Experimental and computational studies of two new mono- and dinuclear iridium complexes containing a Buchwald biphenyl phosphine ligand

Article abstract of DOI:10.1016/j.ica.2007.11.006

The reaction of [(η⁴-1,5-C₈H₁₂)₂Ir₂ (μ-Cl)₂] with 2-di-t-butylphosphino-2′-methylbiphenyl (t-Bu₂Pbiph^Me) in the presence of AgBF₄ afforded the dichlorido-bridged Ir-Ag complex [(η⁴-1,5-C₈H₁₂)Ir(μ-Cl) ₂Ag(t-Bu₂Pbiph^Me)] (1) which was fully characterized by a single crystal X-ray diffraction study. Sequential treatment of the diiridium precursor first with the silver salt and then with the phosphine yielded cyclometalated [(η⁴-1,5-C₈H₁₂)Ir(t-Bu₂P biph^Me-H⁺)] (2). Detailed DFT calculations gave evidence that the phosphine ligand of 2 forms a strained four-membered iridaheterocycle through orthometalation rather than a sterically congested six-membered chelate structure through C-H activation on the remote phenyl ring. The phosphonium salt [t-Bu₂P(H)biph^Me]BF₄ was isolated as a by-product of the preparations of 1 and 2; its crystal structure was determined.

Full text of DOI:10.1016/j.ica.2007.11.006

Available online at www.sciencedirect.com
Inorganica Chimica Acta 361 (2008) 2623–2630
www.elsevier.com/locate/ica
Experimental and computational studies of two new
mono- and dinuclear iridium complexes containing a Buchwald
biphenyl phosphine ligand
a,
a
a
Lutz Dahlenburg *, Ralf Menzel , Ralph Puchta a,b, Frank W. Heinemann
a
Institut fu¨r Anorganische Chemie, Friedrich-Alexander-Universita¨t Erlangen-Nu¨rnberg, Egerlandstrasse 1, D-91058 Erlangen, Germany
Computer-Chemie Centrum (CCC), Friedrich-Alexander-Universita¨t Erlangen-Nu¨rnberg, Na¨gelsbachstrasse 25, D-91052 Erlangen, Germany
b
Received 14 August 2007; received in revised form 25 October 2007; accepted 3 November 2007
Available online 7 November 2007
Abstract
The reaction of [(g4-1,5-C8H12)2Ir2(l-Cl)2] with 2-di-t-butylphosphino-20-methylbiphenyl (t-Bu2PbiphMe) in the presence of AgBF4
afforded the dichlorido-bridged Ir–Ag complex [(g4-1,5-C8H12)Ir(l-Cl)2Ag(t-Bu2PbiphMe)] (1) which was fully characterized by a single
crystal X-ray diffraction study. Sequential treatment of the diiridium precursor first with the silver salt and then with the phosphine
yielded cyclometalated [(g4-1,5-C8H12)Ir(t-Bu2PbiphMe–H+)] (2). Detailed DFT calculations gave evidence that the phosphine ligand
of 2 forms a strained four-membered iridaheterocycle through orthometalation rather than a sterically congested six-membered chelate
structure through C–H activation on the remote phenyl ring. The phosphonium salt [t-Bu2P(H)biphMe]BF4 was isolated as a by-product
of the preparations of 1 and 2; its crystal structure was determined.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: 2-Di-t-butylphosphino-20-methylbiphenyl; Iridium complexes; Cyclometalation; DFT calculations; X-ray structure analysis
1. Introduction
tivity (e.e.max: 24%) [1]. Looking for iridium catalysts with
possibly improved properties, we therefore set out to syn-
*
In previous work, we described some P2/N2-coordinated
cis-dihydridoiridium(III)
thesize cis-dihydrides [IrH2(P*R3)2(H2N\ NH2)]BF4 bear-
compounds
[IrH2(PPh3)2(1,
ing one chiral diamine along with two chiral monodentate
phosphorus ligands as represented by, e.g., the sterically
demanding atropisomeric Buchwald biphenyl phosphines
R2Pbiph (R = t-Bu, cyclo-C6H11) [3].
