708(2) A. The four Ir-N distances are also very similar, the average value

Fig. 1. Ball-and-stick view of Ir,(I)L-NH(p-tolyl)]~(CO), (2).

922 M. K. KOLEL-VEETIL er al.

Table 2. Selected bond distances (A) and angles (“) for 2 * OSCHCl, : the two values given for each distance and angle represent those observed for the two independent

molecules

Ir( 1)-h(2)

Ir(l)_I(l) Ir(2>-I(2) Ir(l)-N(1) Ir(l>-N(2) Ir(2)_N(l) Ir(2)-N(2) Ir(l)--C(l) Ir(l)--C(2) Ir(2)_C(3) Ir(2)-C(4)

N(l)_C(l1) N(2)-C(21) C(l)---o(l) C(2FW2) C(3>-0(3) C(4Wx4)

2591(l), 2607(l) 2.727(2), 2.727(2) 2.689(2), 2.729(l) 2.116(13), 2.106(9) 2.094(13), 2.086(14) 2.104(U), 2.113(15) 2.099(10), 2.104(14) 1.869(17), 1.888(13) 1.883(20), 1.907( 19) 1.894(14), 1.877(21) 1.884(20), 1.866( 18) 1.412(15), 1.429(20) 1.434(13), l/449(18) 1.145(22), 1.121(17) 1.156(28), 1.133(15) 1.131(19), 1.145(27) 1.137(28), 1.159(23)

Ir(2)-Ir(l)--I( 1) Ir(l+Ir(2)--I(2)

I(l)--Ir(l)_N(l) I(l)_Ir(lk-N(2) N(l)-Ir( 1)-N(2)

N(l)-Ir(l)--C(l) N(2Wr(l)--C(l) I(l)_Ir(l)--c(2) N(l)_Ir(WC(2) N(2Wr(l)--C(2) C(l)_Ir(l)--C(2) N(l)-Ir(2)_N(2) I(2)_Ir(2)--C(3) N(l)-Ir(2)-C(3) N(2)-Ir(2)-C(3)

I(2)_Ir(2)_C(4) N(l)_Ir(2>--C(4) N(2)_Ir(2)--C(4) C(3)_Ir(2)--C(4) Ir( 1 )-N( 1 )-Ir(2)

Ir(l)_N(l)--C(ll) Ir(2)--N( I)--C( 11) Ir(l)--N(2)-Ir(2) Ir( l)-N(2)--C(21)

Ir(2>--N(2)--C(21)

136.5(l), 138.1(l) 134.9(l), 136.6(l) 94.7(3), 99.9(4) 96.9(4), 94.7(3) 74.5(S), 74.7(5) 99.1(7), 97.6(5) 166.4(7), 166.9(8) 92.7(6), 91.1(6) 169.4(7), 166.9(8) 97.1(7), 99.7(7) 87.7(8), 86.8(8) 74.6(4), 74.2(5) 92.4(8), 91.0(S) 99.0(7), 97.6(7) 170.5(8), 165.7(7) 95.2(7), 93.5(5) 168.8(7), 171.4(6) 98.1(5), 98.2(6) 87.1(8), 88.9(9) 75.8(4), 76.3(4) 123.7(11), 124.7(11) 126.2(9), 122.8(12) 76.3(3), 76.9(5) 126.6(13), 121.3(8) 122.0(10), 124.7(12)

being 2.103( 12) A. Note that the latter distances are almost identical to those observed for the Ir’-Ir’ dimer (l), although the two complexes have metals in two different oxidation states. However, when the Ir2N2 cores in 1 and 2 are compared, the differences become quite apparent. To aid in the comparison, ball-and-stick diagrams of the two cores are pro- vided in Fig. 2. In going from 1 to 2, the formation of a single bond between the iridium atoms pulls