2-diamine)]BF4 containing the enantiomerically pure N,N
chelates (1R,2R)-1,2-diaminocyclohexane and (R)-2,20-dia-
mino-1,10-binaphthyl in addition to two achiral triphenyl-
phosphine ligands [1]. They were made by substituting the
chelating diamines for the loosely bound solvate ligands
of [IrH2(OCMe2)2(PPh3)2]BF4, which in turn was obtained
by the hydrogenation of [(g4-1,5-C8H12)Ir(PPh3)2]BF4 in
acetone [2]. If activated by strong base in methanol, the chi-
ral chelate complexes [IrH2(PPh3)2(1,2-diamine)]BF4 acted
as enantioselective catalysts for the hydrogenation of aceto-
phenone to (S)-1-phenylethanol, but only with poor selec-
We expected that such compounds might result as
enriched diastereomers from reactions between enantio-
pure 1,2-diamines and the racemates of weakly solvated
dihydridobis(biphenyl phosphine) complexes [IrH2(solv)2-
{(R,S)-R2Pbiph}2]BF4 or their direct precursors [(g4-1,
5-C8H12)Ir{(R,S)-R2Pbiph}2]BF4. In a first effort we there-
fore addressed the preparation of a (cyclooctadiene)irid-
ium(I) complex with additional Buchwald ligands
through the Ag+-assisted bridge-opening reaction of [(g4-
1,5-C8H12)2Ir2(l-Cl)2] with 2-di-t-butylphosphino-20-meth-
ylbiphenyl (referred to as ‘‘t-Bu2PbiphMe’’ in the following)
as an exemplary biphenyl phosphine. This approach did
*
Corresponding author. Tel.: +49 9131 8527353; fax: +49 9131
8527387.
E-mail address: dahlenburg@chemie.uni-erlangen.de (L. Dahlenburg).
0020-1693/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2007.11.006

2624
L. Dahlenburg et al. / Inorganica Chimica Acta 361 (2008) 2623–2630
not afford the hoped-for target compound but rather
launched quite an unexpected coordination chemistry of
the t-Bu2PbiphMe ligand toward IrI and AgI, which is the
subject of the following.
2.3. (2-Di-t-butylphosphino-20-methylbiphenyl-3-yl)(g4-1,
5-cyclooctadiene)iridium(I), [(g4-1,5-C8H12)Ir(2-t-Bu2-
PC6H3C6H4Me-20-jC6,jP)] (2)
A
solution of 242 mg (0.36 mmol) of [(g4-1,5-
2. Experimental
C8H12)2Ir2(l-Cl)2] and 140 mg (0.72 mmol) of AgBF4 in
10 mL of acetone was stirred overnight at room tempera-
ture. Precipitated silver chloride was removed by filtration
and the filtrate was treated with 225 mg (0.72 mmol) of
t-Bu2PbiphMe dissolved in 10 mL of acetone. The mixture
slowly changed color from orange to dark red and depos-
ited a fine precipitate. Evaporation to dryness followed
by trituration of the residue with 20 mL of diethyl ether left
some [t-Bu2P(H)biphMe]BF4 which was filtered off. Con-
centration of the filtrate gave 275 mg (63%) of 2 as a red
solid. 1H NMR (CDCl3): d = 1.18, 1.41 (both d,
3J(P,H) = 13.7 Hz each, 9 H each, both t-butyl CH3),
1.76–2.52 (m, 8H, diene CH2), 2.28 (s, 3H, biphenyl
CH3), 4.55, 4.63, 4.71, 4.90 (all m, 1H, each, all diene
CH), 6.97–7.50 (m, 7H, biphenyl H). 13C{1H} NMR
(C6D6): d = 19.36 (s, biphenyl CH3), 29.42, 30.52 (both d,
2J(P,C) = 6.1 Hz each, both t-butyl CH3), 28.33, 30.90,
32.69 (all s, 1 + 2 + 1C, all diene CH2), 34.76, 34.98 (both
2.1. General remarks
All manipulations were performed under nitrogen using
standard Schlenk techniques. Solvents were distilled from
the appropriate drying agents prior to use. – NMR: Bruker
DPX 300 (300.1 MHz for 1H, 75.5 MHz for 13C, and
121.5 MHz for 31P) at 20 2 ꢁC with SiMe4 (or the sol-
vent) as internal or H3PO4 as external standards (downfield
positive). – Racemic 2-di-t-butylphosphino-20-methylbi-
phenyl and silver tetrafluoridoborate were used as pur-
chased from Strem. [(g4-1,5-C8H12)2Ir2(l-Cl)2] was
prepared as reported in the literature [4].
2.2. Di-l-chlorido[(g4-1,5-cyclooctadiene)iridium(I)][(2-
di-t-butylphosphino-20-methylbiphenyl)silver(I)], [{(g4-1,
5-C8H12)Ir}(l-Cl)2{Ag(2-t-Bu2PC6H4C6H4Me-20)}] (1)
1
d, J(P,C) = 10.5 and 9.1 Hz, both t-butyl Cquart), 68.21,
Solid silver tetrafluoridoborate (62 mg, 0.32 mmol) was
added to a mixture of 108 mg (0.16 mmol) of [(g4-1,5-
C8H12)2Ir2(l-Cl)2] and 200 mg (0.64 mmol) of t-Bu2P-
biphMe in 20 mL of acetone. The yellow solution was stir-
red for 24 h at ambient conditions, which caused the
product to precipitate as yellow microcrystals which were
collected by filtration, washed with diethyl ether
(3 · 2 mL), and dried under vacuum; yield 74 mg (58%).
69.69 (both d, 2J(P,C) = 12.1 Hz each, both diene CH trans
P), 69.06, 70.31 (both s, both diene CH cis P), 123.26 (s),
125.93 (d, J(P,C) = 4.5 Hz), 127.59 (d, J(P,C) = 7.5 Hz),
128.34, 128.55, 128.84, 128.92, 135.53 (all s), 141.88 (d,
J(P,C) = 2.3 Hz), 143.13 (s), 151.78 (d, J(P,C) = 33.2 Hz),
160.46 (d, J(P,C) = 9.8 Hz) all biphenyl C. 31P{1H}
NMR (C6D6): d = 10.67 (s). Anal. Calc. for C29H40IrP
(611.82): C, 56.93; H, 6.59. Found: C, 57.02; H, 6.70%.
3
1H NMR (CDCl3): d = 1.34, 1.39 (both d, J(P,H) = 5.1
and 4.2 Hz, 9H each, both t-butyl CH3), 2.08 (s, 3H, biphe-
nyl CH3), 2.23 (m, 8H, diene CH2), 3.56, 4.00 (both m, 2H
each, both diene CH), 7.05–7.92 (m, 8H, biphenyl H).
13C{1H} NMR (CDCl3): d = 20.92 (s, biphenyl CH3),
2.4. X-ray structure determinations
The specimen used for the structure determination of 1
was picked from a sample of the complex that was re-crys-
tallized from CHCl3. Single crystals of the phosphonium
salt 3 grew from the acetone mother liquor of the prepara-
tion of compound 1. Diffraction measurements were made
with a Bruker-Nonius Kappa CCD instrument at ꢀ173 ꢁC
(complex 1) and with an Enraf-Nonius CAD-4 MACH 3
diffractometer at ꢀ140 ꢁC (phosphonium salt 3) using Mo
Ka radiation; data of 3 were uncorrected for absorption,
those of 1 were corrected for absorption by an appropriate
semi-empirical procedure [5] (Tmin = 0.599, Tmax = 0.760).
The structures were solved by direct methods and subse-
quently refined by full-matrix least-squares procedures on
F2 with allowance for anisotropic thermal motion of all
non-hydrogen atoms employing both the SHELXTL NT 6.12
[6] and the WINGX [7a] packages with some of the relevant
programs (SADABS [5], SIR-97 [8], SHELXL-97 [9], ORTEP-3
[7b]) implemented therein. Structure 1 was refined in space
group P21 allowing for racemic twinning with a twin com-
ponent ratio close to 1:1. Hence, a structure refinement at
higher symmetry (space group P21/m with the molecule
on a crystallographic mirror plane containing the metal
2
31.12, 31.55 (both d, J(P,C) = 10.6 Hz each, both t-butyl
CH3), 31.77, 32.24 (both s, both diene CH2), 35.53, 35.91
1
2
(both dd, J(P,C) = 9.8 Hz each, J(107/109Ag,C) = 4.5 Hz
each, both t-butyl Cquart), 56.74, 61.76 (both s, both diene
CH), 126.77 (s), 127.23 (unresolved dd), 128.29, 128.59
(both unresolved d), 129.64, 130.90, 132.54 (all s), 132.69,
134.47 (both d, J(P,C) = 6.5 and 8.0 Hz), 135.33 (s),
140.29 (d, J(P,C) = 8.7 Hz), 149.80 (dd, J(P,C) = 18.9 Hz,
J(107/109Ag,C) unresolved): all biphenyl C. 31P{1H} NMR
(CDCl3): d = 43.04 (pair of d, J(107Ag,P) = 619,
J(109Ag,P) = 715 Hz). Anal. Calc. for C29H41AgCl2IrP
(791.56): C, 44.00; H, 5.22. Found: C, 42.92; H, 5.25%.
Concentration of the mother liquor induced the crystalliza-
tion of the phosphonium salt [t-Bu2P(H)biphMe]BF4 (3)
which was formed as a by-product of the above prepara-
tion. 1H NMR (CD2Cl2): d = 1.45, 1.62 (both d,
3J(P,H) = 15.0 Hz each, 9 H each, both t-butyl CH3),
1
2.14 (s, 3 H, aryl CH3), 5.84 (d, J(P,H) = 459.6 Hz, 1 H,
PH), 7.10–8.00 (m, 8 H, aryl H). 31P{1H} NMR (CD2Cl2):
d = 33.22 (s).