them closer by 0.34 A, which causes the Ir-N-Ir bond angles to decrease by an average of 12.8’. The two diagrams of Fig. 2b represent views per- pendicular to the Ir2N2 cores in the two molecules, and the compression in going from 1 to 2 is clearly evident when the dihedral angles of 125” (1) and 102” (2) between the Ir(l)-N(l)-N(2) and Ir(2)--N( 1)--N(2) planes are considered. The com- pression also brings the two pairs of COs on adjac- ent metal centres closer together. It appears that the large separation between these CO groups in 1 allows the p-tolyl groups to reside in between them, resulting in a large ‘hinge angle’ [defined by the dihedral angle between the planes Ir( l)-Ir(2)--N( 1) and Ir( l)-Ir(2)-N(2)] of 118.3”, thus minimizing distortion of the N( 1 )-Ir-N(2) angles. Since the carbonyl pairs are closer together in 2, steric crowding is likely to force the p-tolyl groups to move away from the carbonyl groups. If this were to happen without any change in the N(l)-Ir-N(2) bond angles, then the two bridging nitrogen atoms would show a higher degree of planarity in 2 compared to that in 1. This is not the case, as judged by the virtually identical pyramidality of the nitrogen atoms in the two com- plexes. We have used the angles that the N-C bonds make with the Ir(l)-Ir(2)-N planes to assess pyramidality of the nitrogen atoms. As can be seen in Fig. 2a, the average of the two angles around the nitrogen atoms in 1 is 137”, the same as that in 2. However, the hinge angle in 2 is appre- ciably smaller than that in 1, causing the p-tolyl groups to move away from the CO groups.

NMR and IR spectroscopy of 2

The ‘H NMR spectrum of 2 in CDC& reveals two types of phenyl protons, represented by a doub- let and a broad singlet. The -NH- resonance at 6 5.35 marks a shift of - 2 ppm relative to the same resonance in 1. The shift is possibly indicative of enhanced acidity of the hydrogen due to an increase in the metal’s oxidation state. The solution IR spec- trum reveals a three-band pattern, with bands at 2103, 2082 and 2047 cm-‘. All of the bands have shifted to higher frequencies relative to those in 1 by an average of 39 cm-‘, which can be attributed to reduced metal to CO back-bonding.

Reaction of 2 with PPh3 and PPhzMe

The diiodo complex readily undergoes sub- stitution of one of its carbonyl ligands with PPh3 and PPh,Me. More basic and less sterically hin- dered phosphines produce a mixture of unchar- acterizable products. The addition of one or more

Structure and reactions of an amido bridged Ir” dimer

Table 3. Metal-metal distances in selected dimeric complexes of Ir”

Complex Ir-Ir (A) Ref.

923

[Ir~-NH~-tolyl))(Co),(I)1, Ir,(~-NH(p-tolyl))~(CO),(PPh,)(I), [Ir,(CO)S(CSMe,)(CH,C,Me,l[BF,I [Ir,(CO),(C,Me,),l[BF,I [Ir(CO)(PPhJ(Cl)@-4-Cl-pz)l,

IrZ(Me)(I)(CODWpz) Ir,(COD)(l-a,4-S-r&H,,)@-pz)(p-PPh,) Irz(~-tBuS)2(CO)z(PPhMe,)21z

[Ir(H)(~-‘Bus)(Co)(P(OMe),)l, [WNW(CODk [Ir(H)(Co),(PPh,)lz(~-S02) [IWWPPW12 Ir,(p-C,H,)(C,HS)2(Co),

2.591(l) this work 2.579( 1) 5 2.794(l) 6 2.839( 1) 6 2.737( 1) 7 3.112(l) 3 2.780( 1) 8 2.702( 1) 9 2.673( 1) 10 2.800( 1) 11 2.759(2) 12 2.759(2) 13 2.7166(2) 14

pz = pyrazolyl ; COD = l&cyclooctadiene.

(b)

(4

Fig. 2. Views of the diiridium cores in Ir,~-NH(p-tolyl)]z(CO)~ (1) and Ir,(I)&-NH@-tolyl)Z(CO), (2). All of the angles noted here are dihedral angles. (a) Views along metal-metal axes. (b) Views

perpendicular to metal-metal axes.

924 M. K. KOLEL-VEETIL et al.

equivalents of PPh, to 2 in CH,Cl, causes an immediate change in the carbonyl region of the IR spectrum, with three new bands appearing at 208 1, 2030 and a shoulder at 2018 cm-‘. The resulting PPh,-substituted complex, Ir,(I),b-NH@tolyl)l, (CO),(PPh,) (3), has been assigned the structure shown below. An X-ray crystal structure’ confirms the following assignment. As expected, the addition of basic PPh3 in place of 7~ acidic CO lowers the stretching frequencies of the remaining CO bands by an average of 34 cm-‘.