L. Dahlenburg et al. / Inorganica Chimica Acta 361 (2008) 2623–2630
2625
and phosphorus atoms as well as the phenylene ring and
t-Bu2PbiphMe / AgBF4
Cl
Cl
Ir
Ir
the ipso- and para-C atoms of the tolyl group) was also
tested but lead to heavily disordered t-butyl carbon atoms
with non-positive-definite displacement parameters and
ultimately gave significantly higher residuals (wR2 =
0.0697, R1 = 0.0335) than in P21 (vide infra). In structure
3, the split occupancies for the disordered tolyl carbon
atoms of the biphenyl residue refined to 0.61 for C15a–
C21a and 0.39 for C15b–C21b, respectively. The tetrafluor-
idoborate anion showed threefold disorder about the B1–
F1 bond, which was modelled by assigning site occupation
factors of 0.50, 0.30, and 0.20 to atoms F2a–F4a, F2b–
F4b, and F2c–F4c, respectively. Compound 1 (0.10 ·
0.05 · 0.05 mm): C29H41AgCl2IrP (791.56); monoclinic,
Me
Cl
Cl
Ir
Ag
P
tBu
But
(1)
Scheme 1.
required phosphine ligand in the presence of a soluble silver
salt as a chloride-abstracting reagent. However, stirring a
1:2:4 molar mixture of the chlorido-bridged dimer, AgBF4,
and t-Bu2PbiphMe in acetone did not lead to [(g4-1,5-
C8H12)Ir(t-Bu2PbiphMe)2]BF4 but resulted in the formation
of the heterobimetallic complex [(g4-1,5-C8H12)Ir(l-
Cl)2Ag(t-Bu2PbiphMe)] (1); Scheme 1. The phosphonium
tetrafluoridoborate [t-Bu2P(H)biphMe]BF4 (3) was also
produced, most probably by the reaction of the remaining
phosphine with adventitious water in the solvent or the
hygroscopic silver salt.1
˚
P21, a = 9.736(2), b = 12.659(3), c = 11.816(2) A, b =
3
95.22(3)ꢁ, V = 1450.3(5) A , Z = 2, dcalc = 1.813 g cmꢀ3
,
˚
l(Mo Ka) = 5.514 mmꢀ1; 3.46ꢁ 6 h 6 28.52ꢁ, 31100 reflec-
tions collected (ꢀ13 6 h 6 +13, ꢀ16 6 k 6 +16, ꢀ15 6
l 6 +15), 7323 unique (Rint = 0.0412); wR2 = 0.0472 for
all data and 307 parameters, R1 = 0.0274 for 6415 intensi-
ties I > 2r(I). Compound 3 (0.48 · 0.38 · 0.20 mm):
C21H30P, BF4 (400.23); monoclinic, P21/n, a = 10.133(3),
3
˚
˚
b = 18.102(4), c = 11.427(2) A, V = 2095.8(9) A , Z = 4,
Coordination of the t-Bu2PbiphMe ligand to the silver
rather than the iridium center of 1 became evident from
the 31P{1H} NMR spectrum displaying a pair of doublets
due to the coupling of the 31P nucleus with the two silver
isotopes 107Ag and 109Ag. The coupling constants
1J(107Ag, 31P) = 619 Hz and 1J(109Ag, 31P) = 715 Hz are
close to those previously measured for the tetranuclear
compound [{(p-cumene)Ru(l-Cl)2Ag(l-t-Bu2PCH2CH2-
PPh2)}2][PF6]2 (643 and 743 Hz), where the silver ion is also
triply coordinated by two bridging chlorido ligands and
one terminal t-Bu2P donor function [25].
Compound 1 was fully characterized by single crystal X-
ray crystallography. The specimen studied belonged to the
monoclinic space group P21 and turned out to be a racemic
twin with a twin component ratio close to 1:1. A perspec-
tive view of the molecular model resulting from the refine-
ment is given in Fig. 1.
dcalc
=
1.268 g cmꢀ3
,
l(Mo Ka) = 0.168 mmꢀ1
;
2.11
ꢁ 6 h 6 22.50ꢁ, 2928 reflections collected (ꢀ10 6 h 6 0,
ꢀ19 6 k 6 0, ꢀ12 6 l 6 +12), 2748 unique (Rint = 0.043);
wR2 = 0.2482 for all data and 335 parameters, R1 = 0.086
for 1652 intensities I > 2r(I).
2.5. Computational details
All structures were fully optimized using the B3LYP
hybrid density functional [10–12] and the LANL2DZ basis
set augmented with polarization functions (further denoted
as LANL2DZp) [13–16]. All structures were characterized
as minima by the computation of vibrational frequencies.
The influence of the bulk solvent was probed by single
point calculations employing the IPCM formalism [17]
with water as solvent, i.e., B3LYP(IPCM)/LANL2DZp//
B3LYP/LANL2DZp. The GAUSSIAN 03 suite of programs
was used [18]. Additional structure optimizations were car-
ried out with JAGUAR 6.5 [19] applying LMP2/LACVP*
[14,20,21]. The pure density functional BP86 [22] together
with the LACVP* basis set [14,21] was applied for structure
calculations in the gas phase [23] and the water model [24]
as implemented in JAGUAR 6.5.
Although there exist some heterobimetallics in which
iridium(I) is linked by halide bridges to a different transi-
tion metal [26], the IrI(l-Cl)2AgI complex presented in this
work is the only example so far for which structural data
The stoichiometry [(g4-1,5-C8H12)2Ir2(l-Cl)2]/AgBF4/t-Bu2P-
1
biphMe = 1:2:4 used in the preparation of 1 was originally chosen in view
of the attempted synthesis of [(g4-1,5-C8H12)Ir(t-Bu2PbiphMe)2]BF4 as a
target complex. As one of the referees raised the question whether this
molar ratio is really needed, the reaction was repeated using the 1:1:1
limiting stoichiometry of the reactants; specifically: 323 mg of [(g4-1,5-
C8H12)2Ir2(l-Cl)2], 93 mg of AgBF4, and 150 mg of t-Bu2PbiphMe
(0.48 mmol each) in 20 mL of acetone. Work-up gave a reddish orange
semisolid shown to be a mixture of 1 (31P NMR) and unreacted [(g4-1,5-
C8H12)2Ir2(l-Cl)2] (1H NMR). Formation of the phosphonium salt [t-
Bu2P(H)biphMe]BF4 as an accompanying product was also observed,
implying that for the complete consumption of the starting homodimer an
excess of the highly basic phosphine is indeed required because part of it is
lost by protonation.
3. Results and discussion
3.1. [(g4-1,5-C8H12)Ir(l-Cl)2Ag(t-Bu2PbiphMe)] (1)
The most direct route to cationic cyclooctadiene com-
plexes of iridium(I) with two additional monophosphines
or one chelating diphosphine such as [(g4-1,5-
C8H12)Ir(PPh3)2]+ [2d,2e], [(g4-1,5-C8H12)Ir{(R)-binap}]+
[1], and the like involves the reaction of [(g4-1,5-
C8H12)2Ir2(l-Cl)2] with stoichiometric quantities of the

2626
L. Dahlenburg et al. / Inorganica Chimica Acta 361 (2008) 2623–2630
roofing the IrI(l-Cl)2AgI bridge of 1 range from 2.921(7)
C21
C20
C25
C27
˚
C24
C29
C23
to 3.188(7) A, which suggests that there are no obvious
d10-metal–arene interactions in the compound studied in
C4
C2
C26
C19
C22
this work.
C28
C5
The phosphonium tetrafluoridoborate [t-Bu2P(H)biph-
Me]BF4 (3) which crystallized as an accompanying product
C18
C17
C9
C3
Cl1
C11
1
Ir1
from the mother liquors of 1 was identified by its H and
C6
31P NMR data (see Section 2) and a complete X-ray struc-
ture analysis. The structure determination revealed the
presence of P–Hꢁ ꢁ ꢁF hydrogen-bonded ion pairs consisting
P1
Ag1
C7
C12
C1
C16
Cl2
C8
C10
C14
C13
ꢀ
of threefold rotationally disordered BF4 anions and [t-
C15
Bu2P(H)biphMe +
] cations having twofold disordered tolyl
groups in their biphenyl residues (Fig. 2). P–C bond lengths
and C–P–C angles compare favorably to those previously
measured for the phosphonium ions of the compound [t-
Bu2P(H)Ph]3[Zr6Cl18H5] [33]. This includes the t-Bu–P–
Bu-t angles which in both cations are expanded to 117.4ꢁ
due to steric hindrance between the two bulky substituents.
In the biphenyl unit, the disordered tolyl rings are rotated
with respect to the phenylene group by 72.4ꢁ and 76.2ꢁ.
Fig. 1. A perspective view of the molecule [(g4-1,5-C8H12)Ir(l-Cl)2Ag(t-
Bu2PbiphMe)] (1); hydrogen atoms are omitted for clarity; selected bond
˚
lengths (A) and angles (ꢁ): Ir1–Cl1, 2.385(2); Ir1–Cl2, 2.375(3); Ag1–Cl1,
2.554(2); Ag1–Cl2, 2.598(3); Ag1–P1, 2.366(1). Cl1–Ir1–Cl2, 89.33(4);
Cl1–Ag1–Cl2, 80.97(4); Cl1–Ag1–P1, 137.43(11), Cl2–Ag1–P1, 136.62(11);
Ir1–Cl1–Ag1, 92.51(8); Ir1–Cl2–Ag1, 91.66(10).