Ph,p“‘

: i’q--/‘r::.,‘co

:- \ ptolyi’ ?I

3

The ‘H NMR spectrum of 3 in CDCl, is con- sistent with its solid state structure. Since the mol- ecule lacks any symmetry, two separate resonances, at 6 2.02 and 2.21, are observed for the methyl groups on the p-tolyl moieties. The -NH- hydro- gens appear as two broad resonances at 6 4.81 and

*

Phenyl-H

J **

4.92. The resonance at higher frequency appears broader than the other, possibly due to unresolved coupling to PPh3. Note that these chemical shifts are upfield by -0.5 ppm relative to those in 2. The shift can be attributed to increased electron density at the iridium centres due to the presence of the phosphine ligand. We have considered the possi- bility of significantly different -NH- to I- non- bonded contacts in 2 and 3 that may be reiponsible for the chemical shift differences. However, the non- bonding 1. * *N distances in the two complexes are virtually identical (between 3.56 and 3.63 A), thereby ruling out this possibility. The region between 6 6.48 and 7.55 features all of the aromatic hydrogens. As seen in Fig. 3, only three of the four resonances (doublets) due to the aromatic hydro- gens of the tolyl groups can be observed (between 6 6.48 and 6.98). The coordinated phosphine shows a resonance at 6 -3.74 (relative to H3P04) in the 31P(‘H} NMR spectrum.

The reaction of 2 with PPh,Me affords a similar product, Irons-NH~-tolyl)]*(CO)~(PPh~Me) (4). The NMR (‘H and 3’P) and IR spectroscopic data of this complex are described in the Experimental section.

I I I I I I I 8 1 6 5 I 3 2

Fig. 3. ‘H NMR specmm (250 MHz) of Irons-NH~-toIyl)]~(C~)~(PPh~) (3). Asterisk denotes the resonance for the protio impurity of CDC& solvent and double asterisk denotes the resonance due

to CH2Clz, which is present as solvent of crystallization.

-NH-

J

Structure and reactions of an amido bridged Ir” dimer 925

DISCUSSION

As mentioned in the introduction, the mode of oxidative addition to 1 of I2 is different from those of CIZ, Br,, HCl and Mel. Although IR and NMR spectral analyses of 2 provide strong evidence for the formation of an Ir”-It-” dimer, the X-ray struc- tural characterization reported here provides the definitive proof. As we had anticipated, 2 contains two distorted square-based pyramidal 18’ centres joined by a metal-metal single bond. This is the first example of a dimeric Ir” complex containing amido groups in the bridging positions. Although dimeric It-” complexes are not uncommon, their number is still relatively few. A comparison of the metal-metal distances in 2 and 3 with other structurally char- acterized Ir” dimers (Table 3) reveals a very short bond for both the molecules. The small size of the bridging nitrogen atoms possibly allows closer con- tact of the iridium atoms as compared to the other examples, most of which contain bulky sulphur or phosphorus bridging atoms.

The substitution chemistry of 2 with PPh3 and PPh,Me reflects a strong cooperation between the two metal centres, undoubtedly a consequence of the presence of a metal-metal bond. Under normal conditions, only one CO group can be substituted, even in the presence of excess phosphine. This sug- gests that as soon as one iridium centre undergoes substitution, the other becomes significantly deac- tivated. Contrast these substitution reactions with those between It-,(I)&-NH@-tolyl)],(CO), and phosphines.15 The lack of a metal-metal bond, and thus a weaker coupling, between the two metal cen- tres in the tetraiodo complex results in substitution occurring at both of the metal centres sim- ultaneously.

Further work is in progress to achieve trans- formations of 1,2,3 and 4 to new imido complexes.

Acknowledgements-We are grateful to Johnson Mat- they/Aesar for a generous loan of iridium chloride. J.

F. C. acknowledges a summer undergraduate fellowship from Pfizer, Inc. P. R. Y. is grateful to the VISIT program, administered by the Graduate College, Uni- versity of Vermont.

1.

2.

3.

4.

5. 6.

I.

8.

9.

10.

14.

15.

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