are available [27,28]. Its structure can be described most
simply as having an (g4-1,5-C8H12)IrCl2 building block
to which one Ag(t-Bu2PbiphMe) group is attached in a
slightly asymmetric fashion. The three-coordinate geome-
try around the silver center is strongly distorted in that
the Cl–Ag–Cl angle of 80.97(4)ꢁ and the two Cl–Ag–P
angles [136.6(1)ꢁ and 137.4(1)ꢁ] deviate significantly from
the ideal value of 120ꢁ expected for trigonal planar coordi-
3.2. [(g4-1,5-C8H12)Ir(2-t-Bu2PC6H3C6H4Me-20-jC6,
jP)] (2)
If the diiridium complex [(g4-1,5-C8H12)2Ir2(l-Cl)2] was
combined first with 2 equiv. of AgBF4 and then with
4 equiv. of the t-Bu2PbiphMe ligand, a red product was
obtained which elemental analysis and spectroscopic data
confirmed to be cyclometalated [(g4-1,5-C8H12)Ir(t-Bu2P-
biphMe–H+)] (2); Scheme 2. Again, the phosphonium salt
[t-Bu2P(H)biphMe]BF4 (3) was observed as an accompany-
ing product of the reaction.
t
˚
nation. The Ag–P Bu2 bond length of 1, 2.366(1) A, is sim-
t
˚
ilar to the Ag–P Bu2 distance of 2.359(2) A measured for
the
ruthenium–silver
complex
[{(p-cumene)Ru(l-
Cl)2Ag(l-t-Bu2PCH2CH2PPh2)}2][PF6]2 [25]. While the lat-
˚
ter possesses one short [2.472(2) A] and one long
˚
[2.726(2) A] Ag–Cl bond, the distances between the silver
C7
ion and the bridging chlorides of 1 [2.554(2) and
˚
2.598(2) A] show only little divergence. The two Ir–Cl
C5
bonds are also comparable in length, 2.375(3) and
C6
C8
C1
4
˚
2.385(2) A, and only slightly shorter than those of [(g -
C3
F1
˚
C4
1,5-C8H12)2Ir2(l-Cl)2], 2.399(5) and 2.400(5) A [29].
C10
Although the Ir(l-Cl)2Ag unit is folded with an angle
between the normals to the two MCl2 planes of 24.4ꢁ, this
deviation from planarity is much less than that in the bent
structure of the starting homodimer, where the dihedral
angle between the two IrCl2 planes is 86ꢁ [29]. The l.s.q
planes spanned by the tolyl and the phenylene ring of the
biphenyl group are almost orthogonal to each other, the
value of the associated dihedral angle being 89.2ꢁ. Such
substantial rotation of the two aromatic systems with
respect to each other could facilitate p-bonding between
the terminal ring and the d10 metal center AgI similar to
that observed for a number of structurally characterized
complexes where bulky t-Bu2Pbiaryl or Cy2Pbiaryl ligands
are linked to d10-Pd0 [30–32]. These palladium compounds
F4c
F2b
P1
C9
F4a
F4b
F2a
C11
B1
C2
C21b
C16a
F2c
C12
C15a
C14
F3c
F3b
C17a
F3a
C13
C15b
C20b
C19b
C16b
C21a
C18a
C20a
C19a
C17b
C18b
Fig. 2. A perspective view of the [t-Bu2P(H)biphMe]BF4 ion pair (3)
showing the threefold rotational disorder of the BF4ꢀ ion about the B1–F1
bond and the twofold disorder of the terminal ring of the biphenyl residue;
carbon-bonded hydrogen atoms are omitted for clarity; selected bond
lengths (A) and angles (ꢁ): P1–C1, 1.831(8); P1–C5, 1.856(9); P1–C9,
1.791(8). C1–P1–C5, 117.4(4); C1–P1–C9, 109.3(4); C5–P1–C9, 112.7(4).
1
2
˚
typically feature g - and/or g -contacts of 2.298–2.676 A
between the central metal and, respectively, the ipso- and
ortho-carbon atoms of the remote ring. The corresponding
distances between the AgI ion and the aromatic system
˚
˚
Hydrogen bonding interaction as D–H, Hꢁ ꢁ ꢁA, Dꢁ ꢁ ꢁA, D–Hꢁ ꢁ ꢁA (A, ꢁ):
P–Hꢁ ꢁ ꢁF2a, 1.00, 2.49, 3.37(1), 145.5.

L. Dahlenburg et al. / Inorganica Chimica Acta 361 (2008) 2623–2630
2627
1.) AgBF4
2.) t-Bu2PbiphMe
the IrIII complex [IrH(t-Bu2PC6H4-jC2,jP)(t-Bu2PPh)]
[B{3,5- (F3C)2C6H3}4], d = 39.7 and 6.2 [38], pointing to
Cl
Cl
Ir
Ir
a
31P upfield shift of 33.5 ppm upon the incorporation of
the di-t-butylphosphino group into a four-membered
orthometalated iridaheterocycle. Unfortunately, no coordi-
nation compounds of iridium are known which bear a
cyclometalated t-Bu2PbiphMe ligand together with a non-
cyclometalated one and thus could serve to make more
than a rough estimate of the changes in its shift values
Dcoord and Dring upon coordination and formation of a
four-membered (or six-membered) iridacyclic chelate
complex. To solve these ambiguities, quantum chemical
studies aiming at the computation of the relative stabilities
of the two isomers 2-4r and 2-6r depicted in Scheme 3 were
carried out.
Ir
P
Me
But
tBu
(2)
Scheme 2.
The presence of a direct metal-to-aryl linkage in 2,
which could not be obtained as single crystals, was evi-
denced from the 13C{1H} spectrum, where the 13C reso-
nance of the metal-bound carbon atom displays the
significant downfield shift (d = 160.46, d, J(P,C) = 9.8 Hz)
known to be diagnostic of phenyl carbon coordination to a
transition metal [34]. It is well established that gem-di-tert-
butyl groups on phosphorus ligands assist the formation of
four- to six-membered metalacycles by enthalpy, entropy,
and conformational effects which are similar to those of
the gem-dialkyl or ‘‘Thorpe-Ingold’’ effect on ring closure
reactions in organic chemistry [35]. In principle, the steri-
cally promoted internal metalation of the Ir-coordinated
t-Bu2PbiphMe ligand could involve orthometalation of the
phenylene ring forming a strained four-membered ring as
3.3. DFT studies
Independent of the quantum chemical method
employed, the four-membered ring system 2-4r is seen to
be favored over 2-6r under all conditions studied. Thus,
B3LYP/LANL2DZp and BP86/LACVP* revealed 2-4r to
be, respectively, 15.2 and 15.0 kcal/mol more stable than
the six-membered P,C chelate 2-6r. A somewhat smaller
energy gap of 11.7 kcal/mol between the two isomers was
calculated applying LMP2/LACVP*. The influence of sol-
vent effects was investigated by means of IPCM single
point calculations [B3LYP(IPCM)/LANL2DZp//B3LYP/
LANL2DZp] which still showed 2-4r to be preferred by
9.5 kcal/mol. Similar to the values obtained with B3LYP/
LANL2DZp and BP86/LACVP* in the gas phase, BP86/
LACVP* optimization of the two structures within the
SCRF water model implemented in JAGUAR 6.5 led to an
energy difference of 15.6 kcal/mol in favor of the four-
membered metalaheterocycle without resulting in signifi-
cant structural changes.
indicated in Scheme
2 and known from [{Pd(2-t-
Bu2PC6H3C6H4Me-20-jC6,jP)}2(l-Cl)2] [32b]; alterna-
tively, the activation of a C–H bond on the remote tolyl
group could give an unstrained six-membered metalacycle,
which one might anticipate to be more stable (see Scheme
3) and, hence, has been proposed for the palladacyclic
0
structure of [Pd(2-t-Bu2PC6H4C6H4-jC6 ,jP)(O2CMe)]
[36]. Cyclometalated phosphines featuring four-membered
rings show unusual highfield 31P resonances compared to
their non-cyclometalated counterparts, whereas P donors
incorporated in six-membered chelate rings give rise to
slightly downfield-shifted 31P NMR signals [37]. The 31P
nucleus of 2 resonates at d = 10.7, which is 10.4 ppm
upfield from the chemical shift of the free ligand
(d = 21.1) albeit 23.8 ppm downfield from the 31P singlet
of the structurally established four-membered ring complex
[{Pd(2-t-Bu2PC6H3C6H4Me-20-jC6,jP)}2(l-Cl)2] (d = ꢀ 13.1)
[32b]. Relating the 31P chemical shift observed for 2 to that
of complex 1 (d = 43.0), where the phosphine is coordi-
nated to a silver atom, an upfield shift of 32.3 ppm is
revealed. This comes close to the differences in the shift
values of the structurally related phosphine t-Bu2PPh in
Comparison of the structural parameters calculated for
the two isomers revealed virtually identical bond lengths
and angles (in particular, Ir–P, Ir–C, and P–Caryl distances
Table 1
˚
Calculated bond lengths (A) and angles (ꢁ) for 2-4r and 2-6r
a
Method
Ir–P
Ir–C
P–Caryl
C–Ir–P
Ir–P–C
Isomer 2-4r
B3LYP/LANL2DZp
BP86/LACVP*
LMP2//LACVP*
‘‘Water’’
2.41
2.39
2.39
2.40
2.06
2.06
2.08
2.06
1.87
1.86
1.86
1.87
67.8
68.0
67.3
68.2
83.6
84.4
84.5
83.9
Isomer 2-6r
B3LYP/LANL2DZp
BP86/LACVP*
LMP2//LACVP*
‘‘Water’’
2.45
2.43
2.54
2.43
2.09
2.08
2.11
2.08
1.89
1.88
1.88
1.88
82.4
82.6
82.5
82.7
99.8
101.2
99.3
Me
Ir
Me
P
tBu
Ir
But
P
But
100.9
tBu
a
B3LYP/LANL2DZp: B3LYP/LANL2DZp + ZPE(B3LYP/LANL2-
DZp); BP86/LACVP*: BP86/LACVP* + ZPE(B3LYP/LANL2DZp);
LMP2/LACVP*: LMP2/LACVP* + ZPE(B3LYP/LANL2DZp); ‘‘Water’’:
BP86(water)/LACVP* + ZPE(B3LYP/LANL2DZp).
4-membered iridacycle 2-4r
Scheme 3.
6-membered iridacycle 2-6r

2628
L. Dahlenburg et al. / Inorganica Chimica Acta 361 (2008) 2623–2630
Fig. 3. Calculated (B3LYP/LANL2DZp) structures of 2-4r and 2-6r; hydrogen atoms are omitted for clarity.
as well as C–Ir–P and Ir–P–C angles) irrespective of the
method used – except for 2-6r, where the LMP2/LACVP*
results indicate elongated Ir–P and Ir–C bonds (Table 1).
Both the Ir–P and the Ir–C distances obtained for the ener-
getically preferred structure 2-4r (average values, 2.39 and
rings from rotating about their connecting C–C single
bond, such that they are now inclined to each other at
an angle of only 50.3ꢁ. As a consequence, the phenylene
ortho C–H bond is forced to adopt an orientation which
results in destabilizing steric clash with the tert-butyl sub-
stituents, the shortest repulsive C–Hꢁ ꢁ ꢁH–C contact being
˚
2.06 A) are in good agreement with those previously
˚
measured by X-ray crystallography for a family of ortho-
metalated iridium(I) complexes [L2Ir{R2PC6H4-jC2,jP}-
(R2PPh)] (L = C2H4, CO; R = Ph, i-Pr), where d(Ir–P) =
1.88 A.
4. Concluding remarks
˚
˚
2.370–2.400 A and d(Ir–C) = 2.070—2.141 A [39]. The very
same applies for the Ir–P–C valence angles varying between
83.7ꢁ and 85.7ꢁ in the latter compounds and averaging out
at 84.1ꢁ for 2-4r. As no C–Ir–P data are available for the
structurally established four-membered iridaheterocycles
[L2Ir{R2PC6H4-jC2,jP}(R2PPh)] [27,39], we note that
the average value of 67.8ꢁ calculated for [(g4-1,5-
C8H12)Ir(2-t-Bu2PC6H3C6H4Me-2-jC6,jP)] compares
favorably with the acute C–Pd–P and C–Pt–P angles of
the orthometalated PdII and PtII complexes [{Pd(2-t-
Bu2PC6H3C6H4Me-20-jC6,jP)}2(l-Cl)2] [32b] and trans-
[Pt(ONO2)(t-Bu2PC6H4-jC2,jP)(t-Bu2PPh)] [40], which
were measured as 68.7ꢁ and 69.7ꢁ, respectively.
The reason for the preferred formation of the structure
with the four-membered chelate ring becomes obvious
from Fig. 3, where rotation of the remote tolyl group
about the aryl–aryl linkage of 2-4r is not constrained by
the formation of an iridium–carbon bond. As a result,
the tolyl ring lies almost perpendicular to the phenylene
plane, which avoids steric overcrowding through collision
with the tert-butyl groups. In the less favored six-mem-
bered ring isomer 2-6r, on the other hand, coordination
of the tolyl residue to the metal prevents the two aromatic
An investigation of the reaction of the Buchwald phos-
phine t-Bu2PbiphMe with [(g4-1,5-C8H12)2Ir2(l-Cl)2] has
shown that, in the presence of Ag+ ions, the phosphine
coordinates to silver producing heterobimetallic [(g4-1,5-
C8H12)Ir(l-Cl)2Ag(t-Bu2PbiphMe)]. Precipitation of the
bridging chlorido ligands prior to the addition of the phos-
phine results in the chelation of the remaining (g4-1,5-
C8H12)Ir+ complex fragment through Ir–P coordination
and C–H metalation. Detailed DFT calculations indicate
that the phosphine ligand of the cyclometalated product
[(g4-1,5-C8H12)Ir(t-Bu2PbiphMe–H+)] forms a sterically
uncongested albeit strained four-membered iridaheterocy-
cle through orthometalation, i.e., [(g4-1,5-C8H12)Ir(2-t-
Bu2PC6H3C6H4Me-20-jC6,jP)], rather than an unstrained
but sterically encumbered six-membered chelate structure
through C–H activation on the remote phenyl ring.
Acknowledgments
Support of this work by the Deutsche Forschungsgeme-
inschaft (Bonn, SFB 583) and the Fonds der Chemischen
Industrie (Frankfurt/Main) is gratefully acknowledged.

L. Dahlenburg et al. / Inorganica Chimica Acta 361 (2008) 2623–2630
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2629
We would like to thank Prof. R. van Eldik for support,
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Erlangen (RRZE) for a generous allotment of computer
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obtained free of charge from The Cambridge Crystallo-
